There is no single federal temperature that automatically makes working conditions “unsafe” in the United States. OSHA has no specific number on the books that says, for example, work must stop at 105°F. Instead, the agency uses a sliding scale tied to heat index, workload intensity, and how accustomed workers are to the heat. In practice, though, concrete trigger points do exist, and several states have written hard temperature thresholds into law.
OSHA’s Heat Index Triggers
OSHA’s proposed regulatory framework establishes two tiers of heat protection. The first, called the “initial heat trigger,” kicks in at a heat index of 80°F when measured on-site (or 78°F when relying on a weather forecast). At this level, employers would need to provide drinking water, allow rest breaks as needed to prevent overheating, and maintain regular communication with workers by voice, observation, or electronic means.
The second tier, the “high-heat trigger,” activates at a heat index of 90°F on-site (86°F by forecast). This level requires more aggressive protections: mandatory 15-minute rest breaks at least every two hours, shaded or cooled rest areas, and direct observation of workers for signs of heat illness. One option under consideration is a buddy system; another requires one supervisor for every 20 workers specifically watching for symptoms.
These triggers use the heat index, not raw air temperature. A 92°F day with 60% humidity can produce a heat index well above 100°F, which is why air temperature alone doesn’t tell you whether conditions are safe.
How Workload Changes the Threshold
A desk job and heavy manual labor have very different danger zones. NIOSH guidelines, adapted by OSHA, set exposure limits using a measurement called the Wet Bulb Globe Temperature (WBGT), which factors in temperature, humidity, wind, and sun exposure. For workers already accustomed to the heat, the safe ceiling for light work is 86°F WBGT. For moderate work, it drops to 82.4°F. For heavy labor like shoveling, climbing, or carrying 40-plus pounds, it falls to 78.8°F. For very heavy exertion, the limit is just 77°F.
Workers who aren’t acclimatized, meaning they haven’t gradually built up tolerance over one to two weeks, face even lower thresholds. An unacclimatized worker doing heavy labor hits the danger zone at a WBGT of only 73.4°F.
Work-Rest Schedules at High Temperatures
When temperatures climb into the mid-90s and above, NIOSH recommends specific work-to-rest ratios for heavy labor. At 95°F, a worker in normal clothing can work 45 minutes and rest 15. By 100°F, it’s an even 30 minutes on, 30 minutes off. At 103°F, workers should only be active for 20 minutes per hour, resting for the remaining 40. At 105°F, the ratio flips almost entirely: 15 minutes of work, 45 minutes of rest. Above 105°F, the schedule simply reads “Caution,” meaning conditions may be too dangerous for heavy physical work at all.
These figures assume standard clothing. Direct sunlight and humidity require adjustments. NIOSH gives the example of a 90°F day with partly cloudy skies and 50% humidity: after adding 7 degrees for sun exposure and 6 degrees for humidity, the effective temperature becomes 103°F, cutting allowable work time to just 20 minutes per hour.
California and State-Level Rules
While federal OSHA relies on its General Duty Clause (which requires employers to keep workplaces “free from recognized hazards” likely to cause death or serious harm), some states have written explicit temperature laws. California’s heat illness prevention standard is the most specific. It requires employers to provide shade structures whenever outdoor temperatures exceed 80°F. Below 80°F, shade must still be available on request, but above it, shade areas must be set up proactively and maintained the entire time workers are present. These shaded areas must be open to air or have ventilation or cooling.
Other states including Washington, Oregon, and Colorado have adopted their own heat-specific rules with varying trigger temperatures. If you work in a state with its own occupational safety plan, your protections may be stricter than the federal baseline.
Cold Weather Thresholds
Unsafe working conditions aren’t limited to heat. OSHA recognizes cold stress as a serious workplace hazard, though it hasn’t set a specific temperature cutoff. Instead, the agency relies on the National Weather Service’s wind chill system. A Wind Chill Advisory means prolonged exposure could become hazardous. A Wind Chill Warning means conditions are life-threatening within minutes of exposure.
OSHA requires employers to assess wind chill temperatures to gauge worker risk and to train employees on selecting proper clothing for cold, wet, and windy conditions. Blizzard warnings, issued when sustained winds hit 35 mph or more with visibility at or below a quarter mile for at least three hours, also signal conditions where outdoor work becomes extremely dangerous.
Indoor Workplaces
Indoor workers are not exempt from heat hazards. Warehouses, kitchens, laundries, and manufacturing floors can easily exceed outdoor temperatures. OSHA’s proposed heat triggers apply to both indoor and outdoor settings. The engineering standard used for climate-controlled offices, ASHRAE Standard 55, defines thermal comfort as a relatively narrow band, generally between about 68°F and 76°F depending on clothing and activity level. It also sets an upper humidity limit at a dew point of roughly 62°F. These are comfort guidelines rather than safety laws, but an office that consistently falls well outside this range may violate the General Duty Clause if workers are getting sick.
Hydration Requirements
Across all heat thresholds, OSHA recommends workers drink one cup (8 ounces) of water every 15 to 20 minutes in hot conditions, even before they feel thirsty. That works out to about 32 ounces per hour. There is an upper limit: no more than 48 ounces (1.5 quarts) per hour. Drinking more than that can dangerously dilute sodium levels in the blood. Employers are expected to provide cool water (below 60°F) in accessible, visible locations throughout the work area.
Recognizing Heat Exhaustion vs. Heat Stroke
Knowing the temperature thresholds matters less if you can’t recognize when the heat is already affecting someone. Heat exhaustion produces a body temperature between 101°F and 104°F, pale skin, muscle cramps, headache, dizziness, rapid breathing, and nausea. The person is still sweating and mentally coherent.
Heat stroke is a medical emergency. Body temperature rises above 104°F, the skin turns red and dry because the body has lost the ability to sweat, and neurological symptoms appear: confusion, slurred speech, aggression, hallucinations, or seizures. The transition from exhaustion to stroke can happen quickly, which is why OSHA’s high-heat protocols emphasize direct observation of workers rather than relying on individuals to report their own symptoms.
What the General Duty Clause Means for You
Even without a single magic number, the legal framework is clear. Under Section 5(a)(1) of the Occupational Safety and Health Act, your employer has a legal obligation to provide a workplace free from conditions that are recognized as hazardous and likely to cause death or serious physical harm, as long as a feasible way to reduce the hazard exists. Courts have consistently interpreted this to include heat-related hazards. If your employer knows temperatures are dangerous, has no cooling measures, no water, no rest breaks, and no monitoring in place, they are violating federal law regardless of whether the thermometer reads 95°F or 105°F.