The relationship between extreme heat and increases in mortality has been reported in many parts of the world. A simplified way to understand this relationship is the following. The human body has several mechanisms to maintain a constant internal temperature, but when exposed to high temperatures, this regulation becomes more difficult and puts an extra stress to the body. This extra stress can trigger other complications in frail people, such as the elderly or those with chronic conditions. As a result, for example, we observe increases in myocardial infarctions during or after heat waves.
Some episodes of big heat waves, like the 2003 heat wave affecting Western Europe, resulted in very high death tolls that raised public and political attention to this problem. As a result, nowadays, most of the developed countries have plans in place to prevent the health effects of heat waves, with the first evaluations showing that they are effective to reduce – but not to eliminate – those effects. In fact, the number of heat-related deaths is expected to increase in the future in developed countries, mainly because of global warming but also because of an older –and thus more vulnerable – population. This is the case even in projections that try to account for some degree of acclimatization, physiological or through changes in behavior.
Less is known from developing countries
Studies linking mortality and heat are still very limited in developing countries, but the effects are likely to be stronger. Poorer health, poorer work conditions, lack of water and worse health facilities are some of the reasons. Unfortunately, differences in the ability to cope with heat are expected to become wider in the future, especially due to the lack of resources for adaptation in developing countries. Inequalities also occur in developed countries, where socioeconomic gradients in heat vulnerability have also been found.
Heat-related deaths are starting to be seen as an important public health problem in developed countries. For example, in the United States, heat waves account for more deaths than any other natural disaster. Future projections, with all their uncertainties, point towards important increases in heat-related deaths as a consequence of climate change. This important impact on health could become an important motivation for climate change mitigation.