In the United States, legally enforceable exposure limits come from several federal agencies, each covering a different type of hazard. OSHA sets enforceable workplace air contaminant limits called Permissible Exposure Limits (PELs). The EPA enforces outdoor air quality standards and drinking water contamination limits. The Nuclear Regulatory Commission enforces radiation dose limits. Other agencies publish recommended limits that carry no legal weight on their own. Understanding which limits have the force of law, and which are only guidelines, matters because the gap between the two can be significant.
OSHA Permissible Exposure Limits
OSHA’s PELs are the primary enforceable exposure limits in American workplaces. They apply to any employer covered under the Occupational Safety and Health Act of 1970 and set the maximum concentration of a chemical or substance that workers can be exposed to over an 8-hour shift. Violating a PEL can result in citations and fines.
The catch is that most of OSHA’s PELs are badly outdated. The bulk of them have been in effect since May 29, 1971, based on consensus standards that predated 1968. OSHA attempted a major update in 1989, making 376 limits more protective, but the Eleventh Circuit Court of Appeals struck down the entire update in 1992 in a case brought by the AFL-CIO. The agency reverted to enforcing the original 1971 limits starting in March 1993. OSHA itself has acknowledged that many of these limits are “not sufficiently protective of employee health based on current scientific information.”
Updating individual PELs is still possible but requires a lengthy rulemaking process. One notable success: in 2016, OSHA finalized a new rule cutting the PEL for respirable crystalline silica to 50 micrograms per cubic meter of air (as an 8-hour average) across all covered industries. The previous limits had been roughly 100 micrograms for general industry and 250 micrograms for construction. The new rule issued separate standards for general industry, maritime, and construction to account for differences in how those sectors operate.
Recommended Limits That Are Not Enforceable
NIOSH, the National Institute for Occupational Safety and Health, publishes Recommended Exposure Limits (RELs) based on the latest research. These are not enforceable. OSHA has stated directly that “NIOSH RELs are not for enforcement purposes.” The same is true of the American Conference of Governmental Industrial Hygienists (ACGIH), which publishes Threshold Limit Values (TLVs). These are professional guidelines, not law.
In practice, NIOSH RELs and ACGIH TLVs are often far more protective than OSHA’s PELs because they reflect current science rather than 1960s-era data. Many employers voluntarily follow them, and safety professionals routinely recommend them. But no inspector can issue a citation based on exceeding a REL or TLV alone.
The General Duty Clause: Enforcement Without a Specific Limit
When no specific PEL exists for a hazard, OSHA can still take enforcement action under Section 5(a)(1) of the OSH Act, known as the General Duty Clause. It requires every employer to provide “employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm.” To prove a violation, OSHA must show four things: that a hazard existed, the hazard was recognized (by the employer or the industry), it was causing or likely to cause death or serious physical harm, and a feasible method to correct it was available.
This is the legal tool OSHA uses when a substance has no formal PEL but scientific evidence clearly shows it’s dangerous at the levels workers are experiencing. In these cases, NIOSH RELs or ACGIH TLVs often serve as the benchmark for what a “recognized hazard” looks like, even though they aren’t enforceable standards on their own.
EPA Air Quality Standards
The Clean Air Act requires the EPA to set National Ambient Air Quality Standards (NAAQS) for outdoor air. These are legally enforceable and cover six “criteria” pollutants: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. Each state must develop a State Implementation Plan showing how it will meet these standards. If a state fails, the EPA can issue a “SIP call” requiring the state to revise its plan and demonstrate compliance.
These standards protect the general public, not just workers. They set concentration limits for the air everyone breathes, and areas that exceed them are classified as “nonattainment” zones subject to additional regulatory requirements.
Drinking Water Limits
For public water systems, the EPA sets two types of limits under the National Primary Drinking Water Regulations. Maximum Contaminant Levels (MCLs) are the enforceable ones. An MCL is the highest level of a contaminant allowed in water delivered to any user of a public water system. Water utilities that exceed an MCL face enforcement action.
Maximum Contaminant Level Goals (MCLGs) are the non-enforceable counterpart. An MCLG represents the level at which no known or anticipated health effects would occur, with a margin of safety built in. MCLGs are aspirational. MCLs are set “as close to the MCLGs as feasible using the best available treatment technology,” which means the enforceable limit is sometimes higher than the ideal health goal because of practical or cost constraints.
Radiation Dose Limits
The Nuclear Regulatory Commission enforces annual radiation dose limits for workers at licensed facilities under 10 CFR Part 20. The occupational limit for adults is 5 rem (0.05 sieverts) total effective dose equivalent per year. For any individual organ or tissue other than the lens of the eye, the limit is 50 rem (0.5 sieverts) per year. Licensees are legally required to keep worker doses within these limits, and violations can result in enforcement actions including fines and license suspension.
Chemical Manufacturing Restrictions Under TSCA
The Toxic Substances Control Act gives the EPA authority to regulate new chemicals before they enter the market. Any company planning to manufacture or import a new chemical must submit a premanufacture notice at least 90 days in advance. If the EPA determines the substance may pose an unreasonable risk through inhalation, it can issue a consent order requiring the company to keep workplace air concentrations below a New Chemical Exposure Limit (NCEL). These NCELs are modeled after OSHA’s PEL framework and include requirements for air monitoring, respiratory protection, and recordkeeping. The EPA can extend these requirements to other manufacturers of the same chemical through Significant New Use Rules.
State-Level Limits Can Be Stricter
Federal law allows states to run their own occupational safety programs, called State Plans, as long as they are “at least as effective” as federal OSHA. States with approved plans can set and enforce exposure limits stricter than federal PELs. California is the most prominent example, operating a state OSHA program that covers most private sector workers and all state and local government employees. California’s PELs for many substances are significantly lower than the federal limits.
For indoor air quality specifically, there are no federal enforceable standards for residential buildings. OSHA’s jurisdiction covers workplaces, not homes. Only California and New Jersey have state-level indoor air regulations. In workplaces, OSHA does not have a standalone indoor air quality standard, though it has ventilation standards and can address indoor air problems through existing contaminant-specific PELs or the General Duty Clause.
Quick Comparison of Enforceable vs. Advisory Limits
- Enforceable: OSHA PELs (workplace air), EPA MCLs (drinking water), EPA NAAQS (outdoor air), NRC dose limits (radiation), EPA NCELs (new chemicals under TSCA consent orders), state-level limits in State Plan states
- Not enforceable: NIOSH RELs, ACGIH TLVs, EPA MCLGs, WHO guideline values
The practical consequence of this split is that many workplace exposure limits enforced today reflect science from over 50 years ago, while the more current recommendations from NIOSH and ACGIH remain voluntary. If you’re evaluating whether a specific exposure is within legal limits, the enforceable number and the scientifically recommended number may be very different.