Occupational Exposure Limits

Many procedures generate hazardous air contaminants that can get into the air that people breath. Normally, the body can take in limited amounts of hazardous air contaminants, metabolize them and eliminate them from the body without producing harmful effects. Safe levels of exposure to many hazardous materials have been established by governmental agencies after much research in their short term (acute) and cumulative (chronic) health effects using available human exposure data (usually from industrial sources) and animal testing. When the average air concentrations repeatedly exceed certain thresholds, called exposure limits, adverse health effects are more likely to occur. Exposure limits do change with time as more research is conducted and more occupational data is collected

An occupational exposure limit is the maximum average air concentration that most workers can be exposed to for an 8 hour work day, 40 hour work week for a working lifetime (40 years) without experiencing significant adverse health effects. A very small percentage of individuals experience some discomfort or adverse health effects at or below the exposure limit because of a wide variation in individual sensitivities or pre-existing conditions. When employee's work a shift longer than 8 hours per day, the exposure limit must be adjusted (lowered) to prevent the employee from over exposure.

Exposure limits for hazardous air contaminants are listed on the material safety data sheets for each product in use. Individuals who have questions about exposure limits or exposures to hazardous air contaminants can consult their supervisor or the Environmental Health and Safety Office (EH& S).

Types of Exposure Limits

Many organizations research occupational exposures and establish recommended exposure limits. Recommended exposure limits are not enforceable by law. One of the most active in this research is the American Conference of Governmental Industrial Hygienists (ACGIH) who publish exposure limits called Threshold Limit Values (TLV). A copy of the documentation used to establish the TLV's is available at the EH & S office. The National Institute of Occupational Safety and Health (NIOSH), another research organization, publishes the Recommended Exposure Limits (REL).

Governmental agencies like OSHA set maximum workplace exposure limits into law, known as Permissible Exposure Limits (PELs). In Wisconsin, the Department of Commerce (formerly the Dept. of Industry, Labor and Human Relations, DILHR) enforces occupational safety laws for state employees and has adopted the Permissible Exposure Limits that were updated in 1989 by the federal Occupational Safety and Health Administration (OSHA). Note: Due to a federal court ruling, the OSHA permissible exposure limits updated in 1989 have been set aside, and the private sector is now governed by a PEL list generated in the early 1970's. Wisconsin has decided to adopt the more protective 1989 list of Permissible Exposure Limits for use with Wisconsin Public sector employees.

Units of Measurement

A workplace exposure level, such as a PEL or TLV, is expressed as the concentration of the air contaminant in a volume of air. The most common units are:



parts per million (the number of "parts" of air contaminant per million parts of air)



milligrams of substance per cubic meter of air



fibers per cubic centimeter of air

The smaller the concentration number, the more toxic the substance is by inhalation.

Common Exposure Limits

  • The 8 Hour Time Weighted Average (8 Hr TWA) is the average concentration for an 8 hour work period. Brief periods slightly above and below this value are typical, as long as the average for the 8 hour period does not exceed the TWA.
  • Short Term Exposure Limits, (STELs), are allowable 15 minute exposure levels above the TWA. There may be no more than 4 STEL periods in a work day and there must be 60 minutes between the periods. STELS must be balanced with much lower exposures during the day so the average exposure doesn't exceed the TWA. STELS are not available for all substances.
  • Ceiling (C) limits are concentrations that must never be exceeded during any part of the day.
  • Action levels are found only in certain substance specific standards by OSHA. This is the air concentration that triggers a series of actions the employer must take to protect the employee. Action levels are typically one half of the permissible exposure limit.
  • A "SKIN" notation that follows the exposure limit indicates that a significant exposure can be received if the skin is in contact with the chemical in either the gas, vapor or solid form.

On-Line Versions of Occupational Exposure Limits
for Hazardous Air Contaminants