Heat is a serious threat to human health. The number of people exposed to extreme heat is growing exponentially due to climate change in all world regions.

Heat stress is the leading cause of weather-related death and can exacerbate underlying illnesses including cardiovascular disease, diabetes, psychological distress, asthma, and increase the risk of accidents and infectious disease.


The Lancet Countdown: Heat Stress

The global proportion of people at risk to heat stress is increasing. The Lancet Countdown is tracking our exposure and vulnerability to changes in heat caused by climate change.

The number of people exposed to extreme heat is growing.

Children under 1 year old experienced 600 million more person-days of heatwaves, and adults over 65 years 3.1 billion more person-days, in 2012–2021, compared to 1986–2005. Heat-related mortality for people over 65 increased by approximately 68% between 2000-2004 and 2017-2021. Source 2022 Report of the Lancet Countdown

Populations in Europe and the Eastern Mediterranean are among the most vulnerable to the health effects of heat. However, the Western Pacific, South-East Asian and African Regions have all seen a dramatic increase in vulnerability of >10% since 1990. Source Lancet Countdown: Vulnerability to Extremes of Heat

Due to the urban heat island effect, populations in urban environments often face magnify exposures to heat stress, on top of other conditions of vulnerability such as poor housing in informal urban settlements.


Some populations are more vulnerable than others to physiological stress, exacerbated illness, and an increased risk of death from exposure to hot weather.

Especially vulnerable populations include those over 65 years of age – particularly those with chronic medical conditions, infants and children, pregnant women, outdoor workers, athletes and attendees of outdoor events (e.g. music festivals), and the poor.

Gender can also play an important role in determining heat exposure.

Why are some people more vulnerable than others?

Foetuses, Newborns and Young ChildrenChildren and AdolescentsAdultsOlder Adults (<65 years)
GeneralTemperature extremes adversely impact birth outcomes, including changes in length of gestation, birth weight, still birth, and neonatal stress in unusually hot temperatures. Newborns are also highly susceptible to heat-related morbidity and mortality, such as that caused by malarial febrile seizures. Children are more vulnerable because they are often dependent on adults and others to make the decisions about their exposure and wellbeing. They may not recognize risks, have access to water, can be locked in dangerously hot cars, or may not recognize that they are hot and know to take cooling measures. Depending on their individual circumstances, adults can be vulnerable to heat. See below for some of the situations that can cause heat to become more dangerous for adults.
People with chronic illnesses (breathing problems, mental illness, kidney, and heart problems) Many physical and mental health conditions increase vulnerability to adverse temperatures through a direct effect on the body’s physiology or through the effect of certain medications. Exposure to high temperatures exacerbates and increases mortality rates in 90% of the existing global causes of death, including ischemic heart disease, stroke, COPD, lower respiratory infections, Alzheimer’s disease and other dementias, lung cancers, diabetes mellitus, road injury and diarrheal disease. Although heat-related morbidities are poorly registered worldwide, studies show a marked increase in the risk of death and hospitalization from NCDs when patients are exposed to excessive heat.
People with physical and mental disabilitiesReduced mobility and dependency on others and a reduced ability to understand or act on measures to cool themselves may increase risk for people with physical and mental disabilities faced with high temperatures.
Pregnant womenPregnant women are susceptible to changes in length of gestation and other birth complications in unusually hot temperatures.
Workers, especially outdoor workersWorkplace heat stress is a well-known occupational health hazard. Indoor and outdoor workers exposed to hot and humid conditions are at risk of heat-related illnesses, and long-term exposures can exacerbate underlying health conditions. Core temperature elevation and dehydration cause physical fatigue, irritability, lethargy, impaired judgment, vigilance decrement, and loss of dexterity, coordination and concentration, which can compromise occupational safety and lead to increased accidents. Heat conditions decrease worker productivity during the hottest months of the year, with economic consequences for families, businesses and economies. Learn more: manage and adapt to heat at work.
People living, working or studying in buildings not adapted for extreme heatPeople spend approximately 80% of their time indoors, with the elderly or unwell spending longer periods indoors. Buildings (including homes, hospitals, schools and prisons) are not always adapted for temperature extremes and may have insufficient heating/energy efficiency or cooling measures.
The homeless and low-wage earnersThose with few economic resources may not have access to air conditioning or transportation to access cooling, and may live in inadequate housing conditions making indoor temperatures excessively hot. People who are socially isolated are more at risk from temperature extremes because they are less able to access community support, and may also have additional health issues or other vulnerabilities.
People living in areas with poorly prepared or equipped health servicesHealth services need robust plans in order to manage the potential disruption and increased demand during and following temperature extremes; their ability to respond influences population vulnerability.
People exposed to high levels of air pollutionThere is growing evidence that the effects of heatwave days on mortality are larger when levels of ozone or PM10 are high, particularly among the elderly (75–84 years). The total daily number of deaths in this age group have been shown to increase by 16.2% on heatwave days with high ozone levels and by 14.3% on days with high PM10 levels, compared to an increase of 10.6% and 10.5% on days with low levels of ozone and PM10. The mortality increase due to the combined effect of heat and air pollution can be reduced by decreasing exposure to PM10 and ozone on hot days.
Athletes and attendees of outdoor eventsPeople who exercise in hot weather are more likely to become dehydrated and experience heat-related illness, including exertional heat stress. Those attending outdoor events, including concerts, festivals, sporting and religious events may also suffer health impacts from hot weather. Learn more: manage and adapt to heat in sports.

How Does Heat Affect Health?

Rapid rise in heat gain due to exposure to hotter-than-average conditions compromises the body’s ability to regulate temperature, and can result in a cascade of illnesses including heat cramps, heat exhaustion, heatstroke, and hyperthermia. Even small changes from seasonal average temperatures are associated with increased illness and death.

Deaths and hospitalizations from heat can be rapid or delayed, and can result in accelerated death or exacerbated illness in the already frail – especially during the first days of a heatwave. Temperature extremes can also worsen chronic conditions, including cardiovascular, respiratory, and cerebrovascular disease and diabetes-related conditions.

Heat also has important indirect health effects. Heat conditions can alter human behaviour, the transmission of diseases, health service delivery, air quality, and critical social infrastructure such as energy, transport, and water. Heat influences brain functioning and behaviour, and people with mental health issues and/or prescribed medications which limit the body’s natural cooling functions are especially vulnerable.

The scale and nature of the health impacts of heat depend on the timing, intensity and duration of a temperature event, the level of acclimatization, and the adaptability of the local population, infrastructure and institutions to the prevailing climate. The precise threshold at which temperature represents a hazardous condition varies by region, other factors such as humidity and wind, local levels of human acclimatization and preparedness for heat conditions.

Source: Killer Heat in the United States, UCS, 2019, p.9

Organs Damaged by Heat Exposure

Ischemia (limited blood flow)Heat Cytotoxicity (cell toxicity)Inflammatory ResponseDisseminated Intravascular CoagulationRhabdomyolysis (muscle tissue breakdown)

How Does Heat Affect Health Systems?

Hot weather, alone or in combination with other natural disasters, has the potential to cause wide-scale health emergencies, exacerbate risks of mass gatherings (such as music concerts, religious or sporting events), and disrupt emergency health services.

Periods of high temperatures have the potential to increase the patient burden on emergency health services, increase demand on ambulatory services, and increase emergency admission and hospitalization. Increasing temperatures also mean increased costs for health facilities to provide sufficient cooling and to retrofit health facilities to improve ventilation, cooling, insulation, energy and water resources to function efficiently.

Hot weather also increases heat stress for health workers. To prevent heat strain in health workers, sufficient rest and cooling times, safety while wearing personal protective equipment, and other measures are needed. Tool tip Learn more on how to protect health facility staff from heat strain and heat-related illness, and how health workers and other responders can manage heat stress while wearing personal protective equipment (PPE).

Monitoring and Measuring Impacts of Heat on Health

There are a range of indicators available to help governments monitor and measure the impact that high temperatures have on public health. Below are common indicators used to measure the impact of health and wellbeing. Tool tip Learn more about network priorities for Vulnerability Science (Pillar 2).

Indicators for Monitoring and Measuring Impacts of Heat on Health

MetricGood Practice IndicatorsExamples
Excess all-cause mortality
  • Estimated daily excess all-cause mortality by age group and region (1-64)(Over 65)
  • All-cause mortality, all ages and by ages
UK Heatwave Mortality Monitor

European Monitoring of Excess Mortality

Pascal et al. 2021. Evolving heat waves characteristics challenge heat warning systems and prevention plans. International Journal of Biometeorology volume 65.
Heat-related mortality
  •  Number of summertime heat-related deaths by year;
  • Annual rate for deaths classified by medical professionals as “heat-related” based on death certificate record
US-CDC Environmental Public Health Tracking

US EPA Climate Change Indicators: Heat-Related Deaths
Heat-related illnesses
  • Hospitalization rates for “heat-related” illnesses such as heat exhaustion, heat cramps, mild heat edema (swelling in the legs and hands), heat syncope (fainting), and heat stroke, based on hospital discharge records for patients who are admitted to the hospital for 23 hours or more;
  • Age-adjusted rate of hospitalization for heat stress per 100,000 population;
  • Crude rate of hospitalization for heat stress per 100,000 population;
  • Number of hospitalizations for heat stress;
  • Heat Stress Emergency Department Visits
  • Emergency consultations by general practitioners for heat stroke or dehydration, all ages and by age, as a primary or second cause of diagnosis
US EPA Climate Change Indicators: Heat-Related Illnesses

US-CDC Environmental Public Health Tracking

US-CDC Heat Stress Illness Indicators

Pascal et al. 2021. Evolving heat waves characteristics challenge heat warning systems and prevention plans. International Journal of Biometeorology volume 65.
Emergency department visits for heat stress
  • Annual age-adjusted rate of emergency department visits for heat stress per 100,000 population;
  • Annual crude rate of emergency department visits for heat stress per 100,000 population;
  • Annual number of emergency department visits for heat stress.
  • Emergency room visits for hyperthermia, dehydration or hyponatremia, all ages and by age as a primary or second cause of diagnosis
US-CDC – Environmental Public Health Tracking

US-CDC Indicators and Data

NIHHIS Experimental Heat Health Monitor

Pascal et al. 2021. Evolving heat waves characteristics challenge heat warning systems and prevention plans. International Journal of Biometeorology volume 65.
Energy demand
  • Heating and cooling degree days
US-EPA Climate Change Indicators
Productivity loss
  • Working hours lost to heat stress;
  • Percentage of GDP lost to heat stress
Working on a warmer planet: The impact of heat stress on labour productivity and decent work (ILO)
Exposure measures
  • Number of Extreme Heat Days;
  • Dates of Extreme Heat Days;
  • Number of Extreme Heat Events;
  • Dates of Extreme Heat Events
US CDC Indicator: Historical Extreme Heat Days and Events
Temperature distribution
  • Daily estimates of maximum temperature for summer months (May-September);
  • Daily estimates of maximum heat index for summer months (May-September)
US-CDC Indicator: Temperature Distribution