According to the EPA, approximately 11,000 heat-related deaths occurred in the United States between 1979 and 2018. The actual number may be even higher, as many cases go unreported or are not recognized as heat-related.
How Does Heat Affect the Body?
Extreme heat can have significant effects on the human body's organs and can lead to death. Heat exhaustion and heat stroke are serious conditions that can result from heat exposure and can be fatal. Heat can also exacerbate pre-existing conditions or cardiovascular disease, including stroke and heart attack.
The primary reason heat causes damage to the human body is dehydration. The human body's water balance mechanism is temperature-dependent and interacts with the air surrounding the body through the skin and breathing.
In hot, especially dry, conditions, water evaporates from the skin and through breathing faster than the body can replenish it. The evaporation of sweat cools the body, but excessive sweating can lead to dehydration.
According to the U.S. Centers for Disease Control and Prevention (CDC), extreme heat has been the leading weather-related cause of death in the United States over the past 13 years, surpassing hurricanes and tornadoes.
Hydration Is Key to Regulating Body Temperature
Proper hydration is critical for the functioning of internal organs as well as the cells in the body. Dehydration reduces blood volume and circulation, causing organs to malfunction or fail and potentially leading to cell death. A lack of blood circulation also restricts the delivery of vital oxygen to organs, which is why there is an increased incidence of cardiovascular events during hot weather.
Blood circulation is also influenced by core body temperature. Impaired circulation is similar to running a car out of gas. Circulation is one of the main and only mechanisms to maintain proper core temperature. When a person breathes, they are essentially bringing in outside air that is likely cooler than their body temperature and reducing the temperature of the blood as it passes through the lungs to pick up oxygen, helping regulate body temperature.
What Are the Limits of Human Heat Tolerance?
According to a study published in the American Physiological Society's Journal of Applied Physiology in 2021, the range of temperatures in which the human body can maintain function and homeostasis is between 104 and 122 degrees Fahrenheit (40-50 degrees Celsius). Once the air temperature reaches 122 degrees Fahrenheit, our bodies can no longer dissipate heat effectively, and our core temperature will rise. However, another study cites a much lower limit of approximately 90 degrees Fahrenheit (32 degrees Celsius), the temperature at which humans start to sweat. In addition to air temperature, humidity plays a significant role, as higher humidity makes it harder for the body to cool down.
A more widely accepted measure is that a person can die after spending just six hours in an environment with a "wet-bulb" temperature of 95 degrees Fahrenheit (35 degrees Celsius) (which corresponds to 95 degrees Fahrenheit with 100% humidity, or 115 degrees Fahrenheit with 50% humidity).
Wet-bulb temperature is a measurement that takes into account both temperature and humidity, indicating when the air has become saturated with water vapor, according to the study published in the Journal of Applied Physiology.
Under such conditions, sweating becomes more difficult in environments with both high humidity and temperature, and sweat is responsible for dissipating about 80% of the body's heat. This leads to an inability for the body to cool itself, potentially leading to hyperthermia, which can cause respiratory and cardiovascular problems and even lead to death.
However, heat-related health risks can occur well before wet-bulb temperature thresholds are reached. According to the World Health Organization (WHO), increased temperatures expose the body to higher than normal heat loads, which can affect thermoregulation and result in conditions such as heat exhaustion, heat stroke, and hyperthermia.
Extreme heat can also worsen chronic conditions, including cardiovascular disease, respiratory disease, cerebrovascular disease, and diabetes-related conditions.
High-risk groups who are most susceptible to heat-related illnesses include the elderly, infants and children, pregnant individuals, outdoor and manual laborers, athletes, and many others.
The Limits of Internal Body Temperature Are Even More Narrow
According to MedlinePlus, brain damage can occur when the core body temperature reaches 107.6 degrees Fahrenheit (42 degrees Celsius).
A 2022 Outside Magazine article about heatstroke cites the highest temperature a person has been known to survive: "The highest body temperature that internal organs can survive is only about 15 degrees Fahrenheit warmer than normal. Willie Jones, a 52-year-old Atlanta man, was rescued from his apartment during a 1980 heat wave with a core body temperature of 115.7 degrees Fahrenheit."
According to the same article, the lowest temperature that internal human organs can survive is 56.7 degrees Fahrenheit (13.7 degrees Celsius). Cold temperatures slow down circulation, which can cause severe damage in the form of frostbite when it is cold enough. Without blood to warm the tissues, they will freeze, and the ice crystals that form can destroy cells and tissue.
These are sobering numbers because they demonstrate how temperature impacts the rate of evaporation. As core body temperature gets colder, it causes a different set of problems but does not lead to dehydration. In fact, medical researchers are exploring a new technology called emergency preservation and resuscitation, in which patients are intravenously cooled down to temperatures as low as 50 degrees Fahrenheit (10 degrees Celsius) to slow down blood loss while they undergo emergency surgery for traumatic injuries.
How to Manage Heat Exposure
Humans have very limited physiological adaptations to cope with high temperatures beyond staying hydrated. When we sweat, we lose a lot of salts and minerals. If we only drink more water but do not replace the minerals we have lost, we can actually end up causing further complications. According to the CDC, ways to mitigate the effects of high heat include cooling down the home environment, such as taking cool showers, eating cold foods, and dressing in light and airy clothing.
It is important to note that electric fans do not prevent heat-related illnesses. In fact, they can lead to increased dehydration under the same temperature conditions because of evaporation. The wind carries water vapor away at a faster rate, since the water droplets themselves are essentially blown away by the force of the wind.
As the climate heats up, humans will need to adapt to survive. The year 2023 was the world's hottest year on record, and residents of Phoenix, Arizona, experienced 31 consecutive days of temperatures at or above 110 degrees Fahrenheit (43 degrees Celsius). As climate change continues to worsen, particularly without aggressive action by governments around the world, large numbers of people will become climate refugees, and places like Arizona could become uninhabitable to humans.
The human body's simple physiological responses to heat and evaporation are having a difficult time keeping pace with a rapidly warming environment. Given that the biology of most life on Earth relies on maintaining water balance, the effects of extreme heat could lead to the next mass extinction event on the planet. It is imperative to accelerate efforts to mitigate and reverse climate change.
