Occupational heat stress is the net load to which a worker is exposed from the combined contributions of metabolic heat, environmental factors, and clothing worn, which results in an increase in heat storage in the body. Heat stress can result in heat-related illnesses, such as heat stroke, hyperthermia, heat exhaustion, heat cramps, heat rashes, and chronic kidney disease (CKD). Although heat exhaustion is less severe, heat stroke is a medical emergency and requires emergency treatment, which if not provided, can lead to death.
Heat stress causes illness but also may account for an increase in workplace accidents, and a decrease in worker productivity. Worker injuries attributable to heat include those caused by: sweaty palms, fogged-up safety glasses, and dizziness. Burns may also occur as a result of accidental contact with hot surfaces or steam. In the United States, occupational heat stress is becoming more significant as the average temperatures increase but remains overlooked. There are few studies and regulations regarding heat exposure of workers.
Risk factors Heat-related illnesses from occupational heat stress is impacted by multiple factors. Workers exposed to high temperatures, humidity, and limited air movement, especially outside workers, are vulnerable to heat illness. Physiologic factors can also impact a worker's vulnerability, specifically if their job requires physical exertion, which produces metabolic heat. Workers can also be more vulnerable to heat illness if they are dehydrated from sweating and not drinking enough water or have a low level of physical fitness. Certain medications can also make it more difficult for someone to adapt to high temperatures, such as some common antibiotics, as well as some diabetes, cardiovascular disease, and psychiatric medications. Workers who have cardiovascular diseases, respiratory illnesses, diabetes, hypertension, or obesity, or who are currently pregnant, are also at higher risk for heat illness. Additionally, wearing heavy or thick personal protective equipment and clothing can prevent workers from sweating properly, which prevents the body from effectively cooling. A recent umbrella review suggests that female and relocated workers face greater physiological strain due to occupational heat exposure.
A study estimates that climate change is to blame for an additional 25 billion working hours lost annually in India from 2001 until 2020 compared to the previous 20 years. Due to their physical demands and exposure to the elements, agricultural workers have among of the highest rates of heat-related disease. According to studies, low-income workers are disproportionately affected by heat stress because they are less able to take breaks or leave hazardous work environments.
Acclimatization According to the National Institute for Occupational Safety and Health (NIOSH), acclimatization is a biological process that an individual will go through to adjust to a stimulus following continued exposure. Physiologically, acclimatization to heat will allow a worker's body to more efficiently cool itself when exposed to high temperatures. When a worker has adjusted to working in a hotter environment, they will have a lower heart rate, earlier onset of sweating, and increased blood flow to blood vessels near the skin, allowing their body to more efficiently cool itself than a worker who is not acclimatized. (see below section on acclimatization schedules for specific processes).
Examples of high risk occupations Workers in many occupations are at high risk for exposure to heat stress. Especially vulnerable are outdoor workers who have highly physical tasks to complete, such as firefighters, miners, military personnel, construction workers, landscapers, athletes, delivery persons, and agricultural workers. Additionally, many indoor jobs also require high-exertion work in very hot conditions, for example factory workers, boiler room workers, welders, and kitchen staff. In unusually hot conditions, all workers should be aware of their risk for heat illness and should ensure that they drink plenty of water and take breaks in cool places to avoid any severe impacts.
Vulnerable populations Certain populations face increased risk of occupational heat stress due to a combination of environmental exposure, occupational conditions, and socioeconomic factors. While physiological vulnerability is addressed elsewhere in this article, structural and social determinants also play a significant role in determining who is most affected.
Outdoor and physically demanding work Outdoor workers, including agricultural and construction workers, face disproportionate occupational heat stress risk due to the combination of prolonged sun exposure, physically demanding labor, and structural barriers that limit access to protective resources. The International Labour Organization (ILO) estimates that agricultural work accounts for approximately 60% of productive hours lost globally to heat stress, with construction contributing an additional 19%.
Low-wage and seasonal workers Workers in low-wage or seasonal employment may face additional risk due to limited access to protective resources such as protective clothing, shade, hydration stations, and scheduled rest breaks. In some occupational settings, piece-rate payment structures, in which workers are compensated per unit of output rather than by the hour, may discourage workers from taking breaks, increasing cumulative heat exposure during shifts.
Socioeconomic and structural factors Socioeconomic conditions can contribute to disparities in heat-related illness by affecting exposure both during and outside of working hours. Workers with lower incomes may have limited access to air-conditioned housing or transportation, increasing total heat burden across the day. In the United States, immigrant and migrant workers are disproportionately represented in outdoor, low-wage, and agricultural occupations, and may face additional barriers including geographic isolation, substandard housing, limited access to occupational safety training, and language barriers that affect comprehension of heat illness prevention information. Research indicates that the disproportionate burden of occupational heat stress is concentrated in regions with high rates of informal employment and limited social protections, including parts of West Africa, Southern Asia, and Latin America.
Barriers to care and underreporting Access barriers may limit both prevention and treatment of heat-related illness among vulnerable workers. These include limited access to healthcare services, lack of health insurance, and concerns about job security that may discourage workers from reporting symptoms or seeking care. Underreporting of heat-related illness and injury has been documented in occupational settings, suggesting that surveillance data may underestimate the true burden of disease among affected worker populations.
Climate change and the One Health perspective Climate change has been associated with increases in the frequency, duration, and intensity of heat waves, contributing to a growing risk of heat-related illness among occupational populations globally. The WHO has identified outdoor and manual workers as among the groups most susceptible to exertional heat stress as temperatures continue to rise. The ILO projects that by 2030, the equivalent of 80 million full-time jobs could be lost worldwide as a result of heat-related reductions in productivity.
From a One Health perspective, occupational heat stress reflects the intersection of human health, animal health, and environmental conditions. Rising temperatures affect not only workers directly but also the agricultural ecosystems and food-producing systems on which many of the most heat-exposed workers depend. Disruptions to these systems, including crop failure, soil degradation, and heat-related illness in livestock, can compound occupational risk for agricultural communities in affected regions. Addressing occupational heat stress within this framework requires integrated approaches that consider environmental conditions, labor policy, housing, and health equity alongside workplace-level interventions.
Symptoms of heat stress The Signs and Symptoms of Heat Illness is intended to acquaint participants with OSHA's guidelines for heat-related injuries on the job. When someone is exposed to hot conditions, they will begin to sweat and have an increased heart rate. If they do not drink enough water to replenish the fluid they are sweating out, a worker can become dehydrated. This is why drinking extra water while working in hot environments is critical. If workers continually exert themselves in hot environments, they may develop muscle cramps from sweating too much, which can be relieved by taking a break and drinking extra water and electrolytes. Additionally, workers can develop heat rash, which should be treated in a cool place and soothed with powder. For more severe heat illnesses, it is important for workers to be able to identify the differences between heat exhaustion and heat stroke in order to protect themselves and their colleagues.
Heat exhaustion When a person is experiencing heat exhaustion, they will be actively and heavily sweating, though their skin may feel cool and appear pale. If their temperature is taken, it could be normal or slightly high, and their pulse will be fast, but it may be difficult to detect or feel through the skin. Additionally, the person may exhibit a number of different symptoms such as headaches, muscle cramps, and fatigue, as well as nausea, vomiting, and in extreme cases, fainting. Compared to healthy workers, workers who are older, pregnant, or have previous diseases like diabetes or heart disease are more likely to experience heat exhaustion more quickly. When assigning duties in heated environments, employers should be mindful of these risk concerns.
If a worker is exhibiting these symptoms, it is important that they take a break from work to sit in a cool place, use cool, damp towels on their head and neck to further cooling, and sip water slowly as they recover. If a worker does not recover within an hour or continues to vomit, it is important to seek medical attention for them quickly.
Heat stroke If a worker begins to feel the signs of heat stroke from over exertion in a high temperature environment, they will normally have a very high temperature, which can be measured with a thermometer. Their skin could be hot to the touch and damp with sweat, though they will not be actively sweating. If their pulse is taken, it will be fast and easy to detect. Outwardly, the worker could show signs of dizziness and confusion, and they may have digestive discomfort and nausea. In severe cases, workers can pass out. The most serious heat-related sickness is heat stroke, which, if left untreated, can cause irreversible organ damage or even death. If heat stroke is suspected, workers should never be left alone; instead, emergency services should be contacted immediately.
If a worker is displaying these symptoms, it is important to call 911 to get them immediate medical attention. Additionally, helping the person move to a cooler place and covering their head and neck in cool, damp cloths can help while waiting for emergency services to arrive. It is important to wait for a medical professional to see them before asking them to drink a lot of water.
Measurement and monitoring
Measurements