Occupational Exposure Limits – State of the Science

The process of developing and using occupational exposure limits is a cornerstone of industrial hygiene practice, with a history dating back to the 1880s. Occupational exposure limits, known as OELs, have not—until recently—evolved enough to reflect the advances in related sciences of toxicology, risk assessment, and exposure assessment. Much of the pioneering effort to develop and promote OELs as a risk management strategy occurred in the 1940s, when an organization now known as the American Conference of Governmental Industrial Hygienists (ACGIH) created a list of occupational exposure limits for 132 specific chemicals. While these limits represented a significant step forward in the practice of occupational hygiene, they lacked consistent guidelines, explicit definitions, and technical documentation. Gradually, these OELs and others have evolved to consider toxicological mechanisms of action, and uncertainties associated with the data available for assessing specific chemical hazards. Yet, there has still not been a concerted effort to explore how advances in toxicology, risk assessment, and exposure and risk management might better inform consistent and transparent processes for assessing chemical hazards and establishing OELs. To begin to tackle these issues, researchers at the National Institute for Occupational Safety and Health (NIOSH) worked with outside subject matter experts. They developed a collection of 10 articles published in the Journal of Occupational and Environmental Hygiene [JOEH, December 2015] focusing on the underlying principles for developing and interpreting OELs. The articles also discuss using and interpreting OELs in the context of evolving occupational risk assessment and management practices.

The first article, “State-of-the-Science: The Evolution of Occupational Exposure Limit Derivation and Application,” gives a background on OELs, including how they evolved and how they can be used to assess risk and manage workplace hazards. Other articles explore possibilities for incorporating advances in risk assessment, toxicology, and occupational hygiene into the process to develop OELs. The articles described below are listed in the order they appear in the journal.

• “Historical Context and Recent Advances in Exposure-response Estimation for Deriving Occupational Exposure Limits” describes and contrasts traditional and advanced practices in dose-response modeling from both a toxicological and epidemiological perspective.
• “Advances in Inhalation Dosimetry Models and Methods for Occupational Risk Assessment and Exposure Limit Derivation” focuses on inhalation dosimetry, offering several practical examples of dose estimation and OEL derivation are provided for inhaled gases and particulates.
• “The Scientific Basis of Uncertainty Factors Used in Setting Occupational Exposure Limits” emphasizes that risk managers must understand the sources of variability and uncertainty addressed in an OEL in order to effectively interpret and apply the OEL.
• “Systems Biology and Biomarkers of Early Effects for Occupational Exposure Limit Setting” suggests a systems biology approach that uses biomarkers of effect with computational toxicology tools to enable more accurate and timely assessments of chemical exposures.
• “Considerations of Using Genetic and Epigenetic Information in Occupational Health Risk Assessment and Standard Setting” addresses how genetic and epigenetic information can be used in risk assessments, and it describes a framework to organize thinking about such uses.
• “Setting Occupational Exposure Limits for Chemical Allergens—Understanding the Challenges” discusses the biological principles involved in the allergic response to highlight how the evolving sciences of toxicology and risk assessment techniques are enhancing the ability to address the complexities of allergy for developing health-based OELs.
• “Exposure Estimation and Interpretation of Occupational Risk: Enhanced Information for the Occupational Risk Manager” highlights the key ingredients in an occupational exposure assessment, focusing on important sources of variability and uncertainty that can be useful for characterizing occupational risk in terms of risk probability, rather than deciding based only on “acceptable risk or unacceptable risk.”
• “Aggregate Exposure and Cumulative Risk Assessment—Integrating Occupational and Non-occupational Risk Factors” acknowledges that OELs and other risk management tools need to consider complex exposures to chemicals and other stressors by multiple pathways and exposure routes, combined exposures to multiple stressors (such as chemical and non-chemical), and the possibility that such cumulative exposures may modify the toxic effects observed.
• “The Global Landscape of Occupational Exposure Limits—Implementation of Harmonization Principles to Guide Limit Selection” concisely describes efforts to encourage coordination among OEL-setting organizations and processes. This coordination is needed because of a historically confusing landscape of OELs that reflect significant overlap in coverage among organizations for many chemicals, while other important chemicals have no OELs.

In the preface to the JOEH collection of articles comprising the “State of the Science of Occupational Exposure Limit Methods and Guidance,” the guest editors state the following:

“…the 10 articles in this supplement of the Journal of Occupational and Environmental Hygiene provide a window into the future of OEL development. A combined effort by scientists at the National Institute for Occupational Safety and Health (NIOSH), Toxicology Excellence for Risk Assessment (TERA) and others, they present a systematic approach that begins with an understanding of systems biology, mechanisms of action and the early (i.e., “pre-clinical”) effects of toxic exposures including genetic and epigenetic phenomena. They incorporate novel approaches to exposure assessment and inhalation dosimetry, contemporary methods in risk assessment, statistics and decision logic, and considerations of the need to harmonize standards across the world…Many of these articles describe new technologies and data analytic methods that may be useful in overcoming data issues. The use of better models that address both uncertainty and variability in biological systems and exposure assessment offers particular promise.”

This collection contributes much to the underlying research into establishing OELs. The broad and varied topics addressed by the 10 articles blend modern concepts and advances in toxicology, occupational hygiene, risk assessment, and risk management. The articles offer extensive information and a treatment of the subject that may influence the science of setting OELs. The articles explore the continued relevance of occupational exposure limits, given evolving knowledge and guidance. They give insight into the ways OELs are formed and applied, and how effective they are and can be.

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Thomas J. Lentz, Ph.D., is Chief of the Document Development Branch in the NIOSH Education and Information Division.

 Scott Dotson, Ph.D., is a Lead Health Scientist in the NIOSH Education and Information Division.

Deborah Hornback, MS, is a health communications specialist in the NIOSH Education and Information Division.