Recommended Exposure Limits (RELs)

RELsRecommended Exposure Limits. A level of concern representing the adverse health effects of a hazardous substance on healthy adult workers. RELs are not available in CAMEO 365betÓéÀÖ. (also called NIOSHNational Institute for Occupational Safety and Health (www.cdc.gov/niosh). The federal agency responsible for conducting research and making recommendations for the prevention of work-related disease and injury. NIOSH is part of the Centers for Disease Control and Prevention (CDC). RELs) are exposure limits recommended by the National Institute for Occupational Safety and Health (NIOSH) scientists to the Occupational Safety and Health Administration (OSHAOccupational Safety and Health Administration (www.osha.gov). Federal agency within the U.S. Department of Labor with the responsibility of ensuring worker safety and health.).

Note: RELs are not included in CAMEO 365betÓéÀÖ.

What are RELs?

RELs are science-based recommendations rather than legal standards. They are based on animal and human studies.

A REL is defined up to three ways:

  • Time-weighted average (TWA) concentration: The concentration of a contaminant averaged over a workday (usually 8 hours long). It's measured in a workplace by sampling a worker's breathing zone for the whole workday. NIOSH recommends that the TWA should not be exceeded for up to an 10-hour workday during a 40-hour workweek.
  • Ceiling value: A concentration of a toxicPoisonous; can injure or kill people or other organisms. substance in air that NIOSH recommends should not be exceeded at any time during the workday (unless otherwise noted). This value is often used in conjunction with the TWA.
  • Short-term Exposure Limit (STEL) value: A TWA concentration that NIOSH recommends not to be exceeded for longer than 15 minutes during a workday (unless otherwise noted)—even if the 8-hour TWA is within the standards. TWA-STELs are given for contaminants for which short-term hazards are known.
±·´Ç³Ù±ð:ÌýRELs are often more conservative than the corresponding TLVs, and NIOSH's consideration of available research and studies is regarded as thorough.

Below is a graph of concentration over an 8-hour day at a hypothetical workplace. During this day, the TWA and the ceiling value were not exceeded:

  • Excursions of concentration above the TWA line were balanced out by periods when concentrations were below the line.
  • While the ceiling value was reached, it was never exceeded.

A graph of exposure versus time at a hypothetical workplace.

REL Information in the NIOSH Pocket Guide

The External link. includes RELs, along with other information, for nearly 700 substances and substance groups. (Chemical-specific links to the NIOSH Pocket Guide are included on some chemical datasheets in CAMEO 365betÓéÀÖ.) In the NIOSH Pocket Guide, note that:

  • A TWA REL is designated by "TWA" preceding the value.
  • A ceiling REL is designated by "C" preceding the value.
  • A STEL is designated by "ST" preceding the value.
  • Any substance that NIOSH considers to be a potential occupational carcinogenCapable of causing cancer. is designated by the notation "Ca".

Part of the Benzene datasheet from the NIOSH pocket guide. The NIOSH RELs and OSHA PELs are marked.