Heat & Health

The number of people exposed to extreme heat is growing.

In 2018, 220 million more heatwave exposures Tool tip Calculation: Number of heatwave events x Number of individuals exposed per event affecting vulnerable populations were observed – 11 million more than the previous record set in 2015, increasing risk of heat stress, heart disease, and kidney disease.

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).

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.

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.

Hot weather has the potential to cause wide-scale health emergencies, exacerbate risks of mass gatherings, and disrupt emergency services.

High temperatures increase the patient burden on emergency health services, resulting in increased emergency admission and hospitalization, increased demand on ambulatory services, and increased costs and needs to retrofit health facilities to improve ventilation, cooling, insulation, energy and water resources to function efficiently.

Disaster risk management and preparedness efforts often underestimate the impacts of heatwaves compared to other hydro-meteorological disasters.

Hot weather can also increase heat stress for health workers. 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).