Heat stress law and limits

Heat stress law in the United States is a patchwork. Federal OSHA has no specific heat standard. It enforces heat hazards under the General Duty Clause and through an emphasis program, and it proposed a federal heat injury and illness prevention standard in 2024 that has not been finalized. Meanwhile, several states have adopted their own heat rules. Behind all of these sits a common technical reference: the American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit values for heat stress and strain, and NIOSH's recommended standard. This lesson explains those references and how federal and state rules use them. It describes the law as of October 2026; check current requirements before relying on any detail.

The ACGIH heat stress TLVs

The ACGIH publishes threshold limit values, or TLVs, for heat stress as part of its annual TLV booklet. The heat stress TLV is designed so that nearly all acclimatized, adequately hydrated, unmedicated, healthy workers can be repeatedly exposed without adverse health effects, with core temperature kept at or below about 38 °C. Alongside the TLV, the ACGIH sets a lower action limit intended to protect workers who are not acclimatized; when conditions exceed the action limit, a heat stress management program is expected.

The TLV and action limit are expressed as WBGT values in degrees Celsius that depend on two things: the metabolic rate of the work, from light to very heavy, and the proportion of each hour spent working rather than resting. The heavier the work and the larger the share of each hour spent working, the lower the allowable WBGT. The WBGT is first adjusted for clothing, as described below.

The screening criteria are a first step. ACGIH notes that if the adjusted WBGT exceeds the criteria, or if workers wear clothing that is not covered by the adjustment factors, a more detailed analysis or physiological monitoring of heat strain is needed. Conversely, meeting the criteria does not guarantee that every worker is safe; individuals vary, and workers should still be watched for symptoms.

Clothing adjustment factors

Clothing affects heat stress because it blocks evaporation and adds insulation. ACGIH provides clothing adjustment values, in degrees Celsius WBGT, that are added to the measured WBGT before comparing it with the criteria. Ordinary work clothes, such as a long-sleeved shirt and pants, and cloth coveralls have an adjustment of zero. Double-layer woven clothing adds about 3 °C. Limited-use, vapor-barrier coveralls add about 11 °C, which is why workers in impermeable suits may be at risk even in mild conditions; for such clothing, ACGIH says the screening criteria should not be used and physiological monitoring is needed instead. A hood adds about 1 °C to the value for the garment.

# Find the work-rest band allowed by illustrative ACGIH-style screening criteria
TLV = {   # work fraction band -> {metabolic class: WBGT limit}, acclimatized
    "75-100%": {"light": 31.0, "moderate": 28.0, "heavy": None, "very heavy": None},
    "50-75%":  {"light": 31.0, "moderate": 29.0, "heavy": 27.5, "very heavy": None},
    "25-50%":  {"light": 32.0, "moderate": 30.0, "heavy": 29.0, "very heavy": 28.0},
    "0-25%":   {"light": 32.5, "moderate": 31.5, "heavy": 30.5, "very heavy": 30.0},
}
CAF = {"work clothes": 0.0, "double-layer woven": 3.0, "polyolefin coveralls": 1.0}

def allowed(wbgt, work, clothing, hood=False):
    eff = wbgt + CAF[clothing] + (1.0 if hood else 0.0)
    for band, limits in TLV.items():          # from most work to least
        lim = limits[work]
        if lim is not None and eff <= lim:
            return eff, band
    return eff, "exceeds screening criteria: monitor physiologically"

for wbgt, work, cloth, hood in [(27.0, "moderate", "work clothes", False),
                                (27.0, "heavy", "work clothes", False),
                                (27.0, "moderate", "double-layer woven", False),
                                (29.5, "heavy", "polyolefin coveralls", True)]:
    eff, band = allowed(wbgt, work, cloth, hood)
    print(f"WBGT {wbgt}, {work:8s}, {cloth:20s} hood={hood!s:5s} -> adjusted {eff:4.1f}: {band}")

WBGT 27.0, moderate, work clothes         hood=False -> adjusted 27.0: 75-100%
WBGT 27.0, heavy   , work clothes         hood=False -> adjusted 27.0: 50-75%
WBGT 27.0, moderate, double-layer woven   hood=False -> adjusted 30.0: 25-50%
WBGT 29.5, heavy   , polyolefin coveralls hood=True  -> adjusted 31.5: exceeds screening criteria: monitor physiologically

In the first case, moderate work in ordinary clothes at a WBGT of 27 °C is within the criteria for continuous work. Making the same work heavy drops the allowance to 50 to 75 percent work in each hour. Adding double-layer clothing to moderate work pushes the adjusted WBGT to 30 °C and limits work to 25 to 50 percent of each hour. In the last case, heavy work in coveralls with a hood at 29.5 °C exceeds the screening criteria entirely, so the employer must reduce the heat stress, change the clothing or the work, or monitor workers physiologically.

How the law uses these limits

Federal OSHA

OSHA has no specific heat standard. It cites employers for heat hazards under the General Duty Clause, section 5(a)(1) of the Occupational Safety and Health Act, which requires employers to keep workplaces free from recognized hazards likely to cause death or serious harm. To win a General Duty Clause case, OSHA must show that a hazard existed, that it was recognized, that it was likely to cause death or serious harm, and that a feasible means of abatement existed. Courts have sometimes found this hard to prove for heat. OSHA also enforces related standards, such as those for sanitation and drinking water (1910.141 and 1926.51), training, injury recording and medical services and first aid. OSHA renewed its National Emphasis Program on heat in April 2026 for five years; under it, OSHA conducts programmed inspections in high-risk industries on days with heat advisories and investigates heat complaints and referrals.

The proposed federal standard

On 30 August 2024, OSHA published a proposed Heat Injury and Illness Prevention in Outdoor and Indoor Work Settings standard. As proposed, it would apply to most general industry, construction, maritime and agricultural employers and would require a written heat injury and illness prevention plan, a heat safety coordinator, monitoring of heat conditions, and specific controls at two triggers: an initial heat trigger at a heat index of 80 °F, requiring drinking water, break areas, acclimatization measures and indoor controls, and a high heat trigger at a heat index of 90 °F, adding mandatory rest breaks of 15 minutes every two hours, observation for signs of heat illness and hazard alerts. OSHA held an informal public hearing from 16 June to 2 July 2025, and the post-hearing comment period closed on 30 October 2025. The Department of Labor's 2026 regulatory agenda listed a supplemental proposal for December 2026. As of October 2026, there is no final federal heat standard; check its status before relying on any element.

State heat rules

Several states have their own enforceable heat rules, which apply to employers in those states and are generally more specific than federal law. California has had an outdoor heat illness prevention standard (Title 8, section 3395) since 2005, requiring water, shade at 80 °F, high-heat procedures at 95 °F, acclimatization and training, and added an indoor heat standard (section 3396) that took effect on 23 July 2024, with controls triggered at 82 °F and stronger requirements at 87 °F or in high-radiant-heat areas. Washington has an outdoor heat exposure rule, strengthened in 2023 with triggers at 80 °F and 90 °F. Oregon adopted permanent heat rules in 2022 after the 2021 heat dome. Colorado protects agricultural workers, Minnesota has long had an indoor heat standard based on WBGT, Maryland's heat stress standard took effect on 30 September 2024, and Nevada adopted a heat illness regulation requiring job hazard analyses. Other states are considering rules. If you work in a state with its own plan, its rule applies in addition to federal requirements.

NIOSH recommendations

NIOSH's Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments, last revised in 2016, recommends exposure limits based on WBGT and metabolic rate similar to the ACGIH TLVs, a recommended alert limit for unacclimatized workers, acclimatization schedules, medical monitoring, training and a written heat-related illness prevention program. NIOSH recommendations are not enforceable, but OSHA, courts and states use them as evidence of recognized hazards and feasible controls, and they are a sound basis for a company program.

Duties of employers, supervisors and workers

Whatever the specific rules, the general duties are clear. Employers must identify heat hazards, assess them using recognized methods, put controls and a written plan in place, provide water, rest areas and suitable clothing and equipment, acclimatize new and returning workers, train workers and supervisors, and respond to heat illness. Supervisors must know the plan, watch workers for symptoms, enforce breaks, adjust work when conditions change and act immediately when someone shows signs of heat illness. Workers must follow the plan, drink regularly and report symptoms in themselves and others. Workers have a right to raise concerns and to file complaints with OSHA without retaliation under section 11(c), and in limited circumstances to refuse work that poses an imminent danger. Where there is a safety committee or union, it should be involved in developing and reviewing the plan.

Records and documentation

A heat stress program should be documented. Records typically include the written plan, heat stress assessments and WBGT or humidex measurements, daily weather checks and the actions taken, acclimatization schedules for new workers, training records, and records of heat illness incidents and how they were investigated. Heat illnesses that meet recording criteria, such as those needing medical treatment beyond first aid or causing days away from work, must be recorded and reported under the injury reporting rules that apply. Good records also show inspectors, and the employer's own management, that the program is working.