A heatwave is a period where local excess heat accumulates over a sequence of unusually hot days and nights. Heatwaves and prolonged excess heat conditions are increasing in frequency, duration, intensity and magnitude due to climate change. Even low and moderate intensity heat waves can impact the health and well-being of vulnerable populations.
The frequency and intensity of extreme heat and heat waves will continue to rise in the 21st century because of climate change. Extended periods of high day and nighttime temperature conditions create cumulative stress on the human body, increasing the risk of illness and death from heat exposure. Heatwaves can acutely impact large populations for short periods of time, often trigger public health emergencies, and result in excess mortality and cascading socioeconomic impacts (for example, lost work capacity and labour productivity). They can also cause loss of health service delivery capacity, when power shortages accompany heatwaves and disrupt health facilities, transport and water infrastructure.
Population ageing and the growing prevalence of non-communicable diseases (respiratory and cardiovascular diseases, diabetes, dementia, renal disease and musculoskeletal disease) means that populations are becoming more susceptible to negative heat impacts. Cities are not being designed to minimize the accumulation and generation of urban heat, with a loss of greenspace and inappropriate housing materials (for example, metal roofs) that amplify human exposure to excess heat.
Awareness among health workers and the public remains insufficient of the health risks posed by heat. Health professionals should adjust their guidance, planning and interventions to account for increasing heat exposures, as well as to manage acute increases in admissions associated with heatwaves. Practical, feasible and often low-cost interventions at the individual, community, organizational, governmental and societal levels can save lives.
Many populations in tropical and subtropical climates are chronically exposed to high temperatures. In mid to high latitudes, population exposure excess heat is seasonal.
Individual vulnerability to heat for physiological or clinical factors in adults is well described (4). Heat can affect health through a variety of direct and indirect mechanisms. There is limited research on the impact of chronic (sustained) exposure to high temperatures and humidity.
Outdoor and manual workers, athletes and civil protection employees are exposed to excess heat because of their work and susceptible to exertional heat stress.
Urban and rural poor are often disproportionately exposed to overheating due to low quality housing and lack of access to cooling. Due to building materials, informal settlements are often hotter than other urban areas in some cities. Gender can play an important role in determining heat exposure, for example where women are primarily responsible for cooking indoors during hot weather.
Infographic: Main heat vulnerability factors
The amount of heat stored in the human body is determined by a combination of (a) an inability to eliminate internally generated heat from metabolic processes due to environmental heat stress (for example, high temperature, high humidity, low wind, high thermal radiation), (b) clothing creating a barrier to heat loss, (c) external heat gain from the environment.
The body’s inability to regulate internal temperature and eliminate heat gain in such conditions increases the risk of heat exhaustion and heatstroke. The strain put on the body as it tries to cool itself also stresses the heart and kidneys. As a result, heat extremes can worsen health risks from chronic conditions (cardiovascular, mental, respiratory and diabetes related conditions) and cause acute kidney injury.
Deaths and hospitalizations triggered by extreme hot weather occur rapidly (same day and following days), which means interventions also need to be rapid when a heat alert is issued.
Heat can also disrupt and compromise essential health services, such as the loss of power supply and transport. Heat will reduce working productivity and increases the risk of accidents. It is difficult to complete work or learning in very hot weather and heatwaves may lead schools and other institutions to close. Heatwaves can also be associated with hazardous air pollution events.
The scale and nature of the health impacts of heat depend on the timing, intensity and duration of a heat event, and the level of acclimatization and adaptability of the local population, infrastructure and institutions to the prevailing climate.
Infographic: Scale and nature of the health impacts of heat
Download: Signs of heat stroke
Mitigating climate change by reducing greenhouse gas emissions is imperative and urgent to limit the magnitude of human costs from extreme heat. WHO is addressing climate change through the Alliance for Transformational Change in Climate and Health (ATACH), as well as country support of technical and policy resources to help the health sector and communities adapt to the risks of climate change.
WHO works with the health sector to strengthen governance, preparedness and response to acute impacts of heatwaves by developing heat action plans, heat early warning systems and advisories, and emergency response plans that map the risks, vulnerable populations, available capacities and resources. These plans protect high risk populations such as those in health facilities, nursing homes and schools which do not have access to cooling.
WHO co-sponsors the Global Heat Health Information Network to accelerate sharing and learning about the risks and solutions to address extreme heat. WHO partners closely with the World Meteorological Organization on the development of Heat Health Warning Systems.
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