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.

New Book on AEGLs Published by the National Academies Press

The National Academies Press published a book recently discussing the Environmental Protection Agency’s (EPA’s) Acute Exposure Guideline Levels (AEGLs) for Selected Airborne Chemicals. The book provides comprehensive summaries of studies and experiments that the EPA used when determining exposure limits for certain chemicals.

One chemical discussed in the book is Peracetic Acid (PAA), which we have discussed in detail on this blog before. PAA has been receiving increased attention in recent years, as its use as a disinfectant against bacteria, fungi, and viruses is becoming more widespread in both the healthcare and food industries. PAA can be quite harmful in the event of exposure as it is corrosive and irritating to the eyes, mucous membranes of the respiratory tract, and skin. As the chapter states, “extreme discomfort and irritation” can occur in humans after exposure to just 5ppm for 3 minutes. Lethal concentrations of PAA can cause hemorrhage, edema, and consolidation of the lungs of those exposed.

The EPA typically sets 3 different AEGL values for chemicals. In general, the first level refers to the concentration of a chemical which would cause discomfort, the second long lasting adverse health effects, and the third life-threatening health effects. AEGL 1 for PAA is set at 0.17ppm, which is considered to be the threshold for irritation to mucous membranes and eyes. AEGL 2 is set at 0.5ppm, the concentration at which “slight to tolerable discomfort to nasal membranes and eyes” begins to occur. AEGL 3 is set at 4.1mg/m3 (which is not converted to ppm because values are based on exposure to aerosol). While the data for AEGL 3 was based on studies in animals (rats and mice), there is evidence that humans may be slightly more sensitive to PAA.

As we have pointed out before, there is currently no OSHA Permissible Exposure Limit (PEL) for PAA. With the recent actions taken by the administration, there is reason to believe that a new round of PELs may be issued in the near future. However, it is important to remember that OSHA PELs are not an all inclusive list of chemicals that could be harmful to humans if exposed. Data reported by the EPA in articles and books like this serve as practical reminders that chemicals used to protect us can also hurt us if we don’t protect ourselves from potential exposure.

A link to a free PDF download of the new book about AEGLs can be found here: http://www.nap.edu/catalog.php?record_id=12770