”Age can't be changed, but population fitness can, meaning that there are ways in which we can help people to adapt to cope better with heat stress," said Christian Schuster, who did this study at the Humboldt University of Berlin, Germany, and now works at the nation's North Rhine-Westphalia state environment agency.

As climate change continues apace, heat waves are predicted to increase in frequency and severity. In towns and cities greater numbers of people will be exposed to the urban heat-island effect.

Heat stress has been recognized as one of the major adverse impacts of climate change on urban populations worldwide. Data from previous heat waves, such as the European one during the summer of 2003 where an estimated 70,000 people lost their lives, show that elderly people are particularly vulnerable to heat. However, few studies have investigated what, if anything, makes some people more susceptible to heat stress than others.

With that in mind, Schuster and his colleagues designed a questionnaire to assess people's individual risk, by letting them report on their heat-stress susceptibility and symptoms during hot summer days. The survey also investigated different parameters of physical health. Working in Berlin, a city with a clear urban heat-island effect, the researchers gathered answers from 474 participants following a heat wave in August 2013.

Their results show that the amount of physical activity a person does is a bigger predictor of heat stress than age: the more active a person is, the less vulnerable they are to the impact of heat.

The finding makes sense from a physiological perspective. "The heart and circulation system is responsible for the physical adaptation towards elevated air temperature," said Schuster. "It should react quickly and speed up the blood circulation for cooling the body. Regular physical activity increases the strength of that system and its ability to adapt quickly."

When it came to physical activity, the scientists found that regular "active travel" – walking, cycling, or even brisk walks between metro stations – was associated with reduced body-mass index, increased health and physical fitness, and reduced heat stress.

Most likely the correlation between age and risk of heat-related mortality, as observed in many studies, is due to a tendency to lower levels of physical activity in older populations. "In general, age may mostly be regarded as a proxy for lower cardiovascular capacity," said Schuster, whose findings are published in Environmental Research Letters (ERL). "However, people who keep up a physically active lifestyle are not sensitive to heat stress even when they get older."

Until now, recommendations for adaptation of urban areas to climate change have tended to focus on urban greening, to decrease the urban heat-island effect. "They are probably based on the widely reported assumption that age is the dominant risk factor, which cannot be changed," said Schuster. Although urban greening is a good thing, Schuster and his colleagues believe that increasing urban fitness via active travel could provide greater resilience than urban greening.

"Encouraging active travel appears to be the most effective and efficient adaptation strategy," he said. What's more, it has the added benefit of reducing carbon-dioxide emissions, thereby tackling two root causes of urban heat stress: population health and climate change. "We need to burn fat not oil to keep fit and stay cool!"

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