ACGIH Threshold Limit Values are annually updated occupational exposure guidelines published by the American Conference of Governmental Industrial Hygienists. They are not laws and carry no direct federal enforcement power, but they shape workplace safety decisions because OSHA’s own Permissible Exposure Limits are decades out of date, and regulators, courts, and employers routinely treat TLVs as the best available science on what a safe exposure looks like.
The Three Chemical TLV Categories
Every chemical TLV falls into one of three categories, each built around a different exposure pattern.
The TLV-TWA (Time-Weighted Average) is the concentration averaged over an eight-hour workday and 40-hour workweek to which nearly all workers can be repeatedly exposed across a working lifetime without adverse effects.1ACGIH. TLV Chemical Substances Introduction Because it is an average, concentrations rise and fall during a shift, but ACGIH imposes specific excursion limits on how high those peaks can go.
The TLV-STEL (Short-Term Exposure Limit) is a 15-minute time-weighted average that should not be exceeded at any point during a workday, even when the eight-hour TWA stays within its limit. Exposures at the STEL level should happen no more than four times per day, with at least 60 minutes between each occurrence.1ACGIH. TLV Chemical Substances Introduction The STEL protects against acute effects that a substance can produce during brief, intense contact.
The TLV-C (Ceiling) is the concentration that should not be exceeded during any part of the working exposure. When instantaneous measurement is not possible, ACGIH recommends sampling over the shortest feasible period sufficient to detect exposures at or above the ceiling.1ACGIH. TLV Chemical Substances Introduction This category exists for substances where a momentary spike can cause serious harm or severe irritation.
Excursion Limits When a Substance Has No STEL
Many substances have a TLV-TWA but no assigned STEL. For those chemicals, ACGIH applies a default excursion rule. Transient exposures may exceed three times the TWA for no more than 15 minutes at a time, on no more than four occasions spaced at least one hour apart during the workday. Under no circumstances should the exposure reach five times the TWA, measured as a 15-minute average. The eight-hour TWA must still not be exceeded overall.1ACGIH. TLV Chemical Substances Introduction
Some employers treat the TWA as a simple average and assume that low-exposure periods automatically offset high-exposure spikes. They do not. The 3x and 5x ceilings apply regardless of the daily average, and exceeding them puts the exposure outside ACGIH guidance even if the calculated eight-hour TWA looks acceptable on paper.
Notations and Carcinogenicity Classifications
A TLV is rarely just a number. Most entries carry notations that signal hazards beyond what the airborne limit alone conveys. Missing them is one of the most common mistakes in applying TLV data, because a workplace can meet the airborne limit and still expose workers to dangerous doses through other routes.
The Skin notation warns that a substance can be absorbed through the skin in quantities large enough to cause systemic toxicity. For chemicals carrying this notation, ACGIH has also developed the TLV-SL (Surface Limit), expressed in milligrams per 100 square centimeters, to give hygienists a quantitative benchmark for surface contamination.1ACGIH. TLV Chemical Substances Introduction Air monitoring alone is not sufficient for these substances.
The DSEN and RSEN notations flag agents that can cause dermal sensitization or respiratory sensitization, respectively. When sensitization is the basis for the TLV, the limit is set to prevent workers from becoming sensitized in the first place. It is not designed to protect workers who are already sensitized, because those individuals may react at concentrations well below the TLV.2ACGIH. Operations Manual – Threshold Limit Values for Chemical Substances
ACGIH also assigns carcinogenicity classifications from A1 through A5. A1 designates a confirmed human carcinogen, A2 a suspected human carcinogen, A3 a confirmed animal carcinogen with unknown relevance to humans, A4 an agent not classifiable as to human carcinogenicity, and A5 an agent not suspected as a human carcinogen. These classifications appear alongside the TLV in the published tables and directly influence how strictly an employer should control exposure.
TLVs for Physical Agents
Chemical substances get most of the attention, but ACGIH also publishes TLVs for physical agents across five categories: acoustic, electromagnetic, ergonomic, mechanical, and thermal hazards.3ACGIH. TLV Physical Agents Introduction Noise exposure is among the most commonly referenced. ACGIH recommends a TLV of 85 dBA as an eight-hour TWA with a 3-dB exchange rate, which is more protective than the federal OSHA standard of 90 dBA with a 5-dB exchange rate.4Occupational Safety and Health Administration. Occupational Noise Exposure – Standards
Heat stress TLVs use Wet Bulb Globe Temperature measurements and vary by workload intensity and the proportion of work versus rest in a given hour. Other physical agent TLVs cover ionizing and non-ionizing radiation, hand-arm and whole-body vibration, and static magnetic fields.
Biological Exposure Indices
Biological Exposure Indices are the companion tool to airborne TLVs. Where a TLV measures what is in the air, a BEI measures what has actually entered the worker’s body by analyzing urine, blood, or exhaled air.5ACGIH. Biological Exposure Indices (BEI) Introduction The determinant being measured can be the chemical itself, one of its metabolites, or a reversible biochemical change the chemical induces.
BEIs represent the levels most likely to be observed in healthy workers whose inhalation exposure matches the TLV-TWA. They help occupational health professionals detect absorption through the skin or gastrointestinal tract that air monitoring would miss, assess cumulative body burden, evaluate whether protective equipment is working, and reconstruct past exposures.5ACGIH. Biological Exposure Indices (BEI) Introduction
Sampling timing is critical and varies by determinant. Standardized collection windows include prior to the shift, during the shift after at least two hours of exposure, end of shift, and end of the workweek after four or five consecutive days of exposure.5ACGIH. Biological Exposure Indices (BEI) Introduction Determinants with long biological half-lives may take weeks or months of employment before reaching a steady state that is meaningfully comparable to the BEI.
Legal Standing of TLVs
TLVs are the proprietary intellectual property of ACGIH and are explicitly not developed for use as legal standards. ACGIH does not advocate their use as such and notes that applying them without the professional judgment of a trained industrial hygienist stretches the reliability of the underlying data. They are also not consensus standards. Unlike standards developed by organizations such as ANSI, which canvass views from all interested parties, TLVs represent the scientific opinion of the ACGIH committee based on its review of the literature. The development process deliberately excludes economic cost and technical feasibility.6Occupational Safety and Health Administration. Important Note Regarding the ACGIH TLV
Despite this stated intent, TLVs play a substantial role in enforcement and litigation.
The Gap Between TLVs and OSHA PELs
OSHA’s Permissible Exposure Limits are the only federally enforceable workplace exposure standards. Most were adopted shortly after the Occupational Safety and Health Act passed in 1970 and have not been updated since.7Occupational Safety and Health Administration. Permissible Exposure Limits – Annotated Tables OSHA itself acknowledges that many of these limits are “not sufficiently protective of worker health.” ACGIH TLVs, by contrast, are updated annually. The result is a persistent gap where the legal floor set by OSHA may be far less protective than the scientific recommendation from ACGIH.
TLVs and the General Duty Clause
Section 5(a)(1) of the OSH Act requires every employer to provide a workplace free from recognized hazards that are causing or likely to cause death or serious physical harm.8Occupational Safety and Health Administration. OSH Act of 1970 – Section 5, Duties When no specific PEL covers a hazard, OSHA may cite an employer under this General Duty Clause, and TLVs often enter the picture as evidence of what constitutes a recognized hazard.
A TLV alone is not enough to sustain a citation. OSHA enforcement policy requires Area Directors to prove all four elements: that a hazard existed, that it was recognized, that it was causing or likely to cause death or serious physical harm, and that a feasible correction existed. A measured exposure exceeding a TLV does not automatically satisfy the third element. Absent direct evidence of employee illness, injury, or medical diagnosis, OSHA typically needs additional expert testimony or peer-reviewed studies showing that serious harm could occur at the measured levels.9Occupational Safety and Health Administration. Enforcement Policy for Respiratory Hazards Not Covered by OSHA Standards
Serious violations of the General Duty Clause carry penalties up to $16,550 per violation under the most recently published adjustment. Willful or repeated violations can reach $165,514 per violation. OSHA adjusts these amounts annually for inflation.10Occupational Safety and Health Administration. OSHA Penalties
State OSHA Plans
Roughly half the states and several U.S. territories operate their own OSHA-approved workplace safety programs. These state plans must be at least as effective as federal OSHA in protecting workers, and they can go further.11Occupational Safety and Health Administration. State Plans Some state plans have adopted exposure limits that align more closely with current ACGIH TLVs than with the outdated federal PELs. In those jurisdictions, the “TLVs are just guidelines” framing is misleading, because the enforceable limits already reflect TLV-level science.
TLVs in Civil Litigation
Outside the regulatory context, TLVs regularly appear as evidence in personal injury and toxic tort lawsuits. Plaintiffs use them to argue that an employer knew or should have known that exposures were harmful, particularly when airborne concentrations exceeded the TLV even though they remained below the PEL. Courts may treat TLVs as evidence of the professional standard of care that a reasonable employer should follow. Employers who knowingly allow exposures above TLV levels face a difficult argument that they acted with reasonable diligence when the leading scientific authority in the field had flagged those levels as unsafe.
Where to Find the Actual TLV Values
Specific TLV and BEI values are published in the annual TLVs and BEIs book, which ACGIH updates each year to incorporate newly adopted values, revisions, and changes to the Notice of Intended Changes list.1ACGIH. TLV Chemical Substances Introduction Physical copies and digital subscriptions are available through the ACGIH website, which also offers a searchable database.
You can look up a substance by its common chemical name or its Chemical Abstracts Service registry number, which eliminates confusion when a compound goes by multiple names. Each entry includes the TLV value, any applicable notations for skin absorption, sensitization, or carcinogenicity, and the approximate year the documentation was last substantially reviewed.1ACGIH. TLV Chemical Substances Introduction The multi-volume Documentation of the Threshold Limit Values provides the full scientific rationale behind each value. Because TLVs are copyrighted, the full numerical tables are not freely available online, so practitioners who need the actual numbers should budget for the annual publication.