How does extreme heat affect health?

The world is experiencing record-breaking heat, with 2023 seeing the highest global temperatures in over 100,000 years.

From India to Greece to the US – many countries have been hit by prolonged periods of dangerous heat in 2024.

Extreme heat, or heatwaves, are not a new threat but research shows that climate change is making them more frequent, intense and longer-lasting.

One study found that between 2018 and 2022, people experienced health-threatening heat for an average of 86 days each year. Climate change made it twice as likely for 60% of these events to occur.

It’s putting the health of billions of people at risk, and these conditions will continue to worsen until we reach net-zero emissions.

The human body can only tolerate so much heat and works to regulate our core temperature to about 37°C through sweat evaporation. But, in extreme temperatures, the body can take in more heat than it can release, leading to heat stress. That means it has to work harder to keep internal temperatures down, which can push organs to their limits.

The health effects of extreme heat include dizziness, fainting, sleep disruption, breathing difficulties, heat exhaustion, heart attacks and kidney damage.

In severe cases, extreme heat can lead to heatstroke (when the body temperature rises above 40°C), organ failure and even death.

It can impact mental health too, but more research is needed to understand the causes and severity of this, particularly in low- and middle-income countries.

There are also indirect health impacts of extreme heat – including an increased risk of accidents, exposure to air pollution, food insecurity and the rising spread of some infectious diseases.

Everyone is at risk of the health impacts of extreme heat, but some people are more vulnerable. This includes:

• Pregnant people: pregnancy can make individuals more susceptible to heat-related illnesses because the body has to work harder to cool down both the pregnant person and their fetus. Adverse pregnancy outcomes, like stillbirth and low birth weight, have also been associated with extreme heat.
• Infants: young children can be vulnerable to extreme heat because they sweat less than adults and can’t regulate their body temperature as well as adults.
• Older adults: people over 65 have reduced sweating ability and struggle to cool themselves in extreme heat. Heat-related mortality for those over 65 increased by around 85% between 2000 to 2004 and 2017 to 2021.
• People with underlying health conditions: studies have shown that heatwaves are more dangerous for people with cardiovascular or respiratory diseases. Certain medications can also increase an individual’s risk of negative health effects in extreme heat. For example, antidepressants can interfere with the body's ability to regulate temperature.

As well as physiological risks, systemic factors can influence a person’s vulnerability to extreme heat too. How people live and where they live affects their level of exposure to high temperatures:

Income

An individual with low or no income level may not have access to cooling technologies for their home.

Occupation

Many people work outdoors where they are exposed to hours of heat, or work in buildings without access to cooling devices.

Urban heat islands

More than half of the world’s population live in cities where they face worsened heat extremes than in surrounding areas. Heat in cities is intensified by the insulation from closely constructed industrial buildings and the energy created by high numbers of people and transport, as well as urban layout. This is known as the ‘urban heat island’ effect.

Structural racism

Systemic oppression can increase the risk of the health effects of extreme heat. Several underlying inequalities can lead to people of colour being more likely to live in urban heat islands, work in hot environments or have lower incomes. Data from the US demonstrates higher heat-related mortality rates per 100,000 people among Indigenous, Black and Hispanic populations compared to white populations.

While the health effects of heat are being felt across continents, most of the research on heat and health is based in high-income countries. More data is needed on the impacts of heat in low-income countries, which are the hardest hit by the impacts of climate change even though they contribute the least to it. They also have the least capacity and resources to implement solutions to adapt to extreme heat and protect people's health.

If average global temperatures rise to just under 2°C, the annual number of heat-related deaths will increase by 370%.

Researchers worldwide are developing solutions that can help us prevent heat-related illnesses and deaths.

At Wellcome, we’re funding some of this important work through our Climate and Health programme. We want to find evidence-based interventions that work beyond laboratory conditions and in the real world. Some examples of heat adaptation research we’re funding include:

• whether trees can protect agricultural workers in Tanzania from the health risks of heat exposure
• if a heat-health action plan can help people in Mexico understand and manage the effects of heat
• how to reduce the health risks of extreme heat on pregnant women and newborns in South Africa and Zimbabwe

But research alone on how heat affects health and the interventions that can help is not enough. Scientific evidence needs to be used to influence and change policies.

It’s crucial that world leaders invest in and implement heat adaptation solutions that prioritise health, alongside climate mitigation policies that reduce or prevent greenhouse gas emissions and limit temperature rise. And, as climate risks continue to rise, the cost and challenges of acting now are lower than in the future.

Although many of the impacts of global heating are now irreversible, with more urgent, collective action, the world can prevent the worst of its impacts and protect people’s health.