Limit Values United States of America – REL (NIOSH)

The National Institute for Occupational Safety and Health (NIOSH) is the federal agency responsible for conducting research and making recommendations for the prevention of work-related injury and illness in the USA.

The Occupational Safety and Health Act of 1970 created both NIOSH and the Occupational Safety and Health Administration (OSHA). NIOSH is part of the Centers for Disease Control and Prevention (CDC) in the United States government’s Department of Health and Human Services.

Acting under the authority of the Occupational Safety and Health Act of 1970 (29 USC Chapter 15) and the Federal Mine Safety and Health Act of 1977 (30 USC Chapter 22), NIOSH develops and periodically revises recommended exposure limits (RELs) for hazardous substances or conditions in the workplace. These recommendations are then published and transmitted to OSHA and the Mine Safety and Health Administration (MSHA) for use in promulgating legal standards.

RELs are recommendations based on a critical review of the scientific and technical information available on a given hazard and the adequacy of methods to identify and control the hazard. They are produced under the auspices of the NIOSH Education and Information Division (EID) by a team of experts including engineers, chemists, biochemists, biologists, health physicists, industrial hygienists, medical doctors, and epidemiologists. RELs are based on risk evaluations using human or animal health effects data, and on an assessment of what levels can be feasibly achieved by engineering controls and measured by analytical techniques. To the extent feasible, NIOSH projects not only a no-effect exposure, but also exposure levels at which there may be residual risks.

This policy applies to all workplace hazards, including carcinogens, and is responsive to Section 20 (a) (3) of the Occupational Safety and Health Act of 1970, which charges NIOSH to "...describe exposure levels that are safe for various periods of employment, including but not limited to the exposure levels at which no employee will suffer impaired health or functional capacities or diminished life expectancy as a result of his work experience." The effect of this new policy will be the development, whenever possible, of quantitative RELs that are based on human and/or animal data, as well as on the consideration of technological feasibility for controlling workplace exposures to the REL. Under the old policy, RELs for most carcinogens were non-quantitative values labelled "lowest feasible concentration (LFC)."

Once a draft REL is proposed, NIOSH conducts open hearings with the public to obtain input from stakeholders, and also accepts written materials from interested individuals and organisations, which have to be documented and taken into account. In rare occasions RELs may anticipate matters of economic feasibility.

NIOSH recommendations are published in a variety of documents. Criteria documents recommend workplace exposure limits and appropriate preventive measures to reduce or eliminate adverse health effects and accidental injuries.

NIOSH’s recommended exposure limits (RELs) can be found in the NIOSH Pocket Guide to Chemical Hazards (NIOSH Pocket Guide, 2011). They are listed first in the section “Exposure Limits”. For NIOSH RELs, "TWA" indicates a time-weighted average concentration for up to a 10-hour workday during a 40-hour workweek. A short-term exposure limit (STEL) is designated by "ST" preceding the value; unless noted otherwise, the STEL is a 15-minute TWA exposure that should not be exceeded at any time during a workday. A ceiling REL is designated by "C" preceding the value; unless noted otherwise, the ceiling value should not be exceeded at any time. Any substance that NIOSH considers to be a potential occupational carcinogen is designated by the notation "Ca" (see Appendix A, which contains a brief discussion of potential occupational carcinogens).

Recommendations made through 1992 are available in a single compendium entitled NIOSH Recommendations for Occupational Safety and Health: Compendium of Policy Documents and Statements (DHHS (NIOSH) Publication No. 92-100). Copies of the Compendium may be ordered from the NIOSH Publications office.

November 2013