Heat is a major and growing threat to public health, driving a sharp rise in illness and death, especially among older adults and other vulnerable groups. As climate change accelerates, coordinated action and evidence-based public health policies are crucial to prevent avoidable harm and save lives.
The latest data shows that as the planet heats, people around the world are being affected by the dangerous health consequences of increasing levels of heat stress. Climate change is not just bringing more warmer days – it is ushering in a new era of extreme heat stress with catastrophic impacts on people’s health.
Action on climate change to protect people’s health and to limit further global heating is crucial.
What defines “heat stress”?
The risk of a person’s body overheating is determined by four separate characteristics of the surrounding environment.
Temperature – and radiant temperature
The “temperature” that most people are familiar with through weather reports is actually measured in the shade. But the strain that develops inside the body during heat exposure can very different if a person is directly in the sun or not. ‘Black globe temperature’ helps account for the additional radiant heat absorbed by people directly from the sun and indirectly from reflected radiation from surfaces in the surrounding environment, which can be 10 to 15°C higher on a clear summer day.
Humidity
Levels of humidity directly alter people’s ability to keep cool. Sweat must evaporate to enable body cooling, but higher humidity means sweat just sits on the skin without cooling the body. At lower humidity levels, sweat evaporates freely from the skin taking with it large amounts of heat.
Wind
In hot and humid conditions, a light breeze will provide some relief. This is because wind greatly accelerates sweat evaporation, as air directly above the skin that is saturated with water vapour with replaced with relatively drier air. The ability to shed body heat is therefore oppressively restricted when conditions are still, but much improved with higher air flow even if air temperature and humidity remain the same.
Collectively these four parameters (air temperature, radiant temperature, humidity and wind) define the level of heat stress a person is exposed to.
How does heat stress affect health?
Extended periods of heat stress on the human body increase the risk of heat-related illness and death. As heat stress rises, the human body puts a lot of effort into cooling itself by pushing blood to the skin surface – the need to maintain blood pressure puts particular strain on the hearts of people with underlying cardiovascular conditions. Meanwhile older people and infants are susceptible to heat stress as their bodies sweat less, reducing their ability to cool and increases their risk of heat stroke. Outdoor and manual workers, and athletes face increased exposure to excess heat stress because of being outdoors or in hotter environments, while they must also shed extra heat because of higher levels of exertion- collectively increasing the risk of exertional heat illnesses and injury. Poverty can also be an aggravating factor if people do not have the resources to access cooling services or avoid heat exposure.
Beyond directly affecting people’s health, heat stress can also affect health and wellbeing indirectly, by impacting the social and economic conditions on which good health depends. For example, heat stress reduces labour productivity and harms the livelihoods of workers and their dependents, particularly when reducing access to quality nutrition, healthcare, housing, or health-supporting services. A record-high 512 billion potential work hours were lost in 2023 due to heat exposure across the agricultural, construction, manufacturing, and service sectors.
Extreme heat is particularly dangerous and can acutely impact large populations for short periods of time, often trigger public health emergencies, and result in excess deaths and cascading socioeconomic impacts. Extreme heat can also cause loss of health service delivery capacity, when power shortages accompany extreme heat and disrupt health facilities, transport and water infrastructure.
These impacts are becoming increasingly concerning given that the frequency and intensity of extreme heat will continue to rise in the 21st century because of climate change. This is combining with ongoing and rapid growth of urban centres which are ill-prepared to cope with hotter weather, and exacerbate heat exposure of urban dwellers through the urban heat island effect – whereby densely developed areas experience higher temperatures than nearby rural areas. In 2023, the number of heatwave days that infants and adults over 65 were exposed to, reached a new record high of an average of 13.8 heatwave days per person.
What action is needed to protect health from hotter weather?
It is critical that action to reduce emissions and limit further global heating is accelerated, to avoid a future of extremes which breach the limits of adaptation.
In adapting to rising temperatures today, avoiding dangerous heat exposure should not just about intensifying the use of air conditioning (AC) – after all this solution is out of reach for many of the world’s most vulnerable communities. Furthermore, current levels of AC use produce emissions which continue to contribute to climate change, creating urban heat islands, and threatening power outages in times of peak use during heatwaves.
More sustainable health-protecting cooling solutions are therefore needed and available at landscape, building, and personal levels.
(1/3) Landscape level cooling strategies
Increasing equitable access to safe, adequately designed and quality urban green spaces can help reduce the negative health impacts of climate change, reducing heat exposure and flood risk, while offering physical and mental health co-benefits by improving air quality, and offering spaces for exercise, social interaction, and connection with nature. Currently, only 28% of urban centres have at least moderate levels of greenness.
(2/3) Building level cooling strategies
Heat enters the indoor environment via conduction through walls, roofs, and floors, and by direct radiation through windows. Interventions like reflective coatings on roofs can reflect solar radiation and reduce heat gains, while increasing insulation can reduce net conductive heat flow from the outdoor to indoor environments, while increasing energy efficiency in cooler temperatures.
(3/3) Individual level cooling strategies
During heat extremes and hot weather, cooling strategies that are low cost, accessible, and sustainable can be implemented – even in low resource settings at the individual level to drastically reduce physiological heat strain characterised by rises in core temperature, cardiovascular strain, and dehydration.
Health systems must become resilient to rising temperatures, to ensure they can best anticipate and respond to the health impacts of heat to fulfil their duty to protect people. Surveillance, early warning, and early response systems will be key to prepare for shifts in disease range and incidence and alert people to health risks. Healthcare professionals must also be equipped with the right skills and resources to respond in case of heatwaves.
Want to find out more? Explore the latest Lancet Countdown findings on our data platform
Thanks to the Lancet Countdown’s Professor Ollie Jay, Working Group 1 Co-Chair on Health Hazards, Exposures, and Impacts, for his contributions to this page.