UNDERSTANDING OCCUPATIONAL EXPOSURE LIMITS

Understanding Occupational Exposure Limits Explains the common systems for occupational exposure limits (OELs)—threshold limit values (TLVs) and permissible exposure limits (PELS)—and how to use these data to judge the relative hazard of a chemical. 

The OSHA sets safe and healthy workplace standards. When OSHA was formed, they adopted the then current ACGIH TLV - TWAs and TLV-Cs as occupational exposure limits and made them federal standards. However, instead of calling them TLV -TWAs, OSHA called them PELs. OSHA has both TWA and ceiling values for various chemicals. PELs are listed in Title 29 of the Code of Eederal Regulations (CER), Part 1910, Subpart Z, General Industry Standards for Toxic and Hazardous Substances. Emergency responders should understand that ACGIH and OSHA values are not always the same for each chemical. 

When referencing and/or applying an occupational exposure limit, it is critical to review the source documents and understand the parameters of and the toxicologic basis for the cited limit. If, for instance, an exposure limit is based on subjective complaints of odor, it may be important to understand the possibility of chronic toxicity at exposure levels below the odor threshold. Similarly, an exposure limit based on a chronic toxicologic effect may not be directly applicable to precautions for acute, unexpected exposures. 

This selection of AEL = 0.95 is a convenient means to enhance the intuitive understanding of the sections which follow. Any readers who disagree with my choice of contour are encouraged to select another contour and to make their own comparisons. Contours not plotted can be easily computed from Equation A-7. Also, please note that the AEL concept does not apply to occupational exposures which have ceiling standards. Further, discussion of the relationship between the AEL, the PEL, and short-term excursion limits is beyond the scope of this paper. 

Basic information on the toxicity of many of the most commonly encountered and tox-icologically significant industrial chemicals is provided in Table IV-4. The table Is Intended to expedite the recognition of potentially hazardous exposure situations and therefore provides infomiation such as vapor pressures, warning properties, physical appearance, occupational exposure standards and guidelines, and hazard classification codes, which may also be useful in the assessment of an exposure situation. Table IV-4 is divided into 3 sections health hazards, exposure guidelines, and comments. To use the table correctly, it is important to understand the scope and limitations of the information it provides. 

Knowledge of the pharmacokinetics of volatile compounds is important in understanding the rate of onset, the intensity, and the duration of intoxication with these substances. The UK Maximum Exposure Limit (MEL) or Occupational Exposure Standard (OES) (Table 3) provide information on the relative toxicities of different compounds after chronic exposure to relatively low concentrations of vapor. 

To understand the proper level of respiratory protection, you must conduct a hazard assessment to define your potential workplace exposures. This assessment will define if the hazards are Immediately Dangerous to Life and Health (IDLH) or non-IDLH. This is critical information needed to define whether an air-purifying respirator (APR) is suitable or if an air supplied respirator is required. The National Institute for Occupational Safety and Health (NIOSH) Pocket Guide to Chemical Hazards (www.cdc. gov/niosh/npg/) is a comprehensive resource that will define IDLH levels for most hazards you will encounter. The Pocket Guide will also define specific characteristics of your hazard of concern as well as provide the NIOSH Recommended Exposure Limit (REL) and OSHA Permissible Exposure Limit (PEL). 

The theories describing adsorption and desorption phenomena are well established. Unfortunately, only limited studies have been performed to develop parameters applicable to indoor environments. Small chamber data are usually insufficient to describe the sink behavior (especially desorption rates) in full-scale situations, and only a very small number of test house studies have been conducted to develop sink parameters. Sink effects can have a major impact on the long-term concentrations of pollutants in indoor environments and on the exposure of human occupants to indoor air pollutants. While lAQ models can be used to estimate the effect of sinks on exposures, such estimates can be improved when better data are available on sinks. Additional research is needed to fully understand and describe the behavior of sinks in indoor environments. 

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