Heat stress is the total heat load a worker's body carries from hot conditions, physical work and protective clothing, which can lead to heat illnesses ranging from heat rash to fatal heat stroke.
Heat stress is the total heat load placed on a worker's body by the combination of environmental conditions, physical work and clothing. The environmental side includes air temperature, humidity, radiant heat from the sun or hot equipment, and air movement. The work side is the heat the body generates internally from muscular effort. Clothing and PPE, especially chemical suits, firefighting gear and multiple layers, can trap that heat and prevent sweat from evaporating.
Safety professionals often separate heat stress from heat strain. Heat stress is the load. Heat strain is the body's response to it: higher core temperature, faster heart rate, heavy sweating and fluid loss. When the body can no longer shed heat as fast as it gains it, heat strain progresses into heat-related illness. At the far end of that spectrum is heat stroke, a medical emergency that can kill or cause permanent organ damage.
Heat stress is not limited to outdoor work in summer. Foundries, bakeries, laundries, kitchens, boiler rooms, warehouses without air conditioning and workers in impermeable PPE can face dangerous heat loads year-round. It also has a less obvious cost: hot, fatigued workers make more mistakes, so heat contributes to injuries such as falls and machinery incidents that are never recorded as heat illness.
Heat illnesses range from uncomfortable to fatal. Recognizing the early signs gives supervisors the chance to intervene before a worker collapses.
Clusters of small red bumps caused by blocked sweat glands, common in hot, humid conditions. Not dangerous itself, but a sign of heavy, sustained sweating.
Painful muscle spasms, usually in the legs, arms or abdomen, caused by loss of salt and fluids through sweat. Workers should rest in a cool place and drink water or an electrolyte drink.
Fainting or dizziness from standing for long periods or getting up suddenly in the heat, caused by blood pooling in the legs and dehydration. The worker should sit or lie down in a cool area and rehydrate.
The body's response to excessive loss of water and salt. Symptoms include heavy sweating, weakness, headache, nausea, dizziness, irritability, thirst and an elevated body temperature. Move the worker to a cool area, remove extra clothing, give small sips of water, and cool them with cold compresses. If symptoms worsen or do not improve within about an hour, get medical help. Heat exhaustion can progress to heat stroke.
The most serious heat illness. The body's temperature regulation fails and core temperature can rise above 104°F (40°C). Signs include confusion, slurred speech, loss of consciousness, seizures, and hot skin that may be dry or still sweating. Call 911 immediately. While waiting, cool the worker as fast as possible, ideally by immersing them in cold or ice water, or by soaking clothing with cold water, applying ice and fanning. Stay with the worker. Rapid cooling within the first minutes is the strongest predictor of survival.
A breakdown of muscle tissue linked to heat and prolonged exertion, which releases proteins into the blood that can damage the kidneys. Warning signs include muscle pain, weakness and dark urine.
Early dehydration often underlies all of these conditions, which is why access to water is one of the most basic controls.
The National Weather Service heat index combines air temperature and relative humidity into an "apparent temperature." It is easy to get from a weather app and is widely used to trigger heat plans, but it is calculated for shaded conditions with a light breeze. Full sun can add significantly to the effective heat load.
Wet bulb globe temperature (WBGT) accounts for temperature, humidity, radiant heat and air movement. It is the measure used in the ACGIH threshold limit values for heat stress and NIOSH's recommended limits, which pair WBGT readings with workload and work-rest cycles. WBGT meters are the better tool for indoor hot processes and for high-exposure outdoor work.
For high-risk work, such as hazmat response in fully encapsulating suits, employers may monitor heart rate, body temperature or weight loss across a shift to detect heat strain directly.
Most heat-related deaths at work happen in the first few days on the job, before the body has adapted. NIOSH recommends that new workers start at no more than 20% of the usual duration of heat exposure on the first day, increasing by no more than 20% each day. Workers with previous experience in the job can follow a faster schedule of about 50%, 60%, 80% and 100% over the first four days. Workers returning after a week or more away, and everyone during a sudden heat wave, need to reacclimatize.
Provide cool drinking water close to the work and encourage workers to drink about one cup every 15 to 20 minutes during moderate work in the heat, rather than waiting until they are thirsty. Schedule rest breaks in shaded or cooled areas, with longer and more frequent breaks as heat and workload increase. Electrolyte drinks help during long, heavy sweating shifts.
Engineering options include ventilation, air conditioning, local cooling fans, reflective shielding around hot equipment and insulating hot surfaces. Administrative options include scheduling heavy work for cooler hours, rotating workers, using a buddy system, adding extra workers to reduce individual workload and stopping non-essential work at high heat triggers. Cooling vests and other PPE can help where other controls are not enough.
Workers and supervisors should know the signs of heat illness, first aid steps and how to call for help. A written heat illness prevention plan sets out the triggers, controls, acclimatization schedule and emergency response procedures.
As of fall 2026, federal OSHA does not have a final heat standard. OSHA proposed a heat injury and illness prevention rule in August 2024 and has signaled further rulemaking, but it continues to cite heat hazards under the General Duty Clause and to target them through a national emphasis program. Several states, including California, Washington, Oregon, Maryland and Nevada, have their own heat rules. Check osha.gov and your state plan for current requirements.
A roofing contractor started a new commercial project in early June with a crew that included three workers hired that week. On the second day, the heat index reached 101°F by early afternoon. One of the new hires, working on the reflective membrane roof, became confused and stopped responding clearly to his foreman. The foreman recognized possible heat stroke, called 911, moved him into the shade and soaked his clothing with cold water from the water cooler while a coworker fanned him. He was hospitalized and recovered.
The contractor's review found that new hires had been put on a full schedule from day one with no acclimatization plan, water was kept at ground level and only reached during the lunch break, and the crew had no heat triggers for extra rest. The company adopted a written heat plan with a 20% acclimatization schedule for new workers, rooftop water and shade canopies, mandatory rest breaks above set heat index levels and a buddy system. It also started its workday at 6 a.m. during summer months.
Heat stress is the heat load on the body from the environment, work and clothing. Heat stroke is the most severe illness that can result when that load overwhelms the body's cooling system. Heat stress is the cause; heat stroke is one possible outcome.
There is no single federal cutoff. Risk depends on humidity, sun, workload, clothing and acclimatization, not just temperature. Many heat plans use heat index or WBGT triggers to increase rest and water breaks and to stop strenuous, non-essential work at extreme levels. State rules may set specific triggers.
Yes. Workers near furnaces, ovens, dryers, steam and hot processes, or in poorly ventilated warehouses, can face heat loads as high as outdoor work in summer. WBGT is the better measure for these environments.
Workers who are new or returning, not acclimatized, overweight, older, dehydrated, recently ill, or taking medications that affect sweating or heart rate face higher risk. Anyone wearing heavy or impermeable PPE is also at higher risk.
Part of SafetyIQ's EHS glossary: plain-English definitions of workplace health and safety terms.
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