How Hot Is Too Hot for Humans? The Health Risks of Extreme Heat
Scientists have long studied the upper temperature limit of human survival. One important measure is the wet-bulb temperature, which combines air temperature and humidity to show how effectively the human body can cool itself through sweating. Research has estimated that a wet-bulb temperature of 95 degrees Fahrenheit (35 degrees Celsius) at 100% humidity represents an extreme, potentially non-viable limit for healthy humans.
At and above this level, sweat can no longer evaporate efficiently. As a result, the body’s core temperature continues to rise, placing increasing stress on the brain, heart, kidneys and other organs. Without rapid cooling, severe heat illness, organ failure and death can occur.
However, people do not need to experience the theoretical 35-degree Celsius wet-bulb threshold to face dangerous conditions. Older adults, children, pregnant people, outdoor workers and those with certain medical conditions may become seriously ill at lower temperatures. Physical activity, direct sunlight, dehydration and limited access to air conditioning can further increase the risk.
As global temperatures rise, more regions are experiencing dangerous combinations of heat and humidity. Climate research suggests that some areas could approach critical wet-bulb temperatures later this century if greenhouse-gas emissions remain high. Some regions may experience wet-bulb temperatures approaching 35°C by mid-century.
A recent study also found that conditions can become dangerous well below the theoretical human survival limit. In some circumstances, healthy adults sitting in the shade with a light breeze may still be unable to safely regulate their body temperature. The phrase “I can’t live there” may become a realistic description of increasingly hot and humid environments.
NASA temperature data showed a heat dome across parts of the western United States during a July 2026 heat wave. Heat waves are expected to become more frequent and intense as the climate warms.
(Image credit: NASA Earth Observatory/Michala Garrison)
In North America, parts of the Southwest and Southeast are particularly vulnerable to dangerous heat. Future warming is expected to increase the number of heat-related illnesses and deaths. One analysis estimates that tens of thousands of additional heat-related deaths could occur in the United States by 2100.
A separate study projects that nearly 20,000 people in the United States could die from extreme heat each year by the middle of this century under certain warming scenarios. These estimates vary depending on future emissions, population growth, adaptation and access to cooling. Research published in JAMA Network Open highlights the growing public-health burden of extreme heat.
Heat is more than an uncomfortable weather condition. It can worsen cardiovascular disease, kidney problems, respiratory illness and diabetes. Heat waves may also occur alongside drought, wildfires and water shortages, creating multiple pressures on communities. Marina Romanello, a senior research fellow at University College London’s Institute for Global Health, describes climate change as a stress multiplier because its effects can intensify existing health and social problems.
The health effects of extreme heat are not always immediately visible. Hot nights can disrupt sleep, while prolonged heat can affect concentration, mood and decision-making. “There are no individual indicators that can accurately assess the overall risk to health and well-being” when several stressors occur at once, Romanello told Live Science.
How extreme heat causes illness and death
Heatstroke occurs when the body’s core temperature rises rapidly—often above 104°F (40°C)—and the body can no longer cool itself. It is a medical emergency that can cause confusion, seizures, loss of consciousness, organ damage and death. Anyone showing signs of heatstroke should receive emergency medical attention immediately. Move the person to a cool place and begin cooling them while waiting for help.
Official death records may underestimate the total impact of extreme heat because heat is often not listed as the primary cause of death. Instead, it may worsen an underlying illness such as heart disease, kidney disease or diabetes. U.S. death certificates listed heat as a primary or contributing cause in 2,325 deaths in 2023, compared with 1,069 in 1999. After adjusting for population age, research found that heat-related deaths increased substantially during that period.
The risk of heart attack also rises during very hot weather. Research published in Circulation found that the risk of heart attack can more than double on extremely hot days. Stroke risk may also increase on the hottest days. Cardiovascular research suggests that inflammation, dehydration and the strain placed on the heart may all contribute.
Dehydration can also cause hyponatremia, a dangerous drop in blood sodium levels. In severe cases, hyponatremia may lead to confusion, seizures, coma and death. Drinking fluids is important during hot weather, but excessive water intake without replacing electrolytes can also be harmful.
The people most vulnerable to dangerous heat include older adults, infants, children, teenagers, pregnant people and outdoor workers. As people age, the cardiovascular system may become less efficient at moving blood to the skin, where heat can escape. Older adults can therefore have more difficulty regulating body temperature.
Children may also be vulnerable because their bodies produce heat differently from adults and they have a larger surface-area-to-body-mass ratio. They may sweat less efficiently and can become dehydrated quickly. Outdoor workers face additional risks because strenuous activity increases internal heat production and makes it harder to replace lost fluids.
Certain medical conditions and medications can increase the risk of heat illness. Diabetes may interfere with sweating and blood-vessel dilation, while some diuretics, beta-blockers and antidepressants can affect hydration, circulation or the body’s ability to cool itself. People taking prescription medications should ask a healthcare professional how to stay safe during heat waves.
Extreme heat can affect mental health as well. Studies have associated higher temperatures with increased emergency-department visits for mental-health conditions and with higher suicide rates. Researchers believe discomfort, sleep loss, impulsivity and the physiological stress caused by heat may all play a role. Anyone experiencing a mental-health crisis should contact a local emergency service or crisis line.
Kim Meidenbauer, a neuroscientist at Washington State University who studies heat and behavior, has reported that impulsivity may increase when people struggle to stay comfortably cool. The effect may be particularly serious for people living with conditions such as anxiety, depression or schizophrenia.
Sleep is another major pathway through which heat harms health. For every 1°C increase in nighttime temperature, one large sleep-tracking study found that people lost approximately three nights of insufficient sleep per 100 people each month. Poor sleep can impair emotional regulation, balance, concentration and judgment, potentially increasing the risk of accidents and mental-health emergencies.
“When you have poor sleep quality, it’s much more difficult to control your emotions,” Meidenbauer told Live Science.
Heat waves have become more frequent, intense and longer-lasting since the 1960s.
(Image credit: Socio-Economic Data and Applications Center)
How hot weather affects daily health
Extreme heat can affect the brain before it causes obvious signs of heat illness. When temperatures reach approximately 89.6°F (32°C), people may have greater difficulty thinking clearly. A study of more than 50,000 people in China found that heat exposure was associated with lower performance on mathematics and verbal-reasoning tests.
The heat index accounts for both air temperature and relative humidity. As humidity rises, sweat evaporates more slowly, making it harder for the body to release heat. In the United States, the number of days with a heat index above 100°F (37.8°C) is expected to increase, while days above 105°F (40.6°C) may become much more common in some regions.
Although nearly nine out of 10 U.S. households have air conditioning, access to cooling is not universal. High electricity costs, power outages, poorly insulated homes and health conditions that limit mobility can leave people exposed. Even short periods of extreme heat may affect cognitive performance, especially when exposure occurs repeatedly over several days.
Exercise and other physical activity can increase the risk of heat exhaustion and heatstroke.
(Image credit: FG Trade via Getty Images)
Hot weather can also change how people exercise. When outdoor conditions become unsafe, people may stay indoors and become less physically active. Outdoor exercise is associated with physical and mental-health benefits, so long periods of extreme heat may make it harder to maintain healthy routines. Children’s sports and outdoor activities may also need to be shortened, rescheduled or moved indoors during prolonged heat waves.
Some research has identified a U-shaped relationship between temperature and health outcomes, with risks increasing in both very cold and very hot conditions. Prolonged exposure to temperatures above 86°F (30°C) has also been associated with changes in physical activity, weight and metabolic health, although the effects vary between populations and individual studies.
Can humans adapt to extreme heat?
The human body can gradually acclimatize to warmer conditions over several days or weeks. During heat acclimatization, sweating may begin sooner, sweat glands can become more efficient and the body may improve its ability to circulate blood to the skin. People who live in warmer climates may therefore tolerate heat better than those who are suddenly exposed to it.
Acclimatization has limits, however. Extreme temperatures, high humidity and sudden heat waves can overwhelm the body before it has time to adjust. Children are already experiencing increasing heat-related health problems in some regions. A study of two major hospitals in Texas found a 170% increase in pediatric emergency-department visits for heatstroke between 2012 and 2023.
Practical measures can reduce the danger. Cities may open cooling centers, issue heat warnings, improve access to drinking water and check on vulnerable residents. Individuals can also reduce their risk by avoiding strenuous activity during the hottest part of the day, wearing loose and lightweight clothing, drinking fluids regularly and spending time in air-conditioned or shaded locations.
Marshall Burke, a professor of global environmental policy at Stanford University, and his colleagues developed the Environmental Hazards Adaptation Atlas to collect information about heat exposure, heat-related illness, mortality and potential adaptation strategies.
Researchers continue to study which interventions save the most lives. Cooling centers, shaded public spaces, heat-health warning systems, better housing and stronger protections for outdoor workers may all help, but more evidence is needed to determine which approaches work best in different communities.
As Burke has said, “We need good case studies of what works” and enough systematic evidence to guide future heat adaptation.
Key takeaways about extreme heat
- The wet-bulb temperature combines heat and humidity and measures how difficult it is for the body to cool itself.
- Heatstroke is a medical emergency that requires immediate cooling and professional medical help.
- Older adults, children, outdoor workers and people with chronic illnesses face a higher risk of heat-related illness.
- Extreme heat can affect the heart, kidneys, brain, sleep and mental health.
- Human bodies can acclimatize to heat, but acclimatization cannot eliminate the dangers of extreme temperatures and humidity.
- Cooling centers, air conditioning, hydration, shade and reduced physical activity can help protect people during heat waves.
Source: www.livescience.com