What is the difference between a REL and a PEL?

There is often confusion between exposure limits put out by different agencies even within the same administration. For example, the National Institute for Occupational Safety and Health (NIOSH) establishes Recommended Exposure Limits (RELs) whereas the Occupational Safety and Health Administration (OSHA) issues Permissible Exposure Limits (PELs). Realizing the difference between the two is as easy as looking at the names themselves: What is “recommended” vs. what is “permissible”.

NIOSH RELs are supposed to be based on the best available science (using human or animal health effects data). According to the CDC’s website, “To the extent feasible, NIOSH will project 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.’”

OSHA PELs, on the other hand, are subject to the rulemaking and political process, meaning that the interests of all parties involved are taken into consideration. Thus, OSHA does not have the luxury of relying strictly on science. Establishing PELs sometimes even come down to court rulings.


Take, for example, the extremely popular disinfectant and preservative Formaldehyde, a known human carcinogen according to the International Agency for Research on Cancer (IARC). The OSHA PEL is set at 0.75ppm as an 8 hour time-weighted average. In contrast, the NIOSH REL is a 0.016ppm 8 hour time-weighted average with a ceiling set at 0.1ppm. This difference reflects the disconnect between what the political process deems appropriate for exposure and what hard science says is harmful.

What is the takeaway here? To be frank, the OSHA PEL is not the safe limit below which harm cannot occur. Rather it is the legal limit (i.e. what is “permissible”), below which serious harm should not occur to most people. Thus, while the OSHA PEL represents the legal exposure limit, it does not necessarily represent the desired exposure level. To that extent, the NIOSH REL is the more appropriate number.

Everyday we do activities that carry risk of injury including workplace practices. Formaldehyde use offers many benefits, but the employer must reduce the risk of employee exposure to below the OSHA PEL and should reduce the risk to below the NIOSH REL.

Employers can reduce the risk of formaldehyde exposure through implementation of sufficient engineering controls, monitoring devices, personal protective equipment, worker training, good work practices, regular equipment maintenance etc. For those compounds which are potentially hazardous at levels below that of reliable human perception, continuous gas monitors should be used. Where possible, employers should strive to meet the NIOSH RELS, rather than the OSHA PELs.