A permissible exposure limit (PEL) is the maximum amount of a hazardous substance a worker can legally be exposed to over a workday. Set and enforced by the Occupational Safety and Health Administration (OSHA), PELs cover hundreds of chemical substances, dusts, and physical agents like noise. They are legally binding limits, meaning employers who allow exposures above a PEL can face citations and penalties.
Most PELs are expressed as a concentration of a substance in the air, measured in parts per million (ppm) or milligrams per cubic meter (mg/m³). They apply to the air a worker actually breathes in their work zone, not to ambient room air in general.
How PELs Are Structured
Not all PELs work the same way. There are three types, each designed to control a different pattern of exposure.
- Time-Weighted Average (TWA): The most common type. This is the average concentration of a substance over a full 8-hour shift in a 40-hour workweek. A worker’s exposure can spike above the limit briefly, as long as it averages out below the PEL over the full shift. For example, the TWA for carbon monoxide is 50 ppm.
- Short-Term Exposure Limit (STEL): A 15-minute average that cannot be exceeded at any point during the workday. STELs exist for substances that can cause harm even in brief bursts, regardless of what the 8-hour average looks like.
- Ceiling Limit: The strictest type. A ceiling value cannot be exceeded at any moment. If instantaneous monitoring isn’t possible, the ceiling is assessed as a 15-minute average instead.
Some substances have only a TWA. Others have a TWA paired with a STEL or a ceiling limit, depending on how the substance affects the body. A chemical that causes immediate irritation or acute toxicity is more likely to carry a ceiling limit, while one that causes gradual damage over years of exposure typically has a TWA.
Where PELs Come From
Most of OSHA’s current PELs were adopted shortly after the Occupational Safety and Health Act passed in 1970. The agency used its authority under Section 6(a) of the Act to pull in existing federal standards, which themselves were based on the 1968 Threshold Limit Values published by the American Conference of Governmental Industrial Hygienists (ACGIH). Those values are now more than 50 years old.
OSHA itself acknowledges that many of its PELs are outdated and inadequate for protecting worker health. The agency has struggled to update them because the rulemaking process for each substance is lengthy, expensive, and often legally contested. As a result, the PEL for a given chemical may be far less protective than what current science supports. For a handful of high-priority substances like respirable crystalline silica, lead, and benzene, OSHA has issued standalone standards with updated limits, but the bulk of the PEL tables remain unchanged since the early 1970s.
PEL vs. REL vs. TLV
You’ll often see PELs mentioned alongside two other exposure limits: NIOSH RELs and ACGIH TLVs. They measure the same thing (safe airborne concentrations of a substance) but differ in who sets them, whether they’re legally enforceable, and how current they are.
NIOSH Recommended Exposure Limits (RELs) come from the National Institute for Occupational Safety and Health, the federal research agency that studies workplace hazards. RELs are based on a 10-hour workday and 40-hour workweek, slightly different from OSHA’s 8-hour basis. They are recommendations, not legal requirements. NIOSH evaluates medical, biological, engineering, and chemical data to develop RELs, and they are generally more protective than PELs because they reflect more recent science.
ACGIH Threshold Limit Values (TLVs) are guidelines published by a private scientific organization. TLVs represent concentrations under which nearly all workers could be exposed repeatedly over a working lifetime without adverse health effects. Because ACGIH bases TLVs solely on health data with no consideration for economic or technical feasibility, they are often the most protective of the three. They are not legal standards and cannot be directly enforced, but many employers and safety professionals use them as a benchmark when OSHA’s PELs are clearly outdated.
In practice, if you see a substance where the PEL is 100 ppm but the TLV is 20 ppm, it usually means the science has advanced significantly since the PEL was set. Many safety-conscious employers aim for the lower value.
Examples of Current PELs
OSHA publishes its PELs in what are known as the Z-Tables, found in the Code of Federal Regulations. Here are a few to illustrate the range:
- Carbon monoxide: 50 ppm as an 8-hour TWA (55 mg/m³)
- Respirable crystalline silica: 50 micrograms per cubic meter (μg/m³) as an 8-hour TWA, with an action level of 25 μg/m³
- Benzene and formaldehyde: Covered under their own substance-specific standards with detailed requirements beyond a simple PEL number
The silica standard is a useful example of how modern OSHA standards work. The PEL of 50 μg/m³ is the hard legal ceiling. But at 25 μg/m³, half the PEL, the “action level” kicks in. Once exposures reach the action level, employers must begin periodic air monitoring, medical surveillance, and other protective measures even though the PEL hasn’t been exceeded yet. This action-level concept appears across many OSHA health standards and is typically set at half the PEL.
What Employers Must Do
When a workplace has hazardous substances present, employers are responsible for measuring airborne concentrations in workers’ breathing zones and comparing those results against the PEL. If exposures approach or exceed the limit, a specific hierarchy of responses applies.
Engineering controls come first. These are physical changes to the workplace that reduce the concentration of a hazard at its source: ventilation systems, enclosed processes, wet methods to suppress dust, or substituting a less toxic material. Work practice controls come next, meaning changes in how the work is performed to limit exposure.
Personal protective equipment like respirators is the last resort, not the first. OSHA’s policy is clear on this point: a worker’s exposure is measured without factoring in respiratory protection. If the air concentration exceeds the PEL, the worker is considered overexposed even if they’re wearing a respirator. That said, if an employer has implemented all feasible engineering and work practice controls and still can’t get below the PEL, OSHA’s policy is not to issue a citation as long as the employer maintains an adequate respiratory protection program and meets all other requirements of the relevant standard.
Why PELs Matter Beyond Compliance
Because most PELs are based on 1968 science, meeting the legal limit does not necessarily mean a workplace is safe by modern standards. A number of substances have PELs that are 5 to 10 times higher than what NIOSH or ACGIH currently recommends. For workers exposed to these chemicals daily over a career, the gap between the legal limit and the health-based recommendation can translate into real risk of chronic disease, lung damage, or cancer.
Some states run their own OSHA-approved occupational safety programs and have adopted stricter PELs than the federal standard. California, for instance, maintains its own set of permissible exposure limits through Cal/OSHA that are updated more frequently. If you work in a state-plan state, the limits that apply to your workplace may be lower than what appears in the federal Z-Tables.
For workers and safety managers trying to evaluate whether a workplace is truly healthy, PELs are the legal floor, not the ceiling of protection. Comparing your exposure data against NIOSH RELs and ACGIH TLVs gives a more complete picture of risk, especially for substances whose federal limits haven’t changed in over half a century.