Exposure Limits in US – Alphabet Soup

Exposure Limits in US should be super simple. “Do not exceed #.” But it’s a government regulatory issue… so how simple could it be?

Here is a summary of the primary exposure limits used by our government and industrial hygienists in the United States.

Permissible Exposure Limits (PELs) – OSHA

The Occupational Health and Safety Administration (OSHA) sets PELs as a legal limit for exposure of an employee to a chemical substance or physical agent. PELs for for chemicals are generally expressed in parts per million (ppm) or milligrams per cubic meter (mg/m3). Exposures are further qualified as chronic or acute. Chronic exposures are calculated using time-weighted average (TWA), and acute exposure levels are expressed as short-term exposure limits (STEL) or “ceiling limits.”  TWAs are generally for a nominal 8-hour day, STELs address average exposure over a 15-30 minute period, and ceiling limits may never be exceeded for any period of time. Ceiling limits are the highest allowable, and they define irritants that would have an immediate effect on an employee.

PELs are intended to protect workers from hazardous exposures, and the limits are established for industrial environments. PELs also apply to non-industrial environments, such as offices, but it is very rare that PELs are exceeded outside of industrial settings. PELs can be found in Table Z-1 of 29 CFR 1910.1000 and additional PELs can be found in Table Z-2.

First Few Chemicals from OSHA PELs Table Z-1

PELs were created in the early 70’s, and are official regulatory limits enforceable by law. Most PELs have remained unchanged since the early 70s, in large part because of the litigious nature of chemical regulation and powerful lobbying efforts from companies resistant to change the exposure limits in US.

Threshold Limit Values (TLVs) – ACGIH

In the late 60’s the American Conference of Governmental Industrial Hygienists (ACGIH) preempted OSHA and created exposure limits in the form of Threshold Limit Values (TLVs). Over the years TLVs have evolved based on new scientific evidence and anecdotal evidence from the field.

Excerpt from their website, “ACGIH publishes guidelines known as Threshold Limit Values (TLVs) for use by industrial hygienists in making decisions regarding safe levels of exposure to various chemical and physical agents found in the workplace. In using these guidelines, industrial hygienists are cautioned that the TLVs are only one of multiple factors to be considered in evaluating specific workplace situations and conditions.”

Recommended Exposure Limits (RELs) – NIOSH

The National Institute for Occupational Safety and Health (NIOSH) is under the Center for Disease Control. NIOSH has issued Recommended Exposure Limits (RELs) that are science-based and precautionary, but are not enforceable by law. RELs are expressed as Time Weighted Average (TWA) for 8-10 hour work days. Some chemicals also include a Ceiling Limit or Short Term Exposure Limits.

RELs are published in NIOSH Pocket Guide which available for free download online. The Pocket Guide is great because it lists NIOSH RELs, OSHA PELs, and a lot of other information per each chemical.

NIOSH Pocket Guide – Formaldehyde

Limits to Limits

There is always a lag time between scientific health discoveries and changes to regulatory limits. In the case of PELs, regulatory limits have been pretty much stagnant since their inception in 1971.

There are many, many more chemicals in the world than are covered byPELs, TLVs and RELs, and there are very few exposure limits set for biological contaminants such as mold spores, pollens, microbial VOCs, etc. Additionally, exposure limits are designed on a per-chemical-basis, and do not adequately take into account synergistic effects from multiple chemical exposures.

And there are so many exposure limits… which do you trust? Here is a chart from the EPA summarizing different exposure guidelines for Formaldehyde in the US. You can see how confusing this can get, and how regulatory guidelines are above some known health threats.

Formaldehyde Exposure Limit Chart w/ Health Effects

Highlight Resources

The CDC’s website has a great compilation of research and studies organized by chemical name.

The NIOSH Pocket Guide to Chemical Hazards has an online searchable function that is phenomenal. The Pocket Guide can be ordered for free, and under each chemical it highlights NIOSH REL and OSHA PEL, in addition to other valuable information about exposure pathways, symptoms, etc.

Identifying a “safe” level of anything is rather complicated… but at least we have something to refer to when the question arrises, “how much is too much.” Manyexposure limits in the European Union and other countries are lower than in the US, and many environmentalists consider exposure limits in US too high.