Extreme heat may keep millions from exercising, linked to 500,000 early deaths yearly

A hotter world is quietly changing one of the simplest things we do for our health – moving our bodies. For many people, a walk in the park, a jog around the neighbourhood or a cycle to work is becoming harder, and sometimes unsafe, as temperatures rise.

Scientists are beginning to understand how heat affects physical activity and why this matters for long-term health. A new modelling study in The Lancet Global Health suggests that if rising temperatures lead to sustained reductions in activity, the knock-on effects could contribute to hundreds of thousands of premature deaths each year by the middle of this century.

The underlying behaviour is familiar. As temperatures climb, many people cut back on outdoor exercise. There is no single threshold, but activity often becomes noticeably less comfortable somewhere in the high 20s celsius, especially in humid conditions. Running, cycling or even brisk walking can feel more strenuous.

This is because the body has to work harder to stay cool. More blood is diverted to the skin and sweating increases, which can lead to earlier fatigue, dizziness and dehydration. Faced with this, people may slow down, shorten their exercise or avoid it altogether. Across large populations, this can translate into less movement, more time spent sitting and higher risks of chronic disease.

Some groups are more affected than others. Older adults tend to regulate temperature less efficiently. Women may experience different responses depending on physiology and hormonal factors. People with respiratory conditions such as chronic obstructive pulmonary disease can find breathing more difficult in hot and humid conditions, especially during exertion.

Over time, even small reductions in activity matter. Regular movement helps protect against heart disease, stroke, diabetes and other chronic illnesses. When activity levels fall, those protective effects diminish.

The modelling study estimates that, if warming continues and outdoor activity declines without compensation elsewhere, the resulting increase in inactivity could contribute to a substantial number of additional deaths by 2050. These would not be caused directly by heat itself, but by the gradual development of diseases linked to inactivity. The scale of the estimate depends heavily on how people adapt – for example, whether they move exercise indoors or shift it to cooler times of day.

The poor bear the brunt

The effects of heat are not evenly distributed. In wealthier settings, people are more likely to have access to air-conditioned gyms, indoor sports facilities or shaded green spaces. When it becomes too hot outside, they may have alternatives.

In many low- and middle-income countries, those options are more limited. People may live in densely built areas with little green space or cooling. Outdoor work is also more common, meaning higher overall heat exposure alongside fewer safe opportunities for recreation.

Research from Rio de Janeiro, Brazil, illustrates this imbalance. Heatwaves there are associated with increased deaths from cardiovascular and respiratory disease, particularly among older adults and women. Many deaths occur at home, pointing to gaps in cooling, information and access to care.

At the same time, physical activity remains part of the wider public health response. Evidence suggests that encouraging walking, cycling and public transport can reduce emissions while improving health. A framework in the journal Nature Health highlights how designing cities for active travel can support both cleaner air and more consistent physical activity.

There is also growing recognition that climate change may disrupt sport and recreation directly. The UK body Sport England has warned that heat, flooding and drought could damage facilities and reduce participation unless infrastructure adapts.

Some responses are already being tested. Tree-lined streets and shaded paths can lower urban temperatures. Parks with water features and dense planting offer cooler spaces for activity. Guidance increasingly recommends exercising in the early morning or evening, when conditions are milder, and research supports these adjustments as practical ways to maintain activity safely.

Technology may also play a role. During the COVID pandemic, many people turned to home-based exercise, online classes and simple equipment such as resistance bands. A study I conducted found that even housebound patients with serious lung disease could improve fitness, mood and quality of life through structured virtual programmes.

Similar approaches could help during periods of extreme heat. Online sessions, community “cool hubs” in air-conditioned buildings and guided indoor exercise can provide alternatives when outdoor conditions are unsuitable.

Exercise is not just a lifestyle choice but a core component of health. As the climate warms, the challenge will be to ensure people can remain active in ways that are safe and accessible. That is likely to involve a mix of individual adaptation and changes to the environments in which people live.

Vikram Niranjan does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.