Understanding the public health impacts of heatwaves

According to a joint heat warning from the UK Health Security Agency (UKHSA) and the UK Met Office, parts of the UK are expected to experience heatwaves over the next few days of July with rising temperatures.

A heatwave is defined by the British Met Office as a period of at least three consecutive days in which the regional health-relevant temperature limit is reached or exceeded.

Prolonged exposure to high temperatures can have serious effects on the body. In the summer of 2022, temperatures in England exceeded 40°C for the first time and an estimated 2,985 deaths were linked to five separate heatwaves, peaking between 17 and 19 July.

Periods of extreme weather can also have a significant impact on health systems, leading to increases in the use of services, cancellations of surgeries and, in some cases, power outages or complete failures of energy infrastructure.

As the global climate continues to change, heatwaves are expected to become more frequent, longer and more intense around the world. However, these changes will have different effects in different regions.

Researchers at the London School of Hygiene & Tropical Medicine (LSHTM) are working with partners such as the UKHSA to better identify those most at risk and understand what measures, messages and precautions could be put in place to protect communities.

Associate Professor Sari Kovats from the NIHR Health Protection Unit in Environmental Change and Health at LSHTM explained the health risks associated with extreme heat:

“Extreme temperatures overwhelm our body’s ability to regulate temperature. Heat can strain our cardiovascular and other systems, and the stress it causes can lead to premature death. Heat illnesses can also occur from organ damage due to increased body temperature, dehydration, heat cramps, and exhaustion.

“Both environmental and social factors influence the risk of dying in a heatwave. Older people and those with underlying medical conditions such as diabetes are particularly at risk, as are people with reduced mobility or mental illness who may be less able to take steps to protect themselves.

“Heat is also associated with an increased risk of preterm birth and stillbirth. Researchers at LSHTM in the CHAMNHA project, which examines data from sub-Saharan Africa, have found that high seasonal temperatures increase the risk of complications for the mother and can also compromise infant care.

“It is important to follow public health advice, such as staying hydrated and avoiding unnecessary travel, outdoor exercise or strenuous activity, to reduce health risks during a heatwave. People at highest risk must also have access to help where it is needed and those caring for them should be well informed.”

Malcolm Mistry, Assistant Professor at the Environment and Health Modelling Lab at LSHTM, explained how heatwaves and extreme weather events are measured:

“Our climate system is inherently chaotic, and even the most sophisticated forecasting models still find it extremely challenging to predict the weather over a period of more than two weeks. Air temperature is the most commonly used surface measurement taken by observatories and weather stations. Observations from satellites over the past five decades have also made it possible to monitor the weather even in parts of the world where ground stations are sparse or nonexistent.

“It is important to stress that heatwaves themselves, like many other extreme weather events, are not unusual in any given year. This is because our planet’s climate system has a ‘natural variability’. To determine whether the weather we are experiencing is unusual, we use historical records for a particular region over a long period of time and consider whether the current events are more frequent or more severe than the ‘normal’.

“There is consensus in the climate modelling community that the frequency, magnitude and duration of most extreme weather events will increase worldwide in a warmer climate. However, some estimates still have uncertainties, for example regarding heavy rainfall. Some regions of the world are expected to experience drier conditions than others.

“From a long-term perspective, society and policymakers need to take action to increase our resilience to extreme temperatures and other weather events. We also need to have plans in place to support the most vulnerable when we see precautionary warnings being issued. In addition, the UN’s Early Warning for All initiative is a stark reminder that not every region of the world is able to predict or disseminate such warnings.

“Green spaces, higher quality housing, temporary cooling centres in heat-prone areas and mobile relief centres are all examples of measures that could be taken to protect communities. Major sporting events such as the Summer Olympics, as well as outdoor public events such as the Glastonbury music festival, may also need to take place outside of the peak summer months to protect visitors from the potential impacts of extreme heat.”

For more information on heat warnings in the UK, visit the Met Office website.