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Risk Analysis DOI: 10.1111/risa.12354

Uncertainty Assessment in the Quantification of Risk Rates

of Occupational Accidents

Ioannis A. Papazoglou,1, Olga Aneziris,1 Linda Bellamy,2 B. J. M. Ale,3 and Joy I. H. Oh4

Occupational risk rates per hour of exposure have been quantified for 63 occupational ac-cident types for the Dutch working population. Data were obtained from the analysis ofmore than 9,000 accidents that occurred over a period of six years in the Netherlands and

resulted in three types of reportable consequences under Dutch law: (a) fatal injury, (b) per-manent injury, and (c) serious recoverable injury requiring at least one day of hospitalization.A Bayesian uncertainty assessment on the value of the risk rates has been performed. Annualrisks for each of the 63 occupational accident types have been calculated, including the vari-ability in the annual exposure of the working population to the corresponding hazards. Thesuitability of three risk measuresindividual risk rates, individual annual risk, and numberof accidentsis examined and discussed.

KEY WORDS: Accidents per worker; Bayesian analysis; occupational accident; occupational annualrisk; occupational risk rate; uncertainty assessment

1. INTRODUCTION

Occupational safety is a major concern to manycountries; occupational accidents represent a majorsource of worker deaths and injuries. In 2011, ac-cidents at work killed 3,691 workers in the EU-28,whereas 2.7 million workers were injured, with ab-sence from work of more than three days.(1)

Traditional occupational safety policies arebased on legislation, regulation, standards, safetyguidelines, collection of best practices, accidentstatistics, investigations, inspections, and analysis

1National Centre for Scientific Research DEMOKRITOS,Terma Patriarchou Gregoriou E & Neapoleos, Agia Paraskevi,153-10, Greece.

2White Queen Safety Strategies, Hoofddorp, 2130 AS, TheNetherlands.

3Bosboom-Toussaint Plein 117, Delft, 2624 DJ, The Netherlands.4Ministry of Social Affairs & Employment (SZW), The Hague,

2509 LV, The Netherlands.Address correspondence to Ioannis A. Papazoglou, National

Centre for Scientific Research DEMOKRITOS, TermaPatriarchou Gregoriou E & Neapoleos, Agia Paraskevi 153-10,Greece; [email protected].

of safety management systems. The most signifi-

cant standards used for occupational safety are theBritish(2,3) and ILO(4) internationally applied stan-dards. In addition, various analyses of occupationalaccidents have examined the causes of occupationalinjuries and fatalities such as for the construction,electric power, and extractive industries.(57)

Recently, a new tendency has emerged to quan-tify occupational risk and by doing so to strengthenthe basis for occupational risk management. Inaddition to the identification of causes of accidentsin the workplace, this new approach is strivingto quantify the extent to which various working-environment-shaping factors are present in the work-place and to combine them with workers exposureto hazards to arrive at quantified assessments of risk.Two examples of semi-quantitative risk assessmentmethods for occupational risk assessment(8,9) are: (1)risk matrices with two dimensions, the frequency ofoccurrence and the severity of consequences withsemi-quantified scales; and (2) a proportional riskassessment technique, the so-called RSPE method,based on the functionR = SPE, whereR is the risk,

1 0272-4332/15/0100-0001$22.00/1 C2015 Society for Risk Analysis

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2 Papazoglou et al.

S is the severity of accident occurrence, P is thefrequency of the accident and its consequences, andE is the frequency of employee exposure to hazard.The scales of the RSPE method are arbitrarilychosen and formed on a qualitative basis.

A number of attempts to provide a more sys-tematic and consistent approach to quantitativeoccupational risk assessment has appeared in theliterature. A model has been developed to pre-dict the frequency of occupational accidents inthe offshore oil and gas industry based on direct,corporate, and external factors.(10) Quantified riskfor various occupational groups in Sweden based onthe number of accidents and relevant exposure hasbeen calculated.(11) A method has been proposed forrisk assessment of several trades in the constructionindustry, based on estimating the overall frequencyand severity from historical data of accidents inHong Kong and their consequences regarding in-juries, days lost, and compensation costs.(12) Fuzzymethods have been used for risk assessment ofoccupational accidents in a steel company(13) and atconstruction sites(1416) and workplaces.(17) Artificialneural networks and a fuzzy inference system havebeen proposed to assess occupational injury riskindexes and predict number of injuries.(18)

Besides these approaches, the Ministry of SocialAffairs and Employment in the Netherlands devel-oped a large-scale project during 20032008 to helpimprove the level of safety in the workplace by in-

troducing quantitative occupational risk assessment.This project had four major parts: (1) assembly andanalysis of accident and exposure data, (2) general-ization of these data into a logical risk model, (3) de-riving improvement measures and their costs, and (4)developing an optimizer that supports cost-effectiverisk reduction strategies, as described in Refs. 1921.Results of this Workgroup Occupational Risk Model(WORM) project are presented in Refs. 22 and 23.Its main achievements are twofold. First, there is theconstruction of logic models (bowties) that allow forthe quantitative assessment of the risk of 63 types

of occupational accidents. These bowties include,for example, fall from ladders, scaffolds, roofs,(24,25)

and hit by falling objects(26) and are built on boththe detailed analysis of 9,387 accident reports in theNetherlands, extracted from the GISAI(27) occupa-tional accident database, and data concerning the ex-posure of the Dutch population to various hazards,as reported in Ref. 28 Second, there is the devel-opment of the probabilistic occupational risk model(ORCA), which can be used to calculate the risks

to workers performing various tasks and exposedto several hazards, as presented in construction andmanufacturing case studies.(2931)

Occupational accidents can result in injury ofvarious degrees of severity. Occupational risk from

accidents is quantified when the degree of severity ofthe potential injury can be measured together withthe relative likelihood with which each level of in-jury severity is expected to occur. In the WORMmodel, the injury to the worker was discretized tothree possible values: (1) serious recoverable injurydefined as requiring admission to a hospital within 24hours for a period of at least 24 hours, (2) permanentinjury such as amputation or blindness, and (3) fatal-ity. All such accidents are reportable to the author-ities. Every time a worker performs a work-relatedactivity and faces a particular hazard he or she facesthe possibility of an accident that may result in oneof the above consequence levels. Such a consequencedoes not happen with certainty but rather randomlyin the working population and during the lifetime ofthe workers. The associated risk is quantified whenthe probability with which each of these possible con-sequences of serious recoverable injury, permanentinjury, or fatality that can occur during a specific pe-riod of time is known. Thus, for our purposes, the riskis quantified when the three probabilities have beenassessed or by the triplet {p1,p2,p3}, where it is un-derstood thatp1is the probability of serious recover-able injury,p2is the probability of permanent injury,

and p3 is the probability of death. These probabili-ties should always be thought of in association with agiven period of time during which the accident mighthappen.

The WORM project identified 36 types of acci-dents with the potential for causing serious recover-able, permanent, or fatal injuries. The 36 accidenttypes were further developed into the 63 accidenttypes listed in Table I. Consideration of the divi-sion into 63 accident types was mainly dictated bythe need to identify accidents simple enough to beattributed to various jobs and activities without the

need to differentiate between industrial sectors ortypes of jobs. The classification and codification pro-cess to assign each reported event to the differenttypes was independently checked but errors in a com-plicated process such as this cannot be excluded. Noquantified assessment was made of the magnitude ofthese possible errors.

The purpose of this article is to present the quan-tification of the risk rates, these being the probabilityper unit of time (hour) for each of the 63 accident

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Uncertainty Quantification of Occupational Risk Rates 3

Table I. Types of Occupational Accidents that May Result in Significant Health Effects(23)

Type of Accident Description

1. Fall from heightplacement ladder Fall while climbing or working on mobile (extendable or not) ladders.2. Fall from heightfixed ladder Fall while climbing or working on ladders that are part of the structure to be

climbed.3. Fall from heightsteps Fall while climbing or working on stepladders.4. Fall from heightmobile scaffold Fall while working on mobile scaffold systems with wheels.5. Fall from heightfixed scaffold Fall while working on fixed traditional or system built scaffolding.6. Fall from height(de-)installing scaffold Fall while working on the scaffold while actively helping to erect or

dismantle a fixed or mobile scaffold.7. Fall from heightroof Fall while working on whole roof of a building.8. Fall from heightfloor Fall while working in the immediate vicinity of protected or unprotected

edges of upper floors (floor openings like stairwells under construction orbalconies).

9. Fall from heightplatform Fall while working in the immediate vicinity of protected or unprotectededges of work platforms, gantries, or walkways.

10. Fall from heighthole in the ground Fall while working in the immediate vicinity of holes, wells, or openings inthe ground (whether or not covered or fenced off) that are big enough fora (part of a) person to fall in.

11. Fall from heightmoveable platform Fall while working in or on moveable platforms/cabins and work cages(open lifts for transporting passengers and/or goods, cabins attached tohoisting equipment/cranes, platforms/cabins on scissor lifts, and cherrypickers with an articulated or telescopic boom and platforms attached toshelves with rails or chains, temporary or makeshift cabins on the forks oflift trucks, or other vehicles with fork).

12. Fall from nonmoving vehicle Fall while working on the roof of the cabin of a vehicle, on a cargocontainer, on a trailer, or the floor of an uncovered cargo.

13. Fall from heightworking on height unprotected Fall while working at heights where no specific fall prevention is availableand where one must provide ones own fall protection (e.g., machines,installations, warehouse or other racks, garbage dumps, silos, cargocontainers, waste containers, masts, booms or chains of lift trucks, lifts orcranes, heights aboard ships, windmills, radio masts, weight-bearingstructures, fences, partition walls, shuttering, concrete or steel girders,truss structures, wood piles, haystacks, trees, rocks or large objects on

which one can stand but that are not intended for that purpose,equipment such as trampolines, rope slides, abseiling tracks, and climbingwalls).

14. Fall on same level Fall while walking or standing.15. Fall down stairs or ramp Fall while using of all kinds of fixed stairs.16. Struck by moving vehicle Struck by moving vehicle while working on industrial premises, in

warehouses, at loading/unloading bays, or along the public highway (roadconstruction workers, garbage collectors, roadside vehicle repairmechanics, pick-up services, police officers, etc.).

17. Contact with falling objectcranes, part of cranes,or crane loads

Contact with falling object while working in the immediate vicinity of craneswhereby there is a risk of being hit by falling loads or objects.

18. Contact with falling objectmechanical liftingexcept cranes

Contact with falling object while working in the immediate vicinity of loadsthat are lifted with lift trucks, pallet trucks, or other mechanical liftingdevices (excluding cranes).

19. Contact with falling objecttransportation vehicles Contact with falling object while moving loads with the aid of transport carts

(carts that have to be pushed, carts with their own propulsion, or carts ona conveyer or belt).

20. Contact with falling objectmanual handling Contact with falling object during manual transportation of loads.21. Contact with falling objectother Contact with falling objects owing to the fall of objects that should be firmly

in place (e.g., machine components, piles, or bundles of [building]materials, etc.).

22. Contact flying objectmachine or handheld tool Hit by objects or parts of objects that may fly around, while working with orin the immediate vicinity of running machinery or while using hand toolsthat have a shaving, cutting, pressing, or rolling action and that may causematerial to fly around.

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Table I. (Continued)


23. Contact flying objectobject under pressure ortension

Hit by objects or parts of objects that are under pressure or (mechanical)tension and can fly loose, while working in the immediate vicinity of

objects that may be under pressure or (mechanical) tension (e.g., flexiblehoses, tensioners, swivels, springs, cables, wire, hoisting, tensioning,securing, or lifting equipment, hinged parts, building materials, and otherobjects under tension whereby material can fly around).

24. Contact flying objectblown by wind Hit by objects blown by wind or blown open/shut by the wind (e.g., buildingmaterials and doors).

25. Hit by rolling/sliding object or person Hit by objects that may roll, shift, or slide against a person (e.g., sheets,rods, pipes, containers, boxes, barrels, reels, stones, sand, or other loosematerial).

26. Contact with object person is carrying orusinghandheld tool

Contact with tools like knives, hammers, pincers, pliers, screwdrivers,electric drills, saws, staplers, garden tools, etc.

27. Contact with object person is carrying orusingnot handheld tool

Contact with objects persons/colleagues may carry, move, or use, forinstance, when coupling or uncoupling trailers, manually loading,unloading, or moving heavy objects, manual use of objects with sharpedges or (hinged)parts that one may become trapped between.

28. Contact with handheld tools operated by self Contact with handheld tools like knives, hammers, pincers, pliers,screwdrivers, electric drills, saws, staplers, garden tools, etc.

29. Contact with moving parts of a machineoperating Contact with moving parts of a machine during standard operation of amachine, including starting up, testing, adjustment, feeding, andpositioning products.

30. Contact with moving parts of amachinemaintaining

Contact with moving parts of a machine during reparation and maintenanceof a machine, including the time that one is not actively working with themachine but is within reach of (possible) moving parts.

31. Contact with moving parts of a machineclearing Contact with moving parts of a machine during clearing or unblocking amachine with an interruption of service, including the time that one is notactively working with the machine but is within reach of (possible)moving parts.

32. Contact with moving parts of a machinecleaning Contact with moving parts of a machine during cleaning a machine,including the time that one is not actively working with the machine but iswithin reach of moving parts.

33. Contact with hanging/swinging objects Contact with hanging/swinging objects while working in the immediatevicinity of cranes, rotating arms of excavating machines, lift and pallettrucks, and lifting tools.

34. Trapped between/against Working in an environment where a person could be trapped betweenvehicles, machines, and/or objects.

35. Moving into an object Walking back and forth, stepping onto/off raised platforms, running,cleaning, making repairs, and other manual labor, whereby it is possibleto collide with an object or person with force, to get caught behind anobject (e.g., by wearing a ring), or to bump into or to seize a sharp object.

36. Buried by bulk mass Buried by bulk mass could happen by falling into a silo, by being buried inan intended or accidental discharge from a silo, by work in a ditch or nextto an unstable slope/wall made of earth, clay, or sand, or by working on ornext to a heap or pile of such material. This may occur while working withsolid bulk materials or in the immediate vicinity of places where these arehandled or stored (e.g., powders, sand, soil, granulates, cement powder,

ores, coke, coal, phosphate, artificial fertilizer, grain, and other substancesin which one can suffocate or be buried).

37. In or on moving vehicle with loss of control Loss of control while working in or at moving vehicles (like cars, lorries,buses, military vehicles, fork lift trucks, pallet trucks, bicycles, cherrypickers, cranes, agricultural and earth-moving machinery, or vehicles onrails [monorails, trains], etc.).

38. Contact with electricitywires Contact with electricity wires, while working with vehicles, cranes, aerialwork platforms, or excavating machines in the immediate vicinity of hightension cables, including the overhead cables of trams and trains.

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39. Contact with electricitytools Contact with electricity, while using of electrical equipment, like tools,batteries, industrial equipment, office equipment, lighting, etc.

40. Contact with electricity electrical work Contact with electricity, while installing and/or repairing electrical systemsor equipment (e.g., installing and/or repairing control boxes, distributionsystems, switch boxes, transformer rooms, circuitry, conductive rails,overhead wiring, lighting, plugs, and sockets).

41. Contact with extreme hot or cold surfaces or openflame

Contact with extremely hot or cold surfaces or open flame. For example:products that are very hot or cold in the production process, or emergefrom it very hot or cold (machinery, exhaust pipes, ovens, outlets forcooling water or condensation, heated components [axles, rods, bolts],etc.). An open flame refers to controlled flame, like in burners, furnaces,or welding equipment.

42. Release of hazardous substance out of opencontainment

Release of hazardous substance out of open containment, while workingwith or in the immediate vicinity of hot liquids or other hazardousmaterials in open vessels. Hazardous materials refers to biologicallyhazardous, radioactive, toxic, corrosive, or irritating substances as well aspots and pans containing hot liquids, such as molten metals, cooking fat,

water, steam, coffee, tea, soup, or extremely cold liquids. Open vesselsrefer to tanks, drums, containers, cans, boxes, buckets, pans, and bottlesthat are (with or without lid) not sealed against liquids.

43. Exposure to hazardous substance without loss ofcontainment

Exposure to hazardous substance without loss of containment, during theinvolvement in activities or situations in which people may come intodirect contact with hazardous materials (e.g., cleaning agents, paint withsolvents, degreasing agents, using objects that may be contaminatedremoving hazardous substances such as waste, contaminated soil, orspillages, or other activities in which people may unintentionally comeinto contact with a hazardous substance).

44. Release of a hazardous substance out of closedcontainmentadding/removing a substance

Release of a hazardous substance out of closed containment while adding orremoving hazardous materials and in opening closed systems (e.g.,opening a valve, seal, or tap, opening a manhole or creating an opening invessels, pipes, or similar by grinding through them).

45. Release of a hazardous substance out of a closed

containmenttransport of closed containment

Release of a hazardous substance out of a closed containment while

transporting or moving (portable, mobile) closed systems or supervisingsuch activities.

46. Release of a hazardous substance out of a closedcontainmentclosing a containment

Release of a hazardous substance out of a closed containment while sealingof systems or supervising such activities.

47. Release of a hazardous substance out of a closedcontainment

Release of a hazardous substance out of a closed containment, while beingin the immediate vicinity of it and performing all other activities that canaffect the state of the system. This includes vehicle movements,excavation, or lifting works in the immediate vicinity of a closed vessel orpipe, just like work on its exterior (welding, grinding, cleaning, painting,etc.).

48. Firehot work Performing tasks that involve a fire hazard, in which ignition sources arepresent or may be created by sparks or heating (e.g., working withwelding equipment, burners or grinders, cooking, drilling, or sawing).

49. Fireworking with or being nearflammables/combustibles

Working with or in the immediate vicinity of (systems with) flammablesubstances.

50. Firefire fighting Work in which persons are actively involved in fighting fires and/or rescueactivities in the event of fire.

51. Victim of human aggression Work that brings employees in contact with other people who can becomeaggressive (e.g., working with public, customers, clients, or patients atcounters or tills, in the catering industry, in public transport, security,education, the police, in care institutions, prisons, etc.).

52. Victim of animal behavior Work that may bring people in contact with animals. This refers to possiblecontact with animals in a professional capacity.

53. Exposure to hazardous atmosphere in confinedspace

Activities in confined spaces like freezers, (slurry) pits, storage tanks,transport tanks, ships holds, system components, sewers, etc.

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54. Exposure to hazardous atmosphere throughbreathing apparatus

Activities in which equipment is used that provides breathing gas.

55. Impact by immersion in liquidworking in, on, orunder

Work in which people are intentionally in or under water.

56. Impact by immersion in liquidworking nearby Work involving situations in which people could fall into water or anotherliquid.

57. Extreme muscular exertionhandling objects Activities or situations where extreme force is necessary to handle or moveobjects or persons (e.g., lifting, pushing, pulling, or carrying persons orobjects).

58. Extreme muscular exertionmoving around Activities in which sudden, extreme movements can be made (e.g., climbingon and off vehicles, physical training, and sport during working hours).

59. Physical explosion This refers to work with or in the immediate vicinity of equipment, devices,or objects that may be under pressure. This refers to: (1) closed systemsor vessels that are normally pressurized or are pressurized in the course ofwork. Examples are gas cylinders, storage tanks, (car) tires, industrialcream sprays, and hollow vessels that are compressed as part of theproduction process; (2) closed systems or vessels that may explode on

impact or in response to (planned or unplanned) heating.60. Chemical explosionvapor gas Working with or in the immediate vicinity of inflammable substances

(vapor, [organic]dust, volatile solvents), commercial products containingvolatile solvents (spray cans, paint, adhesives, thinners, or petrol), orother gaseous explosive substances (LPG, natural gas, acetylene, etc.).

61. Chemical explosionsdust Working with or in the immediate vicinity of substances that may reactexplosively in contact with other substances.

62. Chemical explosionssolids Working with or in the immediate vicinity of solid explosives, such asfireworks, dynamite, etc.

63. Chemical explosionsreactions Working with or in the immediate vicinity of substances that may reactexplosively under accidental circumstances.

types. The article is organized as follows: Section 2

outlines the modeling of the occurrence of occupa-tional accidents as a Poisson random process andbriefly describes the procedure for identifying thenumber of accidents and the exposure of the Dutchworking population to the occupational hazards dur-ing a given period of time. With these two sets ofdata the point estimates of the risk rates are obtained.Section 3 presents the assessment of the uncertaintiesassociated with this estimation following a Bayesianapproach. Section 4 addresses the issue of proba-bility of an accident per year. Section 5 discussesthe obtained results. Finally, Section 6 presents the

conclusions.

2. MODEL AND DATA REQUIREMENTS

Occupational accidents occur randomly in time.As time-to-accident (t) one can define the timeelapsing between the start of a certain job with ex-posure to a given hazard and the onset of an ac-cident. This analysis assumes that accidents happenrandomly as the worker performs the job and that

accidents occur according to a Poisson random pro-

cess. This means that the times-to-accidents are ex-ponentially distributed with parameter or that theprobability density function (pdf) of the random vari-abletis given by:

f(t) = et. (1)

If a worker is exposed to an accident hazard for hours during a calendar period (e.g., a year), then theprobability that there will be an accident at any timeduring this period is equal to the probability that thetime of accident occurrence will be less thanor:

p = F(t ) = 1 e. (2)

2.1. Sampling and Likelihood of Sample

The parameter is estimated from observationsof the random process and recording of the times atwhich accidents occur.

This is usually done by observing a population ofNworkers each for a given period of time t0. Duringthis period some of the workers will actually suffer

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Table II. Total Exposure and Number of Observed Injuries for 63 Occupational Accident Types in the Netherlands (23)

Number of Injuries Point Risk Rate (/HR)

Total Serious SeriousExposure Permanent Recoverable Permanent Recoverable

(Hours) Fatalities Injuries Injuries Fatalities Injuries Injuries

1.1.1.1 Fall fromheightplacementladder

7.58E+08 18 159 628 2.38E08 2.10E07 8.29E07

1.1.1.2 Fall fromheightfixed ladder

2.14E+08 4 12 54 1.87E08 5.55E08 2.53E07

1.1.1.3 Fall fromheightstep ladder orsteps

6.33E+08 3 43 141 4.74E09 6.78E08 2.23E07

1.1.2.1 Fall fromheightworking onmobile scaffold

1.42E+08 8 44 193 5.64E08 3.13E07 1.36E06

1.1.2.2 Fall fromheightworking on fixed

scaffold

3.96E+08 7 38 184 1.77E08 9.57E08 4.65E07

1.1.2.3 Fall fromheight(de-)installingscaffold

2.66E+08 5 19 54 1.88E08 7.21E08 2.02E07

1.1.3.1 Fall fromheightroof

1.91E+08 43 93 286 2.26E07 4.86E07 1.50E06

1.1.3.2 Fall fromheightfloor

3.00E+08 19 80 310 6.41E08 2.67E07 1.03E06

1.1.3.3 Fall from height fixed platform

2.75E+08 12 56 166 4.37E08 2.05E07 6.02E07

1.1.4 Fall fromheighthole in theground

1.19E+09 2 14 58 1.68E09 1.20E08 4.85E08

1.1.5.1 Fall fromheightmoveable

platform

2.62E+08 21 70 113 8.02E08 2.66E07 4.33E07

1.1.5.2 Fall from height nonmoving vehicle

1.36E+08 7 49 149 5.15E08 3.61E07 1.10E06

1.1.5.3 Fall fromheightother

3.36E+08 6 65 217 1.79E08 1.94E07 6.46E07

1.2 Fall on same level 2.51E+10 3 84 323 1.20E10 3.36E09 1.29E081.3 Fall down stairs or ramp 5.40E+08 2 22 97 3.73E09 4.08E08 1.80E072. Struck by moving vehicle 4.00E+09 46 189 314 1.15E08 4.72E08 7.85E083.1 Contact with falling

objectcrane or load3.05E+08 11 48 42 3.61E08 1.56E07 1.36E07

3.2 Contact with fallingobjectmechanicallifting

3.46E+08 2 27 26 5.78E09 7.76E08 7.56E08

3.3 Contact with fallingobjectvehicle or load

2.26E+08 0 13 13 2.72E10 5.56E08 5.56E08

3.4 Contact with fallingobjectmanual handling

2.77E+08 0 19 41 2.22E10 6.95E08 1.47E07

3.5 Contact with fallingobjectother

1.10E+08 18 72 117 1.64E07 6.56E07 1.06E06

4.1 Contact with flyingobjectmachine orhandheld tool

1.49E+09 2 147 72 1.34E09 9.84E08 4.86E08

4.2 Contact with flyingobjectobject underpressure or tension

1.19E+09 7 80 72 5.88E09 6.75E08 6.02E08

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Table II. (Continued)




4.3 Contact with flyingobjectblown by wind

5.40E+08 0 4 8 5.69E11 6.54E09 1.39E08

5 Hit by rolling/slidingobject or person

1.68E+09 3 37 62 1.79E09 2.20E08 3.69E08

6.1 Contact with objectused orcarriedhandheld tooloperated by other person

3.80E+09 0 15 20 5.47E13 4.00E09 5.21E09

6.2 Contact with objectused or carried NOThandheld tool

2.48E+09 1 140 71 4.04E10 5.66E08 2.85E08

7 Contact with hand heldtools operated by self

5.54E+09 0 107 127 1.04E12 1.94E08 2.29E08

8.1.1 Contact with movingparts of a machineoperating

1.94E+09 1 401 74 5.17E10 2.07E07 3.80E08

8.1.2 Contact with movingparts of a machinemaintaining

3.82E+08 2 46 14 5.26E09 1.20E07 3.67E08

8.1.3 Contact with movingparts of a machineclearing

1.09E+08 6 120 31 5.51E08 1.10E06 2.81E07

8.1.4 Contact with movingparts of a machinecleaning

1.50E+08 1 81 20 6.73E09 5.41E07 1.33E07

8.2 Contact with hanging/swinging objects

1.13E+09 10 90 114 8.85E09 7.99E08 1.01E07

8.3Trapped between 1.25E+09 15 200 96 1.20E08 1.60E07 7.65E08

9 Moving into object 6.57E+09 0 67 59 1.17E12 1.02E08 8.98E0910 Buried by bulk mass 5.99E+08 0 4 13 2.27E12 7.24E09 2.11E0811 In or on moving vehicle

with loss of control2.66E+09 37 164 332 1.39E08 6.15E08 1.25E07

12.1 Contact withelectricityhigh voltagecable

2.10E+08 4 4 10 1.91E08 1.91E08 4.76E08

12.2 Contact withelectricitytool

7.96E+09 5 9 25 6.28E10 1.09E09 3.18E09

12.3 Contact withelectricity electricalwork

1.26E+09 10 35 101 7.94E09 2.78E08 8.01E08

13 Contact with hot or coldsurfaces or open flame

1.81E+09 0 4 7 1.15E12 2.21E09 3.87E09

14.1 Release of hazardous

substance out of opencontainment

1.21E+09 0 41 127 1.75E11 3.35E08 1.05E07

14.2 Exposure to hazardoussubstance without loss ofcontainment

1.17E+09 2 22 113 1.79E09 1.91E08 9.64E08

15.1 Release of a hazardoussubstance out of a closedcontainmentadding,removing, or opening

6.55E+08 2 47 143 3.06E09 7.15E08 2.19E07

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Table II. (Continued)




15.2 Release of a hazardoussubstance out of a closedcontainmenttransport

9.98E+07 0 2 19 2.57E11 2.01E08 1.90E07

15.3 Release of a hazardoussubstance out of a closedcontainmentclosing

4.14E+07 0 7 8 1.55E11 1.57E07 1.81E07

15.4 Release of a hazardoussubstance out of a closedcontainmentworkingnearby

2.26E+08 2 11 50 8.87E09 4.80E08 2.22E07

17.1 Firehot work 1.27E+09 1 17 66 7.87E10 1.34E08 5.20E0817.2 Fireworking near

flammables/combustibles1.09E+09 2 19 45 1.83E09 1.70E08 4.08E08

17.3 Firefire fighting 1.08E+08 0 8 26 4.00E11 6.95E08 2.36E0720.1 Human aggression 6.58E+09 3 43 28 4.57E10 6.54E09 4.26E0920.2 Animal behavior 5.60E+08 2 13 17 3.58E09 2.38E08 2.98E0822.1 Hazardous

atmosphere in confinedspace

4.80E+08 11 5 34 2.30E08 1.05E08 7.09E08

22.2 Hazardousatmosphere throughbreathing apparatus

9.00E+07 2 1 7 2.23E08 5.60E09 7.23E08

23.1 Impact by immersionin liquidworking in orunder

3.99E+07 1 4 4 2.51E08 8.77E08 8.77E08

23.2 Impact by immersionin liquidworkingnearby

3.09E+08 20 2 6 6.47E08 4.86E09 1.78E08

25.1 Extreme muscularexertionhandlingobjects

5.26E+09 0 10 6 1.22E13 1.81E09 1.05E09

25.2 Extreme muscularexertionmovingaround

1.63E+09 0 6 17 1.96E13 3.62E09 1.05E08

27.1 Physical explosion 1.39E+09 3 11 29 2.16E09 7.85E09 2.09E0827.2.1 Chemical

explosionvapor or gas1.62E+09 15 39 72 9.26E09 2.38E08 4.47E08

27.2.2 Chemicalexplosiondust

9.44E+08 1 7 7 1.06E09 6.89E09 6.89E09

27.2.3 Chemicalexplosionexplosives

1.06E+09 0 7 1 7.52E13 6.58E09 9.41E10

27.2.4 Chemical explosionexothermic reactions

1.45E+09 0 3 14 2.20E13 2.07E09 9.64E09

an accident with given consequences and some willsimply work the whole periodt0without an accident.Letkbe the number of workers suffering an accidentat times (t1, t2, . . . , tk). Then N k is the numberof workers without an accident, that is, each of themworked fort0without an accident.

Assuming that the occurrence of each accidentdoes not depend statistically on the other occur-rences or nonoccurrences, and that if a worker has anaccident atti(s)he is no more exposed to the hazardfor up to t0, it is possible to calculate the probabil-ity of such an outcome, also called likelihood. The

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0 0,950

0 90,0% 5,0%

0

0,2

0,4

0,6

0,8

1

1,2

-0,

2 0

0,

2

0,

4

0,

6

0,

8 1

1,

2

Uniform(0;1)

Fig. 2. Prior distributions for annual accident probability.

0,026 1,498

5,0% 90,0% 5,0%

0

500

1000

1500

2000

2500

-0,5 0

0,5 1

1,5 2

2,5

Values in Thousandths

Prior / 2,00E+03

Prior

Fig. 3. Prior distributions for .

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12 Papazoglou et al.

41,9 53,1

5,0%

100,0%

90,0%

0,0%

5,0%

0,0%

0

0,5

1

1,5

2

2,5

010

20

30

40

50

60

Va

luesx

10^8

Values in Billionths

Posterior

Permanent

Fatalies

Fig. 4. Posterior distributions for risk rates for fatal and permanent injuries for the accident type struck by a moving vehicle.

If there are Nworkers in the sample to be ob-served, then the duration of observation of each oneis set tot0. If in addition there is no replacement of aworker having an accident, then:

Poisson sampling of sizeNwithout replacement

T=

ki=1

ti + (N k)t0 = Nt0 k

i=1

(t0 ti ).(7)

If there is replacement of a worker in the sample

when (s)he has an accident then:

Poisson sampling of sizeNwith replacement

T= Nt0. (8)

In all these cases, the statistic Tis predeterminedand known and the statistic k is randomly determinedafter the sampling ends.

2.3. Number of AccidentsIn the WORM project, the constant acci-

dent rate for each of the 63 accident types hasbeen estimated following the analysis described inSection 2.2. A total of 9,387 accidents, reported ac-cording to the Dutch labor law, that occurred in theperiod 1998Feb. 2004 (6.17 years) and contained inthe GISAI database,(27) have been analyzed usingthe Storybuilder tool.(32) This tool enables the build-ing of scenarios or accident sequences incorporatingall the recorded factors that have contributed to theaccident. It forms the basis for building a logical

model connecting the ultimate consequence (deathor injury) to the underlying factors and the work-ing environment conditions in general. The 9,387 ac-cidents are distributed across 36 Storybuilds, thesebeing graphical structures built using the softwareStorybuilder, each representing a type of occu-pational accident, named in each case by theevent that characterizes the accident, the so-calledCentre Event.(31) The 36 qualitative models were

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Table III. Uncertainty Characteristics of Risk Rates per Hour for a Fatal Injury for Various Accident Types in the Netherlands

Fatality Risk Rate

Type of Accident 5% 50% Mean 95%

1 1.1.1.1 Fall from heightplacement ladder 1.64E08 2.46E08 2.51E08 3.52E082 1.1.1.2 Fall from heightfixed ladder 9.21E09 2.18E08 2.34E08 4.28E083 1.1.1.3 Fall from heightstep ladder or steps 2.16E09 5.80E09 6.32E09 1.23E084 1.1.2.1 Fall from heightworking on mobile scaffold 3.31E08 6.11E08 6.34E08 1.02E075 1.1.2.2 Fall from heightworking on fixed scaffold 1.01E08 1.94E08 2.02E08 3.32E086 1.1.2.3 Fall from height(de-)installing scaffold 9.85E09 2.14E08 2.26E08 3.96E087 1.1.3.1 Fall from heightroof 1.77E07 2.30E07 2.32E07 2.92E078 1.1.3.2 Fall from heightfloor 4.48E08 6.64E08 6.75E08 9.39E089 1.1.3.3 Fall from heightfixed platform 2.80E08 4.62E08 4.74E08 7.08E0810 1.1.4 Fall from heighthole in the ground 6.89E10 2.25E09 2.52E09 5.30E0911 1.1.5.1 Fall from heightmoveable platform 5.69E08 8.28E08 8.40E08 1.16E0712 1.1.5.2 Fall from heightnonmoving vehicle 2.93E08 5.65E08 5.89E08 9.68E0813 1.1.5.3 Fall from heightother 9.79E09 1.99E08 2.09E08 3.53E0814 1.2 Fall on same level 5.46E11 1.47E10 1.60E10 3.09E1015 1.3 Fall down stairs or ramp 1.53E09 4.97E09 5.58E09 1.17E08

16 2. Struck by moving vehicle 9.08E09 1.17E08 1.18E08 1.47E0817 3.1 Contact with falling objectcrane or load 2.27E08 3.83E08 3.94E08 5.97E0818 3.2 Contact with falling objectmechanical lifting 2.36E09 7.73E09 8.67E09 1.82E0819 3.3 Contact with falling objectvehicle or load 2.78E10 3.33E09 4.70E09 1.38E0820 3.4 Contact with falling objectmanual handling 2.27E10 2.72E09 3.83E09 1.12E0821 3.5 Contact with falling objectother 1.13E07 1.70E07 1.73E07 2.43E0722 4.1 Contact with flying objectmachine or handheld

tool5.49E10 1.80E09 2.01E09 4.23E09

23 4.2 Contact with flying objectobject under pressureor tension

3.35E09 6.45E09 6.72E09 1.11E08

24 4.3 Contact with flying objectblown by wind 1.05E10 1.34E09 1.91E09 5.66E0925 5 Hit by rolling/sliding object or person 8.16E10 2.19E09 2.39E09 4.62E0926 6.1 Contact with object used or carriedhandheld tool

operated by other person1.36E11 1.83E10 2.64E10 7.89E10

27 6.2 Contact with object used or carriedNOT

handheld tool

1.44E10 6.78E10 8.07E10 1.91E09

28 7 Contact with handheld tools operated by self 9.45E12 1.26E10 1.82E10 5.43E1029 8.1.1 Contact with moving parts of a

machineoperating1.84E10 8.67E10 1.03E09 2.45E09

30 8.1.2 Contact with moving parts of amachinemaintaining

2.15E09 7.02E09 7.88E09 1.65E08

31 8.1.3 Contact with moving parts of a machineclearing

3.02E08 6.13E08 6.43E08 1.09E07

32 8.1.4 Contact with moving parts of a machinecleaning

2.39E09 1.12E08 1.34E08 3.17E08

33 8.2 Contact with hanging/ swinging objects 5.46E09 9.44E09 9.74E09 1.50E0834 8.3 Trapped between 8.03E09 1.25E08 1.28E08 1.85E0835 9 Moving into object 8.02E12 1.07E10 1.53E10 4.58E1036 10 Buried by bulk mass 8.60E11 1.16E09 1.67E09 5.01E0937 11 In or on moving vehicle with loss of control 1.07E08 1.42E08 1.43E08 1.83E08

38 12.1 Contact with electricityhigh voltage cable 9.40E09 2.23E08 2.38E08 4.36E0839 12.2 Contact with electricitytool 3.28E10 7.12E10 7.54E10 1.32E0940 12.3 Contact with electricityelectrical work 4.90E09 8.47E09 8.73E09 1.35E0841 13 Contact with hot or cold surfaces or open flame 2.85E11 3.84E10 5.54E10 1.66E0942 14.1 Release of hazardous substance out of open

containment4.56E11 5.90E10 8.44E10 2.51E09

43 14.2 Exposure to hazardous substance without loss ofcontainment

7.40E10 2.36E09 2.64E09 5.50E09

44 15.1 Release of a hazardous substance out of a closedcontainmentadding, removing, or opening

1.25E09 4.09E09 4.59E09 9.62E09

(Continued)

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Table III. (Continued)

Fatality Risk Rate


45 15.2 Release of a hazardous substance out of a closedcontainmenttransport 5.19E10 6.97E09 1.00E08 3.01E08

46 15.3 Release of a hazardous substance out of a closedcontainmentclosing

1.24E09 1.68E08 2.42E08 7.24E08

47 15.4 Release of a hazardous substance out of a closedcontainmentworking nearby

3.63E09 1.18E08 1.33E08 2.79E08

48 17.1 Firehot work 2.80E10 1.32E09 1.57E09 3.74E0949 17.2 Fireworking near flammables/combustibles 7.50E10 2.45E09 2.75E09 5.78E0950 17.3 Firefire fighting 4.82E10 6.46E09 9.30E09 2.78E0851 20.1 Human aggression 2.08E10 5.59E10 6.09E10 1.18E0952 20.2 Animal behavior 1.46E09 4.78E09 5.37E09 1.13E0853 22.1 Hazardous atmosphere in confined space 1.45E08 2.43E08 2.50E08 3.80E0854 22.2 Hazardous atmosphere through breathing

apparatus9.11E09 2.98E08 3.34E08 7.00E08

55 23.1 Impact by immersion in liquidworking in or

under

8.92E09 4.21E08 5.02E08 1.19E07

56 23.2 Impact by immersion in liquidworking nearby 4.55E08 6.69E08 6.80E08 9.41E0857 25.1 Extreme muscular exertionhandling objects 9.77E12 1.32E10 1.90E10 5.70E1058 25.2 Extreme muscular exertionmoving around 3.15E11 4.25E10 6.14E10 1.84E0959 27.1 Physical explosion 9.86E10 2.65E09 2.88E09 5.59E0960 27.2.1 Chemical explosionvapor or gas 6.20E09 9.67E09 9.88E09 1.43E0861 27.2.2 Chemical explosiondust 3.76E10 1.78E09 2.12E09 5.03E0962 27.2.3 Chemical explosionexplosives 4.83E11 6.52E10 9.41E10 2.82E0963 27.2.4 Chemical explosionexothermic reactions 3.54E11 4.78E10 6.89E10 2.06E09

subdivided into 63 more specific types of accidentsfor which detailed logical models have been devel-oped. A number of investigation reports had uniden-tified consequences. These unknown accident conse-quences have been redistributed to the permanentor recoverable injuries because it is expected that alldeaths are reported. The results are given in Table II.

2.4. Exposure to Hazards

Exposure is the time that a worker is at riskof having an accident. The exposure for each ofthe 63 hazards has been determined by defining theactivitiesknown as missionsthat can lead to theseaccidents and then conducting a national survey in

the Netherlands using a representative sample of theworkforce to establish the exposure to each mission(expressed as duration in hours). For example, for ac-cident type #2: Struck by moving vehicle the mis-sion is defined as:

Number of hours workers are as a pedestrian at lo-cations with moving vehicles during work. This in-cludes working on industrial premises, in warehousesor at loading/unloading bays, or along the publichighway in the role of (for example) road construc-

tion worker, garbage collector, roadside vehicle repairmechanic/pick-up service or police officer. Normal traf-fic activity as a user of the public highway (including

commuting and business travel) is not included.

In November 2006, the mission survey wascarried out(33) among a random sample of 30,000from the TNS-NIPO Internet Panel(34) that has ap-proximately 200,000 members, of which about 50,000constitute a representative sample of the Dutchworking population. In practice, 30,000 were drawnfrom the 50,000. For each mission, the respondentsfilled out how many hours they were exposed in theweek preceding the survey. To obtain representativeexposure data for the Dutch working population,

major deviations from the survey sample with knownpopulation distributions(22,33) were corrected forover- and underrepresentation by applying a weight-ing factor. Aspects of the population distribution thatwere considered in this weighting procedure wereindustry sector, job type, type of employment, educa-tion level, age, and gender. The weighting procedurealso took into account the changes in compositionof the working population in the period 19982004(the period from which accidents were analyzed)

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Table IV. Uncertainty Characteristics of Risk Rates per Hour for a Permanent Injury Accident for Various Occupational Accident Typesin the Netherlands

Permanent Injury Risk Rate


1 1.1.1.1 Fall from heightplacement ladder 1.85E07 2.11E07 2.11E07 2.39E072 1.1.1.2 Fall from heightfixed ladder 3.55E08 5.86E08 6.02E08 9.01E083 1.1.1.3 Fall from heightstep ladder or steps 5.31E08 6.88E08 6.93E08 8.74E084 1.1.2.1 Fall from heightworking on mobile scaffold 2.46E07 3.18E07 3.20E07 4.02E075 1.1.2.2 Fall from heightworking on fixed scaffold 7.38E08 9.73E08 9.82E08 1.25E076 1.1.2.3 Fall from height(de-)installing scaffold 5.04E08 7.46E08 7.59E08 1.06E077 1.1.3.1 Fall from heightroof 4.11E07 4.89E07 4.91E07 5.77E078 1.1.3.2 Fall from heightfloor 2.23E07 2.69E07 2.70E07 3.21E079 1.1.3.3 Fall from heightfixed platform 1.66E07 2.08E07 2.09E07 2.56E0710 1.1.4 Fall from heighthole in the ground 7.99E09 1.26E08 1.29E08 1.87E0811 1.1.5.1 Fall from heightmoveable platform 2.19E07 2.68E07 2.70E07 3.24E0712 1.1.5.2 Fall from heightnonmoving vehicle 2.87E07 3.66E07 3.68E07 4.58E0713 1.1.5.3 Fall from heightother 1.59E07 1.96E07 1.97E07 2.38E0714 1.2 Fall on same level 2.82E09 3.39E09 3.40E09 4.03E09

15 1.3 Fall down stairs or ramp 2.92E08 4.21E08 4.27E08 5.83E0816 2. Struck by moving vehicle 4.20E08 4.74E08 4.75E08 5.33E0817 3.1 Contact with falling objectcrane or load 1.23E07 1.58E07 1.59E07 1.98E0718 3.2 Contact with falling objectmechanical lifting 5.71E08 7.95E08 8.04E08 1.07E0719 3.3 Contact with falling objectvehicle or load 3.59E08 5.85E08 6.00E08 8.91E0820 3.4 Contact with falling objectmanual handling 4.86E08 7.19E08 7.31E08 1.02E0721 3.5 Contact with falling objectother 5.42E07 6.62E07 6.65E07 7.98E0722 4.1 Contact with flying objectmachine or handheld

tool8.61E08 9.89E08 9.91E08 1.13E07


5.64E08 6.81E08 6.83E08 8.13E08



27 6.2 Contact with object used or carriedNOThandheld tool

4.93E08 5.69E08 5.70E08 6.51E08




9.51E08 1.22E07 1.23E07 1.54E07

31 8.1.3 Contact with moving parts of a machineclearing 9.52E07 1.11E06 1.11E06 1.28E0632 8.1.4 Contact with moving parts of a machinecleaning 4.52E07 5.45E07 5.47E07 6.50E0733 8.2 Contact with hanging/swinging objects 6.74E08 8.05E08 8.08E08 9.52E0834 8.3 Trapped between 1.43E07 1.61E07 1.61E07 1.80E0735 9 Moving into object 8.38E09 1.03E08 1.04E08 1.25E0836 10 Buried by bulk mass 3.64E09 8.36E09 8.91E09 1.60E0837 11 In or on moving vehicle with loss of control 5.42E08 6.17E08 6.19E08 7.00E0838 12.1 Contact with electricityhigh voltage cable 9.40E09 2.23E08 2.38E08 4.36E08

39 12.2 Contact with electricitytool 6.54E10 1.18E09 1.22E09 1.93E0940 12.3 Contact with electricityelectrical work 2.12E08 2.83E08 2.86E08 3.69E0841 13 Contact with hot or cold surfaces or open flame 1.09E09 2.58E09 2.76E09 5.06E0942 14.1 Release of hazardous substance out of open



1.37E08 1.97E08 2.00E08 2.72E08


5.66E08 7.25E08 7.30E08 9.12E08

(Continued)

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Table IV. (Continued)

Permanent Injury Risk Rate




8.77E08 1.73E07 1.81E07 3.02E07


3.01E08 5.10E08 5.24E08 7.98E08


apparatus1.97E09 1.32E08 1.67E08 4.35E08


under

4.17E08 1.05E07 1.13E07 2.12E07


and 2006 (the year the exposure was measured). Be-cause the accident population did not include somespecified occupational groups, these occupationalgroups were excluded from the exposure data. Thesame holds true for the self-employed who are notworking under the authority of others (as they arealso excluded from the available accident data).

The weekly exposure was extrapolated to ayearly exposure, based on a 42-working-weeks year.The week in November was an average workingweek. However, being November, the impact ofseasonal labor on the total exposure figures is prob-ably omitted. Next, the survey sample was extrapo-lated to the whole of the Dutch working population.

Fig. 1 demonstrates one example of this exercise

for the occupational accident type struck by a mov-ing vehicle. The average exposure time per year inthis case is 506.52 hours. The total number of ex-posed workers is 1.28 106. Hence, the total num-ber of exposure hours in this case is 648 106 peryear. As the observed number of fatalities in thiscase corresponded to a period of six years and twomonths, the total exposure used in Equation (5) is4 109 hours. During the same period the num-

ber of reported fatalities from accidents with movingvehicles was 46. Hence, Equation (5) calculates thefatality risk rate for the occupational accident typestruck by a moving vehicle as equal to 1.15 108

per hour.It should be noted that in this quantification Pois-

son sampling with replacement has been assumed(see Equation (8)). This approximation, however, isvery good. The term

ki=1(t0 ti ) in Equation (7) is

less than kt0, which inthis case is 46 506.52 6.12=142,595 hours; this is a negligible amount when com-pared to the total of 4 109.

3. UNCERTAINTY ASSESSMENTUncertainties owing to the limited size of the

sample or the equivalent sufficient statistics {k,T} canbe quantified following the Bayesian approach.(3537)

3.1. Prior Distribution of Accident Rate

Before obtaining the sampling results, the stateof knowledge is described by assuming that the

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Table V. Uncertainty Characteristics of Risk Rates per Hour for a Serious Recoverable Injury Accident for Various OccupationalAccident Types in the Netherlands

Recoverable Injury Rate


1 1.1.1.1 Fall from heightplacement ladder 7.77E07 8.30E07 8.30E07 8.85E072 1.1.1.2 Fall from heightfixed ladder 2.03E07 2.56E07 2.58E07 3.17E073 1.1.1.3 Fall from heightstep ladder or steps 1.94E07 2.24E07 2.25E07 2.56E074 1.1.2.1 Fall from heightworking on mobile scaffold 1.21E06 1.36E06 1.36E06 1.53E065 1.1.2.2 Fall from heightworking on fixed scaffold 4.12E07 4.67E07 4.68E07 5.25E076 1.1.2.3 Fall from height(de-)installing scaffold 1.63E07 2.05E07 2.06E07 2.54E077 1.1.3.1 Fall from heightroof 1.36E06 1.50E06 1.50E06 1.65E068 1.1.3.2 Fall from heightfloor 9.43E07 1.04E06 1.04E06 1.14E069 1.1.3.3 Fall from heightfixed platform 5.31E07 6.05E07 6.06E07 6.85E0710 1.1.4 Fall from heighthole in the ground 3.92E08 4.90E08 4.93E08 6.03E0811 1.1.5.1 Fall from heightmoveable platform 3.72E07 4.35E07 4.37E07 5.06E0712 1.1.5.2 Fall from heightnonmoving vehicle 9.59E07 1.10E06 1.10E06 1.25E0613 1.1.5.3 Fall from heightother 5.78E07 6.48E07 6.48E07 7.22E0714 1.2 Fall on same level 1.17E08 1.29E08 1.29E08 1.41E08

15 1.3 Fall down stairs or ramp 1.52E07 1.81E07 1.81E07 2.13E0716 2. Struck by moving vehicle 7.16E08 7.87E08 7.88E08 8.62E0817 3.1 Contact with falling objectcrane or load 1.06E07 1.38E07 1.39E07 1.76E0718 3.2 Contact with falling objectmechanical lifting 5.55E08 7.76E08 7.85E08 1.05E0719 3.3 Contact with falling objectvehicle or load 3.59E08 5.85E08 6.00E08 8.91E0820 3.4 Contact with falling objectmanual handling 1.15E07 1.50E07 1.51E07 1.91E0721 3.5 Contact with falling objectother 9.15E07 1.07E06 1.07E06 1.24E0622 4.1 Contact with flying objectmachine or handheld

tool4.02E08 4.90E08 4.92E08 5.91E08


4.98E08 6.08E08 6.11E08 7.33E08



27 6.2 Contact with object used or carriedNOThandheld tool

2.35E08 2.88E08 2.89E08 3.47E08




2.42E08 3.84E08 3.93E08 5.73E08

31 8.1.3 Contact with moving parts of a machineclearing 2.11E07 2.88E07 2.91E07 3.80E0732 8.1.4 Contact with moving parts of a machinecleaning 9.35E08 1.37E07 1.40E07 1.93E0733 8.2 Contact with hanging/ swinging objects 8.65E08 1.01E07 1.02E07 1.18E0734 8.3 Trapped between 6.48E08 7.70E08 7.73E08 9.06E0835 9 Moving into object 7.28E09 9.08E09 9.13E09 1.12E0836 10 Buried by bulk mass 1.37E08 2.23E08 2.28E08 3.38E0837 11 In or on moving vehicle with loss of control 1.14E07 1.25E07 1.25E07 1.36E0738 12.1 Contact with electricityhigh voltage cable 2.94E08 5.08E08 5.24E08 8.08E08

39 12.2 Contact with electricitytool 2.32E09 3.26E09 3.30E09 4.43E0940 12.3 Contact with electricityelectrical work 6.82E08 8.07E08 8.09E08 9.46E0841 13 Contact with hot or cold surfaces or open flame 2.20E09 4.24E09 4.42E09 7.27E0942 14.1 Release of hazardous substance out of open



8.28E08 9.70E08 9.73E08 1.13E07


1.91E07 2.20E07 2.20E07 2.51E07

(Continued)

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Table V. (Continued)

Recoverable Injury Rate




1.05E07 1.97E07 2.05E07 3.33E07


1.77E07 2.25E07 2.26E07 2.81E07


apparatus4.04E08 7.97E08 8.34E08 1.39E07


under

4.17E08 1.05E07 1.13E07 2.12E07


accident rate () is distributed according to a gamma-1 pdf or:

f() = f1 (|, ) =e()a1

() , > 0, > 0.(9)

The gamma-1 distribution has been chosen sim-ply for calculation convenience because the gamma-1pdf is the natural conjugate distribution to thelikelihood of sample from a Poisson random processresulting in a gamma-1 posterior distribution.

Parameters (,) are chosen so that the meanand the standard deviation achieve desirable values.

Even though the accident rate is the basic pa-rameter of the model, the risk measure of interest

is the annual probability of an accident. It is morenatural then to express the state of knowledge onthe probability of an accident during the course of ayear. Complete lack of knowledge on the value of thisprobability would imply a uniform probability den-sity function (pdf) spanning the interval (0,1).

Because p and are connected throughEquation (2), the parameters (,) in the prior off() can be chosen such that the prior ofp is non-informative. This is done as follows.

The pdf of p is uniform over an interval (a,b);hence,

f(p) =1

b a . (10)By virtue of Equation (9) it follows that:

p

= e

and since the pdf of g() is given by g() =f[p()] p

it follows that the prior of is given by:

f() =

0 if < L = ln(1a)

,

1ba

e if L U,

0 if > U= ln(1b)

.

(11)

If (a,b) is the whole interval (0,1) then the priorof is:

f() = e=f1 (|1, ).

That is, the prior ofis a gamma-1 distributionwith parameters:

= 1 and = . (12)

This prior for = 2,000 hours is shown inFig. 2 for the variable p and in Fig. 3 for the variable

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60-E.170-E.180-E.190-E.101-E.111-E.121-E.1

9 Moving into object1.2 Fall on same level

7 Contact with hand held tools operated by self25.1 Extreme muscular exertion handling objects

6.1 Contact with object used or carriedhand held tool operated by other person13 Contact with hot or cold surfaces or open flame

20.1 Human aggression25.2 Extreme muscular exertion moving around27.2.4 Chemical explosion exothermic reactions

12.2 Contact with electricity tool6.2 Contact with object used or carried - NOT handheld tool

14.1 Release of hazardous substance out of open containment27.2.3 Chemical explosion explosives

8.1.1 Contact with moving parts of a machine operating17.1 Fire - hot work

10 Buried by bulk mass4.3 Contact with flying object blown by wind

4.1 Contact with flying object machine or handheld tool27.2.2 Chemical explosion dust

5 Hit by rolling/sliding object or person1.1.4 Fall from height hole in the ground

14.2 Exposure to hazardous substance without Loss of Containment17.2 Fire - working near flammables/ combustibles

27.1 Physical explosion3.4 Contact with falling object - manual handling

15.1 Release of a hazardous substance out of a closed containment adding, removing or 3.3 Contact with falling object vehicle or load

20.2 Animal behaviour1.3 Fall down stairs or ramp

1.1.1.3 Fall from height - step ladder or steps4.2 Contact with flying object object under pressure or tension

8.1.2 Contact with moving parts of a machine maintaining3.2 Contact with falling object - mechanical lifting

12.3 Contact with electricity electrical work17.3 Fire - fire fighting

8.2 Contact with hanging/ swinging objects27.2.1 Chemical explosion vapour or gas

15.2 Release of a hazardous substance out of a closed containment transport2. Struck by moving vehicle

8.3Trapped between15.4 Release of a hazardous substance out of a closed containment working nearby

8.1.4 Contact with moving parts of a machine cleaning11 In or on moving vehicle with loss of control

1.1.2.2 Fall from height - working on fixed scaffold1.1.5.3 Fall from height other

1.1.2.3 Fall from height (de-)Installing scaffold1.1.1.2 Fall from height - fixed ladder

12.1 Contact with electricity high voltage cable15.3 Release of a hazardous substance out of a closed containment- closing

22.1 Hazardous atmosphere in confined space1.1.1.1 Fall from height - placement ladder

22.2 Hazardous atmosphere through breathing apparatus3.1 Contact with falling object crane or load

1.1.3.3 Fall from height Fixed platform23.1 Impact by immersion in liquid working in or under

1.1.5.2 Fall from height - non-moving vehicle1.1.2.1 Fall from height working on mobile scaffold

8.1.3 Contact with moving parts of a machine clearing1.1.3.2 Fall from height floor

23.2 Impact by immersion in liquid working nearby1.1.5.1 Fall from height - moveable platform

3.5 Contact with falling object other1.1.3.1 Fall from height roof

PROBABILITY OF FATALITY PER HOUR OF EXPOSURE

FATALITY RISK RATE (PER HOUR) OF EXPOSURE TO VARIOUS OCCUPATIONAL HAZARDS

Fig. 5. Uncertainty characteristics offatality riskper hour of exposure to various occupational hazards. Each bar represents the 90% spanof the possible values of the risk owing to exposure variability. The left edge of a bar provides the 5th percentile of the risk. The right edgeof the bar provides the 95th percentile. The point in the bar where the color changes provides the mean value of the risk.

. It is noted that the 95th percentile is 2.6 103

whereas the 5th percentile is 1.5 105. This meansthat the 90% probability interval for the accident rateis [2.6 105, 1.5 103]. Equivalently, this meansthat the 90% probability interval for the yearly riskis [0.05, 0.95].

The value of = 2,000 hours has been chosenbecause it represents an upper limit to the amount ofworking hours in a calendar year.

The sufficient statistics of a sample generatedby a Poisson process is the number of accidents ob-served (k) and the total exposure of the population

(T) as described in the Section 2.1.

3.2. Posterior Distribution of

It can been shown that given the sufficient statis-tics of the sample (k,T), the posterior distributionof {f()} is also a gamma-1 distribution withparameters:(35,36)

= + k and = + T (13)

and

f() = f1 (| + k, + T). (14)

As a demonstration of this approach, the poste-rior distributions for the accident rates for perma-nent injuries and fatal injuries for the accident typestruck by a moving vehicle are given. As shown inTable II (#2. Struck by moving vehicle), the observednumber of accidents and the total exposure duringthe observation period are:

Number of TotalAccidents Exposure (Hours)

Fatal injuries 46 4.01 109

Permanent injuries 189 4.01 109

Given these sufficient statistics, Equations (13)and (14) provide the posterior distributions given inFig. 4.

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1.E-11 1.E-10 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05

12.2 Contact with electricity tool

25.1 Extrem e muscular exertion handling objects

27.2.4 Chemical explosion exothermic reactions

13 Contact with hot or cold surfaces or open flame1.2 Fall on same level

25.2 Extrem e muscular exertion moving around

6.1 Contact with object used or carriedhand held tool operated by other person

20.1 Human aggression

27.2.3 Chemical explosion explosives

27.2.2 Chemical explosion dust

23.2 Impact by immersion in liquid worki ng nearby

4.3 Contact with flying object blown by wind

27.1 Physical explosion

10 Buried by bulk mass

9 Moving into object

22.1 Hazardous atmosphere in confined space

1.1.4 Fall from height hole in the ground

17.1 Fire - hot work

22.2 Hazardous atmosphere through breathing apparatus

17.2 Fire - working near flammables/ combustibles

7 Contact with hand held tools operated by self

14.2 Exposure to hazardous substance without Loss of Containment

5 Hit by rolling/sliding object or person

12.1 Contact with electricity high voltage cable

27.2.1 Chemical explosion vapour or gas

20.2 Animal behaviour

12.3 Contact with electricity electrical work

15.2 Release of a hazardous substance out of a closed containment transport

14.1 Release of hazardous substance out of open containment

1.3 Fall down stairs or ramp2. Struck by moving vehicle

15.4 Release of a hazardous substance out of a closed containment working nearby

6.2 Contact with object used or carried - NOT handheld tool

3.3 Contact with falling object vehicle or load

1.1.1.2 Fall from height - fixed ladder

11 In or on moving vehicle with loss of control

4.2 Contact with flying object object under pressure or tension

1.1.1.3 Fall from height - step ladder or steps

15.1 Release of a hazardous substance out of a closed containment adding, removing or opening

3.4 Contact with falling object - manual handling

1.1.2.3 Fall from height (de-)Installing scaffold

17.3 Fire - fire fighting

3.2 Contact with falling object - mechanical lifting

8.2 Contact with hanging/ swinging objects

1.1.2.2 Fall from height - working on fixed scaffold

4.1 Contact with flying object machine or handheld tool

23.1 Impact by immersion in liquid working in or under

8.1.2 Contact with moving parts of a machine m aintaining

3.1 Contact with falling object crane or load

8.3Trapped between

15.3 Release of a hazardous substance out of a closed containment- closing

1.1.5.3 Fall from height other

8.1.1 Contact with moving parts of a machine operating

1.1.3.3 Fall from height Fixed platform

1.1.1.1 Fall from height - placement ladder

1.1.5.1 Fall from height - moveable platform1.1.3.2 Fall from height floor

1.1.2.1 Fall from height working on mobile scaffold

1.1.5.2 Fall from height - non-moving vehicle

1.1.3.1 Fall from height roof

8.1.4 Contact with moving parts of a machine cleaning

3.5 Contact with falling object other

8.1.3 Contact with moving parts of a machine clearing

PROBABILITY OF PERMANENT INJURY PER HOUR OF EXPOSURE

PERMANENT INJURY RISK RATE PER HOUR OF EXPOSURE TO VARIOUS OCCUPATIONALHAZARDS

Fig. 6. Uncertainty characteristics ofpermanent injury risk per hour of exposure to various occupational hazards. Each bar represents the90% span of the possible values of the risk owing to exposure variability. The left edge of a bar provides the 5th percentile of the risk. Theright edge of the bar provides the 95th percentile. The point in the bar where the color changes provides the mean value of the risk.

It is noteworthy that the uncertainty in the pos-terior assessment of the risk rates is extremely smallgiven the size of the available evidence (sufficient

statistics). Characteristic values of the assessed un-certainty are:

5th 95th MaximumPercentile Mean Percentile Likelihood

(Hour) (Hour) (Hour) (Hour)

Fatal injuries 0.9 108 1.17 108 1.47 108 1.15 108

Permanentinjuries

4.2 108 4.74 108 5.3 108 4.71 108

It follows that the uncertainty owning to thelimited amount of experiential evidence is rela-tively small. Additional evidence would not improve

the uncertainty range significantly because the 90%probability interval spans a factor of 1.6 for fatalinjuries and only 1.26 for permanent injuries. Sim-ilar results are obtained for all 63 accident typesand are given in Table III for fatalities, Table IVfor permanent injuries, and Table V for recoverableinjuries. Results are also graphically summarized inFigs. 57. The risk rates representing the probabilityof an accident per hour of exposure are by their na-ture very small numbers. They are given in terms of

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1.E-10 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05

25.1 Extreme muscular exertion h andling objects

27.2.3 Chemical explosion explosives

12.2 Contact with electricity tool

20.1 Human aggression

13 Contact with hot or cold surfaces or open flame6.1 Contact with object used or carriedhand held tool operated by other person

27.2.2 Chemical explosion dust

9 Moving into object

27.2.4 Chemical explosion exothermic reactions

25.2 Extreme muscular exertion moving around

1.2 Fall on same level

4.3 Contact with flying object blo wn by wind

23.2 Impact by immersion in liquid working nearby

27.1 Physical explosion

10 Buried by bulk mass

7 Contact with hand held tools operated by self

6.2 Contact with object used or carried - NOT handheld tool

20.2 Animal behaviour

5 Hit by rolling/sliding object or person

8.1.1 Contact with moving parts of a machine operating

8.1.2 Contact with moving parts of a machine maintaining

17.2 Fire - working near flammables/ combustibles

27.2.1 Chemical explosion vapour or gas

4.1 Contact with flying object machine or handheld tool

1.1.4 Fall from height hole in the ground

12.1 Contact with electricity high voltage cable

17.1 Fire - hot work

3.3 Contact with falling object v ehicle or load

4.2 Contact with flying object obje ct under pressure or tension

22.1 Hazardous atmosphere in confined space

8.3Trapped between

3.2 Contact with falling object - mechanical lifting

2. Struck by moving vehicle

12.3 Contact with electricity electrical work

22.2 Hazardous atmosphere through breathing apparatus

14.2 Exposure to hazardous substance without Loss of Containment

8.2 Contact with hanging/ swinging objects

14.1 Release of hazardous substance out of open containment

23.1 Impact by immersion in liquid working in or under

11 In or on moving vehicle with loss of control

3.1 Contact with falling object crane or load

8.1.4 Contact with moving parts of a machine cleaning

3.4 Contact with falling object - manual handling

1.3 Fall down stairs or ramp

15.2 Release of a hazardous substance out of a closed containment transport

15.3 Release of a hazardous substance out of a closed containment- closing

1.1.2.3 Fall from height (de-)Installing scaffold

15.1 Release of a hazardous substance out of a closed containment adding, removing or opening

1.1.1.3 Fall from height - st ep ladder or steps

15.4 Release of a hazardous substance out of a closed containment working nearby

17.3 Fire - fire fighting

1.1.1.2 Fall from height - fixed ladder

8.1.3 Contact with moving parts of a machine clearing

1.1.5.1 Fall from height - moveable platform

1.1.2.2 Fall from height - working on fixed scaffold

1.1.3.3 Fall from height Fixed platform1.1.5.3 Fall from height other

1.1.1.1 Fall from height - placement ladder

1.1.3.2 Fall from height floor


1.1.5.2 Fall from height - non-moving vehicle

1.1.2.1 Fall from height working on mobile scaffold

1.1.3.1 Fall from height roof

PROBABILITY OF RECOVERABLE INJURY PER HOUR OF EXPOSURE

RECOVERABLE INJURY RISK RATE (PER HOUR) OF EXPOSURE TO VARIOUSOCCUPATIONAL HAZARDS

Fig. 7. Uncertainty characteristics ofrecoverable injury riskper hour of exposure to various occupational hazards. Each bar represents the90% span of the possible values of the risk owing to exposure variability. The left edge of a bar provides the 5th percentile of the risk. Theright edge of the bar provides the 95th percentile. The point in the bar where the color changes provides the mean value of the risk.

two decimal places to allow for the comparison of al-most equal numbers.

4. OCCUPATIONAL RISK PER YEAR

Another risk measure that is widely used to char-acterize occupational risk is the number of accidentsper worker (or per 100,000 workers) and year. This isa measure close to the annual probability of an acci-dent given the average exposure to a particular haz-ard during the year. If is the average number ofhours that a worker is exposed to a particular haz-ard per calendar year, y is the observation period in

calendar years, Kis the number of accidents in theobservation period, andNis the worker population,by virtue of Equations (2) and (5) it follows that the

annual occupational risk is:

p = 1 exp[K

Ny] or

for K

Ny 1, itis p =

K

Ny. (15)

Fig. 8 presents the top occupational accidentsranked with respect to the occupational risk rate

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1.3.1Fallfromh

eightroof

3.5Contactwithfallingob

jectother

1.1.5.1Fallfromheight

-moveable

23.2

Impactbyimmersio

ninliquid

1.1.3.2Fallfromheightfloor n

ognikrow

thgiehmorfllaF1.2.1.1

afostrapg

nivomhtiwtcatnoC3.1.8 1

.1.5.2Fallfromheight-non-moving

1.1.3.3FallfromheightFix

edplaorm

roenarct

cejbognillafhtiwtcatnoC1.3

23.1

Impactbyimmersio

ninliquid

1.1.1.1Fallfromheight-placementladder

22.1Hazardousatmosphere

inconfined

22.2

Hazardousatmosph

erethrough hgihyticir

tcelehtiwtcatnoC1.21

1.1.2.3Fallfromheight(d

e-)Installing

1.1.1.2Fallfromheight-

fixedladder

1.1.5.3Fallfromhe

ightother

1.1.2.2Fallfromheight-

workingon

11Inoronmovingvehicle

withlossof

8.3Trapp

edbetween

2.Struckbymo

vingvehicle

27.2.1Chemicalexplosionva

pourorgas

15.4Releaseofahazardou

ssubstance

RISK RATE

# of ACCIDENTS/100000workers

number of accidents

Fig. 8. Prioritization of occupational accident types with respect to risk rate (blue bars), annual risk (red bars), and number of accidentsover the observation period (green bars) for fatalities (colors visible in online version). Values have been normalized so the highest is set to100%.

along with the corresponding values of the annualrisks and the number of observed fatal accidents. Itcan be seen that this ranking does not correspondto the ranking according to the annual occupationalrisk. For example, the type of accident presenting thehighest annual occupational risk for a fatal accidentis working in an environment where loads can falland hit the worker. The differences in the rankingsbased on the occupational risk rates and annual riskare due to the different average annual exposures tothe corresponding hazards. Consequently, compar-

ing number of accidents per worker and per year (ap-proximately equal to annual risk) is meaningful onlywhen risks per year are compared and under the as-sumption that all workers are exposed to the hazardfor a period equal to the average exposure. Compar-ing hazards involving exposures different from theaverage yearly exposures requires the use of the oc-cupational risk rates.

Collective occupational risk can be measured bythe total number of accidents observed during a spe-cific period. As seen in Fig. 8, the accident type rep-resenting the highest collective fatality risk is struckby a moving vehicle, producing a total of 46 deathsover the six-year observation period. Next comesfall from height roof with 43 deaths and third inor on a moving vehicle with loss of control. For asingle worker facing two alternative tasks involvingwork on a building roof and work in an area wherehe can be struck by a moving vehicle, the individual

fatality risk per unit of exposure is almost 20 timeshigher in the former case than in the latter.

Another point of interest is that the averageyearly exposure to a particular occupational hazardmust be used with care because all the workers arenot exposed to the hazard for this average dura-tion each year. Fig. 1, for example, shows the vari-ability of the yearly exposure time of the working

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30-E.140-E.150-E.160-E.170-E.180-E.190-E.1

25.1 Ext reme muscular exertion handling objects1.2 Fall on same level

9 Moving into object7 Contact with hand held tools operated by self

1.3 Fall down stairs or ramp6.1 Contact with object used or carriedhand held tool operated by other person

25.2 Extreme muscular exertion moving around13 Contact with hot or cold surfaces or open flame

20.1 Human aggression6.2 Contact with object used or carried - NOT handheld tool

14.1 Release of hazardous substance out of open containment27.2.4 Chemical explosion exothermic reactions

12.2 Contact with electricity tool8.1.1 Contact with moving parts of a machine operating

27.2.3 Chemical explosion explosives1.1.4 Fall from height hole in the ground

17.1 Fire - hot work10 Buried by bulk mass

4.3 Contact with flying object blown by wind5 Hit by rolling/sliding object or person

14.2 Exposure to hazardous substance without Loss of Containment4.1 Contact with flying object machine or handheld tool

3.4 Contact with falling object - manual handling27.2.2 Chemical explosion dust

1.1.1.3 Fall from height - step ladder or steps27.1 Physical explosion

3.3 Contact with falling object vehicle or load17.2 Fire - working near flammables/ combustibles

8.1.4 Contact with moving parts of a machine c leaning20.2 Animal behaviour

15.1 Release of a hazardous substance out of a closed containment adding, removing or opening8.1.2 Contact with moving parts of a machine maintaining

3.2 Contact with falling object - mechanical lifting17.3 Fire - fire fighting

1.1.5.2 Fall from height - non-moving vehicle15.2 Release of a hazardous substance out of a closed containment t ransport

4.2 Contact with flying object object under pressure or tension2. Struck by moving vehicle

1.1.1.2 Fall from height - fixed ladder8.2 Contact with hanging/ swinging objects

12.3 Contact with electricity electrical work27.2.1 Chemical explosion vapour or gas

15.3 Release of a hazardous substance out of a closed containment- closing1.1.1.1 Fall from height - placement ladder

11 In or on moving vehicle wit h loss of control8.3Trapped between

15.4 Release of a hazardous substance out of a closed containment working nearby1.1.5.3 Fall from height other

1.1.2.3 Fall from height (de-)Installing scaffold22.1 Hazardous atmosphere in confined space

22.2 Hazardous atmosphere through breathing apparatus1.1.2.2 Fall from height - working on fixed scaffold

23.2 I mpact by immersion in liquid working nearby8.1.3 Contact with moving parts of a machine clearing

1.1.3.3 Fall from height Fixed platform12.1 Contact with electricity high voltage cable

1.1.2.1 Fall from height working on mobile scaffold3.1 Contact with falling object crane or load

23.1 Impact by immersion in liquid working in or under1.1.3.2 Fall from height floor

1.1.5.1 Fall from height - moveable platform1.1.3.1 Fall from height roof


PROBABILITY OF FATALITY

FATALITY ANNUAL RISK FOR VARIOUS OCCUPATIONAL HAZARDS

Fig. 9. Variability characteristics offatality risk per year of exposure to various occupational hazards including variability in the annualexposure. Each bar represents the 90% span of the possible values of the risk owing to exposure and risk rate variability . The left edge ofa bar provides the 5th percentile of the risk. The right edge of the bar provides the 95th percentile. The point in the bar where the colorchanges provides the mean value of the risk.

population to the accident type struck by a mov-ing vehicle. The average yearly exposure time is506.5 hours. However, there is a substantial numberof workers who are exposed to about 1,700 hours peryear whereas an even higher number of workers areexposed for less than 200 hours (Fig. 1). Obviously,the former group is faced with greater occupationalrisk per year than the latter. The variability in theexposure time introduces a corresponding variabil-ity to the annual occupational fatality risk. Assess-ing the effect of the variability in the exposure time(as shown in Fig. 1) and the uncertainty in the fatal-

ity risk rate in Equation (2) provides a 90% prob-ability interval of (3.3 107, 1.7 105) with meanvalue 5.8 106. This variability is much larger thanthat due to the uncertainty in with fixed exposureat the average value of 506.5 hours. In this case, the90% probability interval is (4.6 106 7.5 106)with mean value 5.9 106. Similar assessments havebeen made for all 63 occupational accident types andthe results are schematically depicted in Fig. 9.

We therefore maintain that for comparison ofindividual occupational risks the risk rates are suit-able for general comparisons while detailed expo-sure profiles are necessary for comparing specific jobsinvolving exposures to different occupational haz-ards for different durations.

5. DISCUSSION

Occupational risk rates constitute a quantitativemeasure for prioritizing individual occupational risk.The results given in Fig. 5 (and in Table III) indi-

cate that, even after consideration of the associateduncertainty, the riskiest occupation per unit of timewith respect to fatal accidents is working on the roofof a building and working in a space where loadsstacked in piles or on shelves are present. Whileworking on a roof a worker can fall from protectedor unprotected edges or through existing holes on theroof. The mean value of the fatality risk rate is 2.32 107 per hour, whereas the 90% probability interval

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is extremely narrow (1.77 107 per hour, 2.92 107 per hour) with an error factor (95th/5th) of 1.65.In an area where loads are piled up or stacked upon shelves a worker can be hit by a falling load. Themean value of the fatality risk rate is 1.73 107 per

hour whereas the 90% probability interval is (1.13 107 per hour, 2.43 107 per hour) with an errorfactor of 2.15.

In general, working at heights is among theriskier occupational activities for causing fatalinjuries while handling (lifting) objects, falling on thesame level, and moving into objects are the leastrisky.

Probability intervals are narrow for types of ac-cidents that have generated incidents (injuries) anddecrease in size as the number of incidents increases.The relative position of the interval in thex-axis de-

pends on the size of the exposure. For example, nodeaths have been observed for work involving clos-ing vessels containing a hazardous substance (acci-dent #15.3, Table I) with a total exposure of 420 106 hours. The 90% probability interval is (1.24 109 per hour, 7.24 108 per hour) with an errorfactor of 58. Similarly, no deaths have been recordedfor the hazard working in an area where one can beburied by a bulk mass with an exposure of 600 106

hours. The 90% probability interval is (8.60 1011

per hour, 5.01 109 per hour) with an error fac-tor of 58. This time the probability interval althoughhaving the same size is moved to the left by an order

of magnitude.It should be emphasized that occupational risk

rates refer to individual risk per unit of time of ex-posure. Full comparison among alternative hazardsshould include the exposure to these hazards. For ex-ample, working on the roof of a building for 1 hour(accident type #7) is less dangerous than working ona moveable platform (accident type #11) for 5 hours.The former has a probability for a fatal injury equalto 2.32 107 whereas the latter has a fatality proba-bility equal to 4.2 107 (5 hours

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