Heat safety fundamentals
Heat stress occurs when the body takes on more heat than it can comfortably release. That heat can come from the environment, from physical activity, from clothing or equipment, or from a combination of all three.
Heat stress matters because the human body has to keep core temperature within a narrow range. When cooling becomes difficult, the body works harder to maintain balance. Heart rate rises, sweating increases, fatigue can build, and performance can decline. If heat strain continues, heat-related illness can develop.
Wet Bulb Globe Temperature, or WBGT, is one way to assess the environment that contributes to heat stress. But WBGT is only part of the picture. The body’s response depends on the person, the activity, and the conditions.
Key takeaways
- Heat stress describes the heat load on the body, while heat strain describes the body’s response.
- Humidity, radiant heat, sun, wind, workload, and clothing all affect whether cooling is effective.
- Acclimatization, fitness, medications, illness, and access to recovery can change personal heat risk at the same WBGT level.
- Prevention works best when planning, workload changes, hydration, recovery, and symptom recognition happen before serious illness develops.
Heat stress and heat strain
Heat stress and heat strain are related, but they are not the same thing.
Heat stress is the heat load placed on the body. It includes environmental heat, metabolic heat from work or exercise, and the effect of clothing or equipment.
Heat strain is the body’s response to that heat load. It can include increased heart rate, heavy sweating, fatigue, dizziness, reduced coordination, confusion, or a rising core temperature.
This distinction is important because two people can experience the same WBGT conditions and respond differently. A heat-acclimatized outdoor worker doing light activity may experience less strain than a new worker doing heavy labor. A trained athlete may respond differently than a child at camp, an older adult, or someone taking medication that affects fluid balance or sweating.
How the body gains heat
Environmental heat
Hot air, warm surfaces, direct sun, and radiant heat from pavement, buildings, roofs, machinery, or athletic fields can all increase heat load. A person standing on artificial turf or asphalt may experience more radiant heat than someone standing in grass or shade, even if the air temperature is the same.
Metabolic heat
The body produces heat during physical activity. Walking, lifting, digging, sprinting, climbing, carrying equipment, marching, and repeated drills all generate internal heat. The harder the activity, the more heat the body produces.
This is why heat stress is not only a weather problem. A moderate day can become high-risk during intense exertion, especially if humidity is high, wind is low, or recovery time is limited.
Clothing and equipment
Clothing and equipment can trap heat or reduce sweat evaporation. Uniforms, pads, helmets, turnout gear, chemical-resistant suits, rain gear, body armor, and other protective equipment can all increase heat burden.
Protective equipment may be necessary for safety, but it changes how heat stress should be interpreted.
How the body loses heat
Evaporation
Evaporation is the cooling that occurs when sweat turns into vapor. It is one of the body’s most important cooling mechanisms during hot conditions.
Humidity makes evaporation harder. When the air already holds a lot of moisture, sweat does not evaporate as efficiently. Sweat may drip off the skin without providing the same cooling benefit.
Convection
Convection is heat transfer from the skin to moving air or water. Wind can help cooling when it moves heat and moisture away from the body. But wind may help less when clothing blocks airflow or when the surrounding air is extremely hot.
Radiation
Radiation is heat exchange between the body and surrounding surfaces. The body can gain radiant heat from the sun, pavement, rooftops, machinery, vehicles, buildings, or other hot surfaces.
This is one reason direct sun can feel much more stressful than shade at the same air temperature.
Conduction
Conduction is heat transfer through direct contact. It is usually less important than evaporation, convection, and radiation for outdoor heat exposure, but it can matter when people contact hot or cool surfaces.
Why humidity, sun, wind, and radiant heat matter
Air temperature alone does not explain heat stress.
A humid, cloudy day can be stressful because sweat evaporation is limited. A sunny, dry day can be stressful because radiant heat is high. A calm day can reduce airflow and make cooling harder. A windy day may help cooling in some situations but may not offset heavy exertion, high radiant heat, or restrictive clothing.
WBGT is useful because it accounts for several of these environmental factors together. It is especially relevant for outdoor work, sports, events, and other activities where exertion and exposure matter.
Acclimatization and re-acclimatization
Heat acclimatization is the body’s gradual adjustment to repeated heat exposure. With repeated exposure over time, many people sweat more efficiently, maintain a lower heart rate during the same task, and tolerate heat stress better than they did at the start of a hot period.
Acclimatization does not happen immediately. Many occupational and sports guidance documents describe meaningful heat adaptation as developing over roughly one to two weeks, although timing varies by person, activity, and conditions.
People may need extra caution when they are new to outdoor work or training, returning after vacation, returning after illness, starting a more intense workload, coming back after time in cooler weather, or beginning a heat wave early in the season.
Re-acclimatization matters. A person who handled heat well earlier in the summer may not have the same tolerance after time away from hot conditions.
Individual risk factors
The same heat environment can affect people differently. Personal risk factors can include:
- low recent exposure to heat,
- poor sleep,
- recent illness, fever, vomiting, or diarrhea,
- dehydration or limited access to fluids,
- alcohol use,
- intense or unfamiliar activity,
- low fitness for the task,
- pregnancy,
- older age,
- young age,
- chronic health conditions,
- certain medications,
- limited access to cooling,
- heavy clothing, uniforms, or PPE.
Some medications can affect heat risk by changing sweating, thirst, fluid balance, electrolyte balance, blood pressure, or alertness. People should not stop or change medication on their own because of heat. Medication questions should be discussed with a clinician, pharmacist, or other qualified health professional.
Early warning signs deserve fast action
Heat-related illness does not always progress neatly. Fatigue, dizziness, confusion, loss of coordination, cramps, vomiting, or behavior that seems out of character should be treated as warning signs, especially when exertion, protective equipment, and limited cooling are part of the situation.
Heat-related illness: symptoms and response
Heat-related illnesses do not always follow a neat sequence. A person does not have to experience heat rash or cramps before developing more serious illness. Symptoms can also vary by person.
Heat rash
Heat rash can develop when sweat ducts become blocked. It often appears as red or irritated skin in hot, sweaty conditions.
Heat cramps
Heat cramps may occur during or after exertion in the heat. They are painful muscle cramps that can be associated with heavy sweating and fluid or electrolyte loss.
Heat syncope
Heat syncope refers to fainting or near-fainting related to heat, standing, or circulatory strain. It can occur when blood pools in the extremities and the body struggles to maintain blood pressure.
Heat exhaustion
Heat exhaustion can include heavy sweating, weakness, dizziness, nausea, headache, fast heart rate, reduced exercise tolerance, and a feeling of being overwhelmed by the heat. It can progress if the person is not cooled and supported.
Heat stroke
Heat stroke is a medical emergency. It can include altered mental status, confusion, collapse, seizure, loss of consciousness, or a dangerously high core temperature. Immediate emergency response and rapid cooling are critical.
Any emergency plan should define who calls emergency services, who starts cooling, what cooling resources are available, and how the person is monitored while help is on the way.
Preventing heat stress
Prevention is better than relying on last-minute reaction. Many heat-related problems can be reduced by planning before the highest-risk conditions arrive.
Planning and scheduling
Use weather information, WBGT, heat index, or other relevant tools to identify the highest-risk periods. Schedule strenuous work, practices, or events for cooler parts of the day when possible. Build in flexibility when forecasts show rising heat stress.
Hydration
Access to fluids matters, but hydration alone does not solve heat stress. People still need rest, shade, cooling, supervision, and workload adjustment when conditions are stressful.
Shade, cooling, and recovery
Provide shaded rest areas, air-conditioned spaces where possible, fans or air movement when appropriate, cooling towels or ice where relevant, and enough recovery time for the workload and conditions.
Monitoring and supervision
Supervisors, coaches, event staff, and workers or participants should know the warning signs of heat-related illness and what action to take. The ability to recognize symptoms early can matter as much as the forecast itself.
Acclimatization procedures
New or returning workers, athletes, and participants often need a graduated increase in heat exposure and workload. The first days of a hot period, preseason, or return-to-work cycle can be higher-risk.
Prevention planning checklist
- Identify the highest-risk time window before strenuous activity begins.
- Match workload, practice structure, or event timing to the expected heat conditions.
- Confirm shade, hydration, cooling, and recovery resources are actually in place.
- Use extra caution for people who are new, returning, ill, under-conditioned, or wearing protective equipment.
- Make sure staff know who watches conditions, who watches people, and who can stop activity.
What WBGT can and cannot tell you
Klimo Insights focuses on practical heat-stress information, including WBGT conditions and forecasts. These tools can help people understand when heat stress is likely to increase and plan around the highest-risk periods.
But WBGT does not replace onsite judgment, policy, medical guidance, supervision, acclimatization planning, or direct observation of symptoms. The same environmental conditions can produce very different outcomes depending on the person and the activity.
FAQ
What is the difference between heat stress and heat strain?
Heat stress is the total heat load placed on the body. Heat strain is the body’s response to that heat load, such as rising heart rate, sweating, fatigue, dizziness, or confusion.
Can someone have heat strain even when the weather does not seem extreme?
Yes. Intense activity, humidity, limited airflow, protective clothing, poor acclimatization, illness, or dehydration can make a moderate day become high-risk.
Does acclimatization eliminate heat risk?
No. Acclimatization can improve tolerance, but it does not eliminate risk. Workload, hydration, symptoms, equipment, health status, and access to recovery still matter.
Can medications affect heat risk?
Yes. Some medications can affect sweating, thirst, fluid balance, electrolytes, blood pressure, or alertness. Medication questions should be discussed with a clinician or pharmacist.
How does WBGT fit into heat-stress prevention?
WBGT helps describe the heat environment more completely than air temperature alone, especially when humidity, radiant heat, and wind matter. It is useful for planning, but it still needs to be interpreted with workload, clothing, acclimatization, and personal risk in mind.
Sources and notes
Show sources and notes
- OSHA Technical Manual, Section III, Chapter 4 defines heat stress as net heat load and heat strain as the physiological response, and describes environmental factors, workload, clothing, and acclimatization as important contributors.
- CDC/NIOSH heat-related illness guidance describes heat-related illnesses, including heat stroke, heat exhaustion, heat cramps, and heat rash, and provides first-aid guidance.
- CDC clinical guidance on heat and medications notes that some medications can increase heat risk through effects on thermoregulation and fluid balance.
- Korey Stringer Institute guidance on heat acclimatization describes heat acclimatization as a series of adaptations that typically develop over about 7 to 14 days.
- NATA guidance on exertional heat illnesses supports careful language around exertional heat stroke, emergency action planning, rapid recognition, and cooling.