When you think of someone dying from extreme temperatures, you probably imagine someone collapsing from heat stroke or dying from hypothermia. But most people don't die directly from "heat" or "cold." Instead, it's usually cardiovascular or kidney disease, respiratory infections, or diabetes that are exacerbated by extreme temperatures and lead to death.

Almost no one has "heat" or "cold" written on their death certificate, but suboptimal temperatures lead to a large number of premature deaths. Older people are particularly vulnerable to extreme temperatures - most deaths occur over the age of 65. It is important to understand what "death" means in this context and how temperature-related deaths compare to other causes. Being too warm or too cold increases the risk of developing certain health problems or worsening existing ones, which can ultimately lead to an earlier death than would be the case at "optimal" temperatures.

How much lifespan do hot or cold conditions steal? This is difficult to quantify precisely. A common method researchers use is to analyze excess mortality rates - that is, how many more people die in a particularly warm or cold year compared to an "average year." This excess mortality provides information about whether temperature-related deaths significantly shorten life or not.

One study examined mortality patterns in the United Kingdom over 50 years and found that most cold-related deaths occurred in people who would not have died in the next six months. A later study examining the effects of high and low temperatures in a larger sample of countries showed that most temperature-related deaths reduced lifespan by at least one year, although there were also some who lost less life time.

The "Sweet Spot": Where temperatures are most comfortable and healthiest

To estimate temperature-related deaths, researchers calculate what is known as the temperature-mortality relationship, which shows how our risk of death varies at different temperatures. Using real death data, scientists calculate "excess mortality" - that is, how many "extra" deaths occur at certain temperatures over an expected baseline. When this increased risk is plotted against temperature, a U-shaped curve is obtained.

The risk is lowest at the “optimal temperature” or “minimum mortality threshold” (MMT). This is the “sweet spot” - the point where the temperature is optimal: not too hot, not too cold, just right.

On both sides of this optimum, the health risk increases. The shape of this curve is crucial. In most regions, the risk increases relatively slowly in "moderately cold" temperatures before rising sharply in extremely cold conditions. In very warm temperatures, however, the risk increases steeply.

The relationship between local temperatures and the risk of premature death

Interestingly, MMT is not the same everywhere, but varies from city to city around the world. In fact, a look at the example curves below shows that the relationship between temperature and relative risk of death varies by location because people acclimate to certain climates and respond through natural heat adaptation - where our bodies physiologically adjust to hotter or colder temperatures - or through technological adaptations such as heating or air conditioning. For example, people in Vancouver are well acclimated to very cold temperatures, but not adapted to warm days.

Accordingly, mortality rates in very hot temperatures are low in Cape Town because most people have air conditioning. This is not the case in Paris or London; in many European countries, air conditioning is still rare.

One example that clearly shows this is the great heatwave that hit Europe in 2003. France was particularly hard hit - around 15,000 people died. The temperatures reached - up to 40°C in some cities for a week - were unusual for the region in the early 2000s. In other parts of the world, however, they would have been barely noticeable and would not have caused particularly high deaths. The problem was that local populations, accustomed to milder summers, did not know how to react to extreme heat. Even simple adaptation measures such as rehydration could have saved some people's lives.

Most people die in “moderately cold” conditions

If you look at many of the "optimal temperature" curves, you'll see that people in Germany, for example, spend a large portion of the year in temperatures that are colder than "optimal." This means that most temperature-related deaths occur in "moderately cold" conditions, not on extremely cold or hot days. This is not because the risk of death is highest in this range, but because we spend the most time there.

There are now a large number of studies examining how many people die from suboptimal temperatures. A large global study by Prof. Qi Zhao and colleagues estimated that between 2000 and 2019, 5 million people died prematurely each year from cold or heat. This corresponded to 9.4% of deaths from all causes. Almost one in ten deaths.

An earlier study by Dr. Antonio Gasparrini and colleagues estimated that 7.7% of deaths in selected countries were due to temperatures. This cannot be directly compared to the 9.4%, nor can it be used to estimate a global figure as not all countries were included. But it does show that a significant proportion of deaths in Australia, Brazil, Canada, China, Italy, Japan, South Korea, Spain, Sweden, Taiwan, Thailand, the United Kingdom and the United States are related to suboptimal temperatures.

Some researchers estimate that up to 1.8 million deaths annually are due to short-term temperature fluctuations alone. Large fluctuations from cold to warm or vice versa can stress our organ systems and increase health risks. Although estimates vary depending on methodological differences, data quality and assumptions about how people respond to temperature changes, the numbers are not small - they range from 1.7 to 5 million.

Cold-related deaths exceed heat deaths in all countries

What is consistent across these studies is that cold-related deaths far exceed heat-related ones. The study that estimated 7.7% of deaths were due to temperature found that 7.3% were due to cold temperatures; 0.4% to heat. In the "5 million deaths" study, 9.4% of deaths were due to suboptimal temperatures. 8.5% were cold-related and 0.9% were heat-related. This imbalance was seen across all regions.

Globally, cold deaths are nine times more common than heat deaths. In no region is this ratio lower than 3, and in many it is over 10. Cold is more deadly than heat, even in the hottest parts of the world. To be clear, most of these deaths occur in "moderately cold" conditions, not extreme temperatures. But it is a robust and consistent finding in the scientific literature: more people die today from cold than from heat. This may change in the future due to climate change.