Approximately 30% of the world’s population is exposed to catastrophic heat events every year.^{4 4}WHO, 2018. COP24 Special Report: Health and Climate ChangeX  In all world regions the characteristics of extreme temperatures are changing in similar ways, including longer heat seasons, more frequent heat waves, and more intense heat days. ^{5 5}Changes in regional heatwave characteristics as a function of increasing global temperature. Perkins-Kirkpatrick and Gibson, 2017X

Due to the Urban Heat Island (UHI) effect, people in urban environments face magnified exposure to heat stress. This exposure can be compounded by other vulnerability factors such as poor housing and inadequate access to clean drinking water or cool spaces.

Heat causes a wide range of health impacts, the majority of which are related to pre-existing health conditions. Extreme heat directly impacts human health in the following ways:

• Dehydration
• Heat cramps
• Heat fatigue
• Rash and dermatitis
• Heat syncope (fainting)
• Heat edema (swelling)
• Heat exhaustion (body temperature rises above 38°C)
• Heat stroke (body temperature rises above 40.6°C)

Heat also affects human health indirectly by:

• Stressing underlying physiological systems, resulting in renal insufficiency, acute cerebrovascular disease, pulmonary disease and respiratory illnesses;
• Altering the transmission of infectious diseases such as malaria, dengue, cholera and tuberculosis;
• Increasing exposure to and proliferation of environmental toxins such as algal blooms;
• Disrupting infrastructure and causing power shortages that disrupt health care facilities and delivery, transport and water services, which may trigger secondary health risks due to lack of access to essential services.

Hot environments can overwhelm the body’s heat-dissipating mechanisms. At high temperatures, the body increases blood flow to the skin and starts to sweat to increase evaporation, cool down and maintain a safe body temperature. This physiological response can strain the body’s circulatory system and lead to its collapse, causing negative health impacts or even death in people with pre-existing health conditions. High humidity can limit evaporation of sweat, limiting the effectiveness of this mechanism for cooling.

Negative health effects of heat often occur early in the heat season, before people have acclimatized to the heat.

No population can avoid exposure to rising ambient temperatures, but some populations are more exposed to or more vulnerable to physiological stress, exacerbated illness, and an increased risk of death from exposure to excess heat than others. 

Populations in Europe and the Eastern Mediterranean are among the most vulnerable to the health effects of heat as a result of ageing populations, the high prevalence of chronic disease, and rising levels of urbanisation. However, the Western Pacific, South-East Asian and African Regions have all seen a dramatic increase in vulnerability of >10% since 1990.^{6 6}Lancet Countdown: Vulnerability to Extremes of Heat; The 2020 Report of the Lancet Countdown on Health and Climate X

Especially vulnerable populations include the elderly, infants and children, pregnant women, physical labourers, athletes and attendees of outdoor events (e.g. music festivals), and the poor. Gender can also play an important role in determining heat exposure.

• Pregnant women face heightened metabolic demands, which can increase core body temperature, and are also at greater risk of dehydration – making them more susceptible to heat related illness. Furthermore, extreme heat can increase risk of adverse pregnancy outcomes.
• Elderly people have reduced thermoregulatory responses, are less mobile and are more likely to be disabled or suffer from illnesses that limit the body’s ability to respond to heat and may be socially isolated.
• Indoor and outdoor workers undertaking regular strenuous activity for long periods of time, sometimes wearing Personal Protective Equipment (PPE) that can limit heat shedding, as well as attendees of mass outdoor gatherings such as concerts, sporting events and pilgrimages during high temperatures are at heightened exposure to heat and sunlight.
• Infants and children are at risk of heat-related death due to their small body mass to surface area ratio. They are at increased risk of electrolyte imbalance, fever, respiratory disease and kidney disease.
• People with pre-existing illnesses and/or taking certain medications are at increased risk of heat-related illness.

Other determinants of vulnerability include:

• Behavioural/cultural factors: When temperatures become more extreme, most people take some action to adapt to the conditions. However, some factors limit the ability to adapt, such as age, poor health or economic circumstances, and certain belief or value systems may also mean that appropriate action is not taken in response to the temperature conditions.
• Institutional capacity: Health 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.
• Physical adaptation: People 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.
• Socioeconomic status: 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 or other vulnerabilities.

There is robust evidence that climate change is increasing the frequency, intensity, and duration of heatwaves. Heat-related risk is destined to increase for decades to come, due to greenhouse gases already in the atmosphere that are rapidly warming the Earth’s climate. 

Mitigating climate change by reducing greenhouse gas emissions is imperative and urgent, as the degree and rate of future warming and impacts beyond 2100 will depend on the success of climate change mitigation efforts.

Practical, feasible, and often low-cost measures can save lives. Some of these measures include:

• enhancing the capacity of health systems to reduce risks and respond to emergencies;
• including heatwave health risks in emergency risk management plans across all sectors;
• preparing and protecting hospitals and other health infrastructure from heatwaves;
• strengthening surveillance and control of health impacts of heatwaves;
• improving the use of heatwave early warning systems by the health sector;
• Crafting local and national policies to protect workers from occupational exposure to heat, and providing energy subsidies and retrofit/efficiency programs to make homes safer for lower income individuals.
  reducing the effects of urban heat islands through policy and infrastructural interventions such as cool surfaces and urban greening / tree planting;
• building public health measures at the local level to increase community resilience to heatwaves;
• opening cooling centres, where people without access to air conditioning or water can cool off.

Heat risk management requires organization and coordination of many actors. Heat Health Action Plans are an important mechanism to strategically coordinate planning, preparedness, and action. They provide a portfolio of measures that can be taken over different time scales to increase preparedness and reduce heat health impacts. The core elements of a heat plan include:

• Accurate and timely alert systems
• A heat-related health information plan
• A reduction in indoor heat exposure
• Targeted care for vulnerable groups
• Preparedness of the health and social care system
• Long-term urban planning
• Real-time surveillance and evaluation

Heat Health Warning Systems are an integral part of Heat Health Action Plans, and are provided by National Meteorological Services. They provide heat advisories, warnings, and watches to alert decision-makers and the public so that timely action can be taken.