Intended for healthcare professionals

Feature Climate and Health

How hot weather kills: the rising public health dangers of extreme heat

BMJ 2022; 378 doi: (Published 14 July 2022) Cite this as: BMJ 2022;378:o1741
  1. Sally Howard, freelance journalist1,
  2. Geetanjali Krishna, freelance journalist2
  1. 1London
  2. 2New Delhi
  1. indiastoryagency{at}

Heatwaves are increasing in frequency around the world, taking a toll on individuals and healthcare systems. Some countries are rising to the challenge better than others, write Sally Howard and Geetanjali Krishna

Paris, 9 August 2003: Eugénie Thievent, 88, dies alone in her top floor apartment in the Marais district. Alone and confused, Thievent suffers a fatal blood clot as her blood thickens from dehydration. A neighbour finds the retired teacher’s body five hours after her death.

On that day the French capital was in the midst of a 20 day heatwave that saw the country’s mean maximum temperature exceed the seasonal norm by 11°C on nine consecutive days, reaching an overall temperature of 37°C, while night time temperatures nudged 26°C. Thievent’s apartment was four degrees hotter than those on lower floors of her four storey block.

Thievent’s was one of 35 000 deaths attributed to that single extreme weather event.1 The World Health Organization has reported that 166 000 people died of heat stress worldwide in the 20 years to 2017, and the number of people exposed to heatwaves increased by around 125 million from 2000 to 2016. The UN agency estimates that from 2030 to 2050 climate change will cause around 250 000 additional deaths a year worldwide, from malnutrition, malaria, diarrhoea, and heat stress.2

Exposure to extreme heat can cause exhaustion and heatstroke, a severe condition that occurs when body temperature rises to 40°C or higher and if untreated can quickly damage the brain, heart, kidneys, and muscles,3 being fatal in 10-50% of all cases. Other symptoms of heat stress include swelling in the lower limbs, heat rash on the neck, cramps, headache, irritability, lethargy, and weakness. Heat can also cause severe dehydration and acute cerebrovascular accidents, and it can contribute, as in Thievent’s case, to thrombogenesis (blood clots).

Much public health planning by governments has been reactive. And despite deaths and hospital admissions due to extreme heat making headlines each year, only now are steps being taken for different countries to learn from each other about this increasingly common health danger.

Who is at risk of heatstroke?

Older people (including those in residential care homes) and children are at greater risk of complications and death as a result of heat stress, as well as people with chronic diseases or who take daily medicines and may not know to adjust the dose. Studies have also found risks in pregnant women, linking heat exposure to preterm births and low birth weight.4

The risk from heat stress depends on both temperature and humidity and is indicated using “wet bulb globe temperature,” a measurement that considers temperature, humidity, and direct or radiant sunlight.5 A wet bulb globe temperature above 32°C is defined as “extreme risk” (fig 1).

Fig 1

Areas where a wet bulb globe temperature higher than 32°C (indicating “extreme risk”) would occur for more than 10 days a year at 4°C global warming

Met Office7

People in low and middle income countries are those most at risk from heat stress,6 as are city dwellers. In 2021 the UK Met Office estimated that a 2°C rise in global temperatures would lead to a billion people living in extreme heat stress, up from 68 million people today.7 A June 2022 study found that a 1.0°C rise in global heat could be linked to a million deaths in Latin America.8 Many of the world’s people most affected are located on the populous Indian subcontinent, in Brazil, and in central Africa.

People with physical outdoor jobs have a greater risk of adverse health effects9 (known as exertional heatstroke) because physical activity increases the core body temperature, compounding the effects of heat and humidity. Evidence also shows an increase in acute and chronic kidney disease in people who experience occupational heat stress.10 Early research has also pointed to a worse physiological toll exacted by heat stress in polluted environments,11 emphasising that “global warming increases the health effects of outdoor air pollution, resulting in more heat waves, during which levels of air pollutants raise and high temperatures and air contamination act in synergy, causing more serious health impacts than those estimated from heat or pollution alone.”12

Action plans

The 2003 European heatwave, which killed 20 000 people in France including Thievent, was a turning point for the western world, says Franziska Matthies-Wiesler, who has worked on climate change and health for WHO’s regional office for Europe and the EuroHEAT project, which developed the guidance for heat health action plans in 2007.13

“Europe began taking heat seriously as a public health risk and planning accordingly,” says Matthies-Wiesler. It’s a pattern in many countries: taking action only when extreme temperatures lead to sudden and alarming deaths. The US developed plans after events such as the 1995 heatwave in Philadelphia, which killed over 1000 people.14 Even in Asia, where high temperatures are commonplace, it took a deadly 2010 heatwave that led to 4462 excess deaths15 before any public health plans were drawn up.

The EuroHEAT project of 2005-07, coordinated by the World Meteorological Organization Europe and funded by the European Commission, quantified the health effects of heat in European cities and identified options for improving health systems’ preparedness and their response to the effects of heatwaves. France instituted a heat health warning system in 2003—a system of “graded” alerts generated by the country’s meteorological information systems—and a heat health action plan (HHAP) in 2012. Germany did likewise in 2008, as did Portugal in 2010 and Italy in 2012,16 while England and Wales had produced plans in 20041718 (guidance is in place for Northern Ireland or Scotland, but there are no formal HHAPs). By 2014, 18 of 51 WHO European region member states had instituted a HHAP.19

The many ways to tackle heat stress include adaptations to the built environment and architecture,20 public awareness campaigns, monitoring vulnerable people, and linking warning systems with healthcare readiness. HHAPs in the US engage GPs, health centres, and civil protection and social services. As most people vulnerable to heat are managed in general practice, a 2018 Australian paper21 argued that GPs had a role in identifying vulnerable people in their practices and working with these patients and their relatives to incorporate primary and secondary prevention strategies, while advocating for mitigating greenhouse gas emissions.

A groundbreaking 2013 HHAP implemented by the Indian city of Ahmedabad22—the first in Asia, triggered by the 2003 tragedy—emphasised capacity building among healthcare professionals, including training medics to treat heatstroke and ensuring enough intensive care beds to deal with heatstroke patients during heatwaves. And a 2018 pilot evaluation of the impact of Ahmedabad’s HHAP on all cause mortality found a correlation between ensuring that the healthcare system was forewarned of anticipated heatwaves and lower summertime all cause mortality rates, with the largest declines seen at the highest temperatures.23 As of 2022, HHAPs had been implemented in 23 Indian states.

Sharing knowledge

WHO works with national health sectors to strengthen governance, preparedness, and the response to heatwaves by developing contingency plans to map risks, vulnerable populations, available capacities, and resources. “These plans also include early warning systems and ensure that vulnerable populations, such as those in health facilities, nursing homes, and schools, have adequate provision of cooling equipment,” a spokesperson said.

Despite the growth of HHAPs, however, knowledge about heat stress and the risks of extreme heat is not yet shared globally, increasing the toll of deaths that might have been prevented by learning from effective interventions. Although WHO provides a fact sheet on heat stress,24 its climate change initiatives focus on the environmental sustainability of healthcare facilities25 and healthcare resilience (ensuring that healthcare systems can perform essential functions when climate stressed26) rather than population heat stress and population messaging.

In 2018 the UN and public health specialists formed the Global Heat Health Information Network, designed to increase knowledge sharing around heat health information, warning systems, and practices at a local, national, and global level. ENBEL, a project funded by the EU’s Horizon 2020 programme, aims to connect health and climate change research.27

Matthies-Wiesler says that one problem with HHAPs, which are typically led by environmental departments, is that they don’t yet fully integrate healthcare providers. A 2021 comparative study of HHAPs found that confusion and overlap between stakeholder roles and governance, including health agencies, hindered many current action plans.28

In Germany at least, medical and social care professionals have joined forces to tackle these shortcomings. A group including the Berlin Medical Association and the Senate Department for Science, Health, Care and Equality launched the ongoing Action Alliance for Heat Protection Berlin project on 20 June 2022,29 with interventions led by healthcare professionals rather than civic officials. The project will include heat protection of healthcare facilities, such as adapting buildings and increasing water availability in hospital wards. The results will inform other German cities and federal states.

Peter Bobbert, president of the Berlin Medical Association, said, “Between 2018 and 2020 there were around 1400 heat deaths in Berlin and Brandenburg alone. Heat is a real hazard for people with pre-existing medical conditions, pregnant women, young children, and anyone who works outdoors. Heatwaves are life threatening for the elderly and those with pre-existing conditions, and especially for those who live alone.

“As a society, we must protect vulnerable groups from this danger. It is underestimated how many people are already dying here in Berlin as a result of the climate crisis.”

Heat warnings in England: patients resist “nanny stateism”

Amina Albeyatti, a Surrey GP with an interest in sun damage and heat stress, fears that the health risks of hot weather are not taken seriously in the UK despite an estimated 2000 deaths a year from heat stress, which are projected to treble by 2050.30

“We use the term ‘heatstroke’ very liberally in this country, but when you have a patient with heatstroke it is very dramatic,” she told The BMJ. “The indoor risks of heat are poorly understood, and the emphasis with elderly people is often on retaining heat in the home, which puts them at risk. They often don’t know how to adjust medications—such as diuretics, anticholinergics, antipsychotics, and beta blockers, which can make the body more sensitive to heat—to lower the risk.

“At the moment, I don’t think public health messaging is working for either the young people exposed to hot work environments or for frail and vulnerable people. Often patients don’t want to hear these messages from their GPs, either. It’s hard to get through.”

An architect of England’s heat health action plan who wishes to remain anonymous told The BMJ that heat health warnings in England should be couched and timed very carefully because of public resistance to the concept of a nanny state.

“The problem is that people love heat in this country, so it’s a very unpopular message,” they said. “The literature also shows that people don’t identify themselves as vulnerable to heat.”


  • Competing interests: None.

  • Provenance and peer review: Commissioned, not externally peer reviewed.