Effects on health
Heatwaves and Health Risks: Who Is Vulnerable and How to Stay Safe
Who is most vulnerable in a heatwave, what the health risks are, how to recognise heat illness, and how to stay safe when temperatures soar.
Heatwaves and Health Risks: Who Is Vulnerable and How to Stay Safe
Editorial advisor: Mr. John Smith, Specialist Environment, PhD in Environmental Engineering.
Heatwaves kill more people than most other weather extremes in many countries, yet they often receive less attention than floods or storms because their damage is less visible. There are no dramatic images of collapsed buildings — only emergency departments filling quietly, excess deaths recorded in statistics weeks later, and workers, older people, and children suffering behind closed doors.
As the climate warms, heatwaves are becoming more frequent, more intense, and longer. Understanding how heat harms the body, who is most vulnerable, and what actually protects people is therefore essential public-health knowledge, not just summer advice.
How heat harms the body
The human body maintains a core temperature around 37 degrees Celsius. When the surrounding air is hot, the body cools itself mainly by sweating and by increasing blood flow to the skin, where heat can be released. Both mechanisms strain the heart, which must pump harder to move blood to the skin while maintaining blood pressure.
When heat overwhelms these mechanisms, illness progresses in stages. Heat cramps and heat exhaustion — heavy sweating, weakness, dizziness, nausea, headache — signal that the body is struggling. Heatstroke is a medical emergency: core temperature rises above 40 degrees, sweating may stop, confusion and loss of consciousness occur, and organs can be damaged rapidly. Heatstroke can kill if cooling is delayed.
Heat also worsens chronic disease. It increases the risk of heart attacks and strokes, worsens kidney disease — especially among outdoor workers who become repeatedly dehydrated — and triggers asthma and chronic lung disease exacerbations. Certain medicines increase risk: diuretics, beta-blockers, and some psychiatric medicines impair sweating or fluid balance, making people on these medicines more vulnerable without realising it.
Humidity is critical. Sweat cools the body only when it evaporates. In humid air, evaporation slows, so the body cannot shed heat effectively even at moderate air temperatures. Meteorologists combine temperature and humidity into heat index or wet-bulb temperature measures. A wet-bulb temperature approaching 35 degrees Celsius is considered near the limit of human survivability for sustained exposure, even for healthy, acclimatised people with shade and water — because at that point sweat no longer cools the body.
Who is most vulnerable
Older adults are at highest risk. Ageing reduces sweating efficiency, thirst perception, and cardiovascular reserve, and older people are more likely to live alone, take risk-increasing medicines, and have chronic illness. During major heatwaves, excess deaths are concentrated among people over 65.
Young children are also vulnerable: they depend on adults for fluids, shade, and rest, and their bodies heat up faster relative to size. Infants left in parked vehicles — which can reach lethal temperatures in minutes — are a recurring tragedy during heatwaves.
Outdoor workers — farm workers, construction workers, delivery workers — face prolonged exposure combined with physical exertion and sometimes piece-rate pressure that discourages rest and hydration. Studies of chronic kidney disease among agricultural workers in hot regions suggest repeated heat stress and dehydration may cause lasting kidney damage, a condition sometimes called heat-stress nephropathy.
Pregnant people face increased risks of preterm birth and low birth weight during heat exposure. People with disabilities may face barriers to cooling, transport, or communication of warnings. Low-income households may lack air conditioning, or be unable to afford to run it, and may live in hotter urban neighbourhoods with less tree cover and more heat-absorbing surfaces.
Why nights and cities make it worse
Heatwaves are not only about daytime peaks. Warm nights are increasing faster than hot days in many regions, and they matter enormously: the body recovers overnight when temperatures fall. When nights stay hot, recovery fails, and cumulative heat stress builds over successive days. Mortality during multi-day heatwaves often rises sharply after the second or third hot night.
Cities amplify heat through the urban heat island effect. Asphalt, concrete, and rooftops absorb heat by day and release it at night. Lack of vegetation and water bodies removes natural cooling. Dense, low-income neighbourhoods with little green space can be several degrees hotter than leafy suburbs in the same city, creating a within-city inequity that maps closely onto income and historical planning decisions.
What individuals can do
Practical protection starts with the basics, done consistently. Drink water regularly, before feeling thirsty, and avoid alcohol, which dehydrates. Stay in shade or indoors during the hottest hours, typically late morning to late afternoon. Wear light, loose clothing and a hat outdoors. Take cool showers or baths, and use wet cloths on skin if air conditioning is unavailable.
Never leave children, older people, or pets in parked vehicles, even briefly. Check on neighbours who live alone, especially older adults, during heatwaves — social isolation is a major risk factor for heat death. Plan outdoor work and exercise for cooler hours, take frequent breaks in shade, and stop and seek help at the first signs of heat illness rather than pushing through.
Air conditioning saves lives during extreme heat, but it is not available to everyone, and it increases electricity demand and emissions if powered by fossil fuels. Where air conditioning is unavailable, community cooling centres, libraries, shopping centres, and shaded public spaces can provide relief. Fans help up to a point, but in very hot, humid conditions they cannot substitute for cooling, and in extreme heat above body temperature they can worsen dehydration if fluids are not replaced.
Know the warning signs that require emergency help: confusion, slurred speech, hot dry skin or stopped sweating, fainting, seizures, or a body temperature that does not fall with cooling. Call emergency services immediately and cool the person rapidly with water, ice, and shade while waiting.
What communities and governments must do
Individual advice is insufficient without systems. Heat-action plans — now adopted in cities from Ahmedabad to Paris — combine early warnings, public messaging, cooling centres, adjusted work hours, hospital preparedness, and outreach to vulnerable people. Evidence shows they reduce deaths when implemented well.
Longer-term measures matter more: urban tree planting and green roofs that cool neighbourhoods, reflective roofs and pavements, building designs that allow passive cooling and ventilation, and housing standards that prevent new homes from overheating. Workplace regulations — mandatory rest breaks, shade, water, and acclimatisation for new workers — protect those who cannot choose to stay indoors.
Early-warning systems must reach everyone, in languages and formats people actually use, and must be tied to action: opening cooling centres, checking on vulnerable residents, and preparing hospitals before the peak, not after. Heat warnings that arrive without services help those who already have options and leave behind those who do not.
Heatwaves will worsen with further warming, but heat deaths are largely preventable. The gap between a dangerous heatwave and a deadly one is often not the temperature itself, but whether a society planned for it.
Sources
- World Health Organization, Heat and Health — https://www.who.int/health-topics/heat-and-health
- World Health Organization, Climate Change and Health — https://www.who.int/health-topics/climate-change
- US Centers for Disease Control and Prevention, Extreme Heat — https://www.cdc.gov/extreme-heat/
- IPCC, Sixth Assessment Report, Working Group II — https://www.ipcc.ch/report/ar6/wg2/
- National Weather Service, Heat Safety — https://www.weather.gov/safety/heat
Frequently asked questions
How does heat harm the body?
The human body maintains a core temperature around 37 degrees Celsius, and when the surrounding air is hot, it cools itself mainly by sweating and by increasing blood flow to the skin, where heat can be released. Both mechanisms strain the heart, which must pump harder to move blood to the skin while maintaining blood pressure. When heat overwhelms these mechanisms, illness progresses in stages, from heat cramps and heat exhaustion, with heavy sweating, weakness, dizziness, nausea, and headache, to heatstroke, a medical emergency in which core temperature rises above 40 degrees, sweating may stop, confusion and loss of consciousness occur, and organs can be damaged rapidly.
Why does humidity make heat more dangerous?
Sweat cools the body only when it evaporates, and in humid air evaporation slows, so the body cannot shed heat effectively even at moderate air temperatures. Meteorologists combine temperature and humidity into heat index or wet-bulb temperature measures, and a wet-bulb temperature approaching 35 degrees Celsius is considered near the limit of human survivability for sustained exposure. That limit applies even for healthy, acclimatised people with shade and water, because at that point sweat no longer cools the body.
Who is most vulnerable in a heatwave?
Older adults are at highest risk, since ageing reduces sweating efficiency, thirst perception, and cardiovascular reserve, and older people are more likely to live alone, take risk-increasing medicines, and have chronic illness, with excess deaths during major heatwaves concentrated among people over 65. Young children are also vulnerable, since they depend on adults for fluids, shade, and rest and their bodies heat up faster relative to size, and infants left in parked vehicles, which can reach lethal temperatures in minutes, are a recurring tragedy. Outdoor workers face prolonged exposure combined with physical exertion and sometimes piece-rate pressure that discourages rest and hydration, pregnant people face increased risks of preterm birth and low birth weight, and low-income households may lack air conditioning or be unable to afford to run it.
Why do warm nights and cities make heatwaves worse?
Warm nights are increasing faster than hot days in many regions, and they matter enormously because the body recovers overnight when temperatures fall, so when nights stay hot, recovery fails and cumulative heat stress builds over successive days. Mortality during multi-day heatwaves often rises sharply after the second or third hot night. Cities amplify heat through the urban heat island effect, since asphalt, concrete, and rooftops absorb heat by day and release it at night, and dense, low-income neighbourhoods with little green space can be several degrees hotter than leafy suburbs in the same city.
How can individuals and communities stay safe in a heatwave?
Practical protection starts with the basics, done consistently: drink water regularly, before feeling thirsty, avoid alcohol, which dehydrates, stay in shade or indoors during the hottest hours, typically late morning to late afternoon, wear light, loose clothing and a hat outdoors, and take cool showers or baths or use wet cloths on skin if air conditioning is unavailable. Never leave children, older people, or pets in parked vehicles, even briefly, and check on neighbours who live alone, especially older adults, since social isolation is a major risk factor for heat death. Know the warning signs that require emergency help, including confusion, slurred speech, hot dry skin or stopped sweating, fainting, and seizures, and call emergency services immediately and cool the person rapidly with water, ice, and shade while waiting.