How Heatwaves Affect Our Health

A heatwave is a period of at least three consecutive days during which the temperatures forecast are higher than the maximum temperatures recorded for the season

Recently, the weather has become a very common topic of conversation. And the reason is that this year we have had the warmest late Spring weather in decades, with temperatures similar to those recorded in 2003. And for those of us who study the effects of temperatures on health, 2003 stands out as a catastrophic year when a period of unusually high temperatures in Western Europe lasted numerous days and caused serious health problems. Studies in Spain estimated that the excess mortality during that heatwave was as high as 6,500 deaths.

Plans for the prevention of heat-related health effects usually define a heatwave as a period of at least three consecutive days during which the temperatures forecast are higher than the maximum temperatures recorded for the season. The health effects of heat depend on both the duration of the heatwave and its intensity (the maximum temperatures reached).

Surface air temperature anomaly for May 2017 relative to the May average for the period 1981-2010. Source: ERA-Interim. (Credit: ECMWF, Copernicus Climate Change Service)

Heat as a Triggering Factor in Mortality

Every 1ºC increase in the maximum temperature is associated with a 3.3% increase in mortality

When temperatures rise, so does the number of deaths. This association is not due to deaths caused by heat stroke, of which, there are fortunately very few, but rather because heat can aggravate the symptoms of certain diseases and trigger fatal episodes. A study carried out in Spain that analysed data for the period 1990-2004 concluded that mortality increased during the summer months by 14.6%. In other words, every 1ºC increase in the maximum temperature is associated with a 3.3% increase in mortality.

Barcelona is one of the urban areas in Spain where the greatest impact is observed, with mortality soaring by 27% during heatwaves

However, the impact of heat can vary across different geographical areas. The authors of that study reported a higher risk of heat-related death in the provinces in the south and west of Spain (with increases of between 19% and 29%). In the Mediterranean areas, by contrast, mortality on hot days increased but much less (between 2% and 9%). The city of Barcelona, however, is an exception because it is one of the urban areas in Spain where the greatest impact is observed, with mortality soaring by 27% during heatwaves.

Heat-related deaths are often caused by mental problems or complications affecting the nervous system. Mortality attributable to these causes can increase by 30% according to a study carried out in Catalonia. The people affected include, on the one hand, patients with psychiatric problems, but also people  (patients with Alzheimer's disease or dementia, for example) taking psychotropic medicines that can reduce the effectiveness of thermoregulatory mechanisms, making these patients more vulnerable to high temperatures. Other causes of mortality that have been associated with high temperatures include cardiovascular diseases, respiratory diseases, diabetes and impairment of the kidneys and urinary system.

Heat-related deaths are often caused by mental problems or complications affecting the nervous system

The health effects of high temperatures also depend on whether the hot weather occurs early or late in the summer. This was the conclusion of the authors of one international study, who observed that the risk of death decreases as the summer progresses, possibly due to factors such as the population’s acclimatisation to the high temperatures or the implementation of preventive measures.

Some People Are More Vulnerable

Heat does not affect everyone in the same way. Older people are the most vulnerable group, especially those with chronic diseases. According to a study carried out in Catalonia, the risk of mortality during a heatwave rises by 20% among people over 60 and by 40% in people aged between 80 and 90 years. Workers are another group vulnerable to high temperatures. Studies have shown that the risk of on-the-job accidents increases on very hot days. People who work outdoors, such as farm workers and construction workers, are the groups most vulnerable to the effects of heat.

Although few cases occur, the risk of infant mortality increases by 25% during episodes of extreme heat

Infants and pregnant women are two other at-risk groups during heatwaves. Although few cases occur, the risk of infant mortality increases by 25% during episodes of extreme heat. A study carried out in Barcelona on the effects of heat in pregnant women found that, on average, labour occurs up to 5 days early during heatwaves.

The Importance of Preventive Measures

Like other European countries, after the exceptionally hot summer of 2003, which had devastating health consequences, Spain put in place a plan to safeguard the population from the adverse health effects of extreme temperatures. Using a system of alerts issued by the Meteorological Service, the plan defines a series of recommendations and specifies actions that target both the population in general and the most vulnerable groups in order to protect them from the adverse effects of severe heat.

In some areas of Spain the number of deaths due to severe heat could increase eightfold by 2050, which would represent some 2,500 deaths

Avoiding exposure to the sun, drinking lots of water and ensuring good hydration, and refraining from outdoor activities during peak sunlight hours are just some of the recommendations that people should follow during periods of very hot weather. Since it has been estimated that in some areas of Spain the number of deaths due to severe heat could increase eightfold by 2050, which would represent some 2,500 deaths, it is important to apply the recommendations specified in the prevention plans because they represent one of the main strategies for adapting to climate change.