Heat is the number one environmental cause of death in the US – greater than cold, storms, hurricanes, floods, or tornadoes. Worldwide there are about 5 million deaths per year related to extreme heat or cold, four and a half million from cold and half a million from heat. This ratio is slowly shifting, however, due to global climate change. There are fewer cold related deaths, but more than offset by greater numbers of heat related deaths in recent years. We can expect this trend to continue for at least the next 2-3 decades, depending on how successful our efforts are to mitigate climate change.
Most heat-related deaths occur during weather events known as heat waves. In 2022 Europe experienced severe summer heat waves, and a recent study estimates this resulted in almost 62,000 heat-related deaths. Heat waves are getting more frequent, more intense, lasting longer, and with a longer heat wave season. In addition, rising global temperatures are also increasing humidity due to increased evaporation and because warmer air can hold more water vapor. This has significant health effect because the heat index, the effective temperature felt by the body, depends on both the temperature and the humidity.
Humidity is critical because one of the ways the body sheds excess heat is through sweating. When sweat evaporates on the skin it carries away some heat. But as humidity increases, this method of heat loss becomes less and less effective, until it does not work at all. Heat is also lost through radiation, convection and conduction. Conduction of heat is through direct contact with cooler objects. Convection works through air or water moving across the skin, carrying away heat. Radiation carries heat away through infrared rays, without needing direct contact with anything physical. We lose about 60% of our heat through radiation, but this is not something that is easily modifiable.
The body primarily uses evaporation to regulate temperature because that is the one variable it can control through sweating. Blood flow regulation can also bring more blood and heat to the skin to allow for greater convection. However, when air temperatures rise convection becomes less efficient, just as when humidity rises, evaporation becomes less efficient.
The National Weather Service heat index chart shows the impact of humidity on how dangerous high temperatures are. For example, at 40% humidity, you get into the danger zone at 98 degrees F, but at 80% humidity the danger zone begins at only 88 degrees. At 94 degrees you get into extreme danger, where heat stroke is “highly likely”. The highest heat index (effective temperature) this year reached 153 degrees in the Middle East, which is at the limit of human survivability.
What is heat stroke? Heat stroke occurs when the body is no longer able to shed as much heat as it is creating, due to decreased effectiveness of evaporation and convection. When this point is reach, heat accumulates in the body causing body temperature to rise rapidly. Within 10 minutes of reaching this point, body temperature can increase to 106 degrees F. This high temperature causes confusion, can lead to loss of consciousness, and can cause seizures. Heat exhaustion can also occur, which results when the body is depleted of water and electrolytes due to excess sweating. This, in turn, can lead to syncope (fainting) and muscle cramps.
High body temperatures caused by heat stroke can damage most organs, the brain, heart, kidneys, and muscles especially. Death due to heat stroke most often occurs because of cardiac effects. Children, elderly, and those with underlying heart conditions are most susceptible to death due to heat stroke. Repleting water and electrolytes can be critical, but the most important treatment is to reduce body temperature. This requires getting into a cooler environment immediately.
While global climate change is global, it refers to average temperatures. Not everywhere is affected the same. Heat related deaths, for example, are concentrated in Europe, southern Asia, and sub-saharan Africa. But as global temperatures rise, the number of locations around the world susceptible to dangerous heat waves is increasing, as is the number, duration, and intensity of those heat waves. This means the number of people living with the threat of dangerous heat waves is increasing. Some parts of the world may simply become uninhabitable during certain times of the year.
The primary mitigating technology for extreme heat is air conditioning. However, this uses a lot of energy, which ironically can worsen global warming due to increased carbon-intensive energy demand. Also, heat waves can lead to spikes in demand for energy for air conditioning which can stress existing systems. A power outage during a heat wave can be deadly as millions of people may suddenly lose access to air conditioning.
What all of this means is that heat related deaths are a large and growing public health concern. Researchers have found that already about a third of heat related deaths are attributable to climate change (would not have happened without climate change). This proportion is going to rise significantly.
Another study attempted to project heat deaths in the US as a function of climate change. They found that climate change mitigation can have a significant effect. With aggressive mitigation, essentially the best-case scenario, there would be no significant change in heat-related deaths in the US. With the worst case scenario high levels of warming, annual deaths from heat can rise by 97,000. With an average projected warming, deaths would still rise by 36,000. This is for the US alone, with numbers much higher globally. Where we fall in that range is entirely up to the policy decisions we collectively make regarding climate change.