Exposure Estimation and Interpretation of Occupational Risk: Enhanced Information for the Occupational Risk Manager

Figures & data

Table 1 Occupational Exposure Limits (OELs) Developed by Various Organizations^A

OEL^B	Definition	Health Basis	Analytical Feasibility	Economic Feasibility	Engineering Feasibility
ACGIH TLV	Threshold Limit Values (TLVs) refer to airborne concentrations of chemical substances and represent conditions under which it is believed that nearly all workers may be repeatedly exposed, day after day, over a working lifetime, without adverse health effects. TLVs are developed to protect workers who are normal, healthy adults. They are not fine lines between safe and dangerous exposures, nor are they a relative index of toxicology.	Yes	No	No	No
AIHA-OARS^C WEEL	The Workplace Environmental Exposure Levels (WEELs) are health-based airborne chemical OELs established to provide guidance where other OELs are not available. The WEELs provide guidance for protecting most workers from adverse health effects related to occupational chemical exposures. WEELs are expressed as either time weighted averages or ceiling limits.	Yes	No	No	No
DFG MAK	The MAK-values are daily 8-hr time-weighted average values and apply to healthy adults. MAKs give the maximum concentration of a chemical substance in the workplace.	Yes	No	No	No
EC SCOEL	The SCOEL 8-hr time weighted average exposures represent levels to which an employee may be exposed via the airborne route for 8 hr per day, 5 days per week over a working lifetime which will not result in adverse effects on health of the worker or their progeny.	Yes	No	No	No
NIOSH REL	RELs are occupational exposure limits recommended by NIOSH as being protective of worker health and safety over a working lifetime. The REL is used in combination in engineering and work practice controls, exposure and medical monitoring, labeling, posting, worker training, and personal protective equipment. This limit is frequently expressed as a time-weighted average (TWA) for up to a 10-hr workday during a 40-hr workweek.	Yes	Yes	No	Yes
OSHA PEL	OSHA sets enforceable permissible exposure limits (PELs) to protect workers against the health effects of exposure to hazardous substances. PELs are regulatory limits on the amount or concentration of a substance in the air. OSHA PELs are based on an 8-hr time weighted average (TWA) exposure.	Yes	Yes	Yes	Yes

^AFocusing primarily on full-shift time-weighted averages and excluding biological exposure limits and skin notations. Note that the TLVs have traditionally applied a non-probabilistic approach based on non-cancer and cancer endpoints. For OSHA, probabilistic approaches have been more common. PEL development has focuse/d more on carcinogens, and chemicals with large epidemiology databases. Some organizations such as the MAK Commission and NIOSH have historically not set quantitative limits for carcinogens but have provided control-based recommendations. ^BACGIH TLV - American Conference of Governmental Industrial Hygienists (US) Threshold Limit Values; AIHA OARS-WEEL - American Industrial Hygiene Association and Occupational Alliance for Risk Science Workplace Environmental Exposure Levels; NIOSH REL - National Institute for Occupational Safety and Health (US) Recommended Exposure Level; OSHA PEL - Occupational Safety and Health Administration (US) Permissible Exposure Level; DFG MAK - Deutsche Forschungsgemeinschaft (German Research Foundation) Maximale Arbeitsplatz-konzentrationen (Maximum Airborne Concentration); EC SCOEL - European Commission Scientific Committee on Occupational Exposure Limit Values.^CThe WEELs were originally developed and published as a committee under the auspices of the American Industrial Hygiene Association. In 2013 The AIHA signed an agreement to transfer the updating and development of new WEEL values to Toxicology Excellence for Risk Assessment under the Occupational Alliance for Risk Science.

Figure 1 Sources of uncertainty and variability in occupational exposure limit derivation.

Figure 2 Sources of uncertainty in exposure estimation.

Table 2 Exposure Assessment Cases in Which Probabilistic Uncertainty Analyses are Useful (modified from Cullen and Frey⁽¹⁵⁾)

• When consequences of poor or biased exposure estimates are unacceptably high.

• When screening exposure estimates indicate potential risk, but the estimates have substantial uncertainty.

• When it is uncertain whether the collection of additional samples or information will improve the quality of the decision to be made.

• When uncertain information stems from multiple sources.

• When comparing potential interventions with different costs and differential impacts on workers.

• When ranking or prioritizing exposure levels, exposure groups or exposure agents are important.

• When exposure reduction costs are high.

Figure 3 Low-dose extrapolation methods for estimating risk. Approaches are shown for extrapolation to exposures below the range of available health effects data. Panel (A) shows a hypothetical exposure response curve with the probability of adverse effect shown to increase with increasing exposure concentration. Panel (B) shows the application of an uncertainty factor (UF) to a point of departure (POD) in region of low effect incidence to estimate an occupational exposure limit (OEL). Panel (C) shows the linear extrapolation from the POD to an OEL or a hypothetical point of no exposure or excess risk (the graphical origin). The slopes of the resulting lines are used by some organizations as an upper bound estimate of risk. Panel (D) is an alternative to linear extrapolation when the use of the health effects data supports the derivation of an exposure-response curve directly in the exposure region of interest. This is most typical when large data sets are adequate to estimate risk in the 1:1000 range or when a high degree of knowledge of the underlying biology of the adverse effect is available to infer the shape of the curve at low concentrations.

Table 3 Summary of Key Considerations during Phases of Risk Assessment

Phase of Risk Assessment	Critical Question(s)	Key Resources
Problem Formulation	What is the occupational exposure scenario under consideration?	• NRC: Science and Decisions: Advancing Risk Assessment (2009).^(60)
		• Bullock, W.H., and J.S. Ignacio: A Strategy for Assessing and Managing Occupational (2006).^(1)
		• IPCS: Uncertainty and Data Quality in Exposure Assessment (2008).^(5)
OEL Derivation (Toxicity evaluation and exposure-response)	What are the key sources of variability and uncertainty in the OEL?	• Nelson, D.I.: Chapter 9: Risk Assessment in the Workplace In The Occupational Environment: Its Evaluation, Control, and Management (2011).^(71)
		• Haber, L.T. et al: Noncancer Risk Assessment: Principles and Practice in Environmental and Occupational Settings (2012).^(54)
Exposure Assessment	What are the methods for characterizing occupational exposures and their uncertainties?	• Rappaport, S.M. and L.L. Kupper: Quantitative Exposure Assessment (2008).^(3)
	• Sources of variability	• IPCS: Uncertainty and Data Quality in Exposure Assessment (2008).^(5)
	• Uncertainties in exposure estimation	• Cullen, A.C., and H.C. Frey: Probabilistic Techniques in Exposure Assessment (1999).^(15)
	• Methods for estimating exposures	• O’Hagan, A. et al: Uncertain Judgments: Eliciting Experts’ Probabilities (2006).^(6)
	• Measurement-based approaches
	• Modeling approaches
Risk Characterization	What methods are used to compare exposure estimates to OELs for decision-making about the acceptability of exposures?	• Bullock, W.H., and J.S. Ignacio: A Strategy for Assessing and Managing Occupational (2006).^(1)
		• Rappaport, S.M., and L.L. Kupper: Quantitative Exposure Assessment (2008).^(3)
	What tools exist to support risk characterization?	• AIHA: Exposure Assessment Strategies Committee: IH STAT Excel Spreadsheet Tool. https://www.aiha.org/get-involved/VolunteerGroups/Documents/EASC-IHSTAT-V235.xls. (2014).^(44)
		• Drolet, D. et al.: Exposure Assessment Strategies Committee: IH MOD Excel Exposure Models Suite. https://www.aiha.org/get-involved/VolunteerGroups/Documents/IHMOD_Korean-AIHA-MathModel209.xls (2014).^(43)
		• Hewett, P.: IHDA for Bayesian Decision Analysis. http://www.oesh.com/software.php. V1. 27, Exposure Solutions, Inc., Morgantown, WV (2011).^(45)
Risk Management	What problems are associated with this exposure scenario?	• NRC: Science and Decisions: Advancing Risk Assessment (2009).^(60)
	What actions could be taken?	• Bullock, W.H., and J.S. Ignacio: A Strategy for Assessing and Managing Occupational (2006).^(1)
	What additional information is needed to evaluate possible risk management options?

Cullen, A.C., and H.C. Frey: Probabilistic Techniques in Exposure Assessment: A Handbook for Dealing with Variability and Uncertainty in Models and Inputs. New York: Springer Science & Business Media, 1999.

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