Heat & Health

Rising global temperatures have led to an increase in the frequency and intensity of extreme heat and heatwaves, which pose significant risks to individuals and communities.

Prolonged exposure to extreme heat can lead to a range of health issues, including heatstroke, dehydration, respiratory problems, and exacerbation of existing conditions.

Vulnerable populations, such as the elderly, children, and those with preexisting health concerns, are particularly at risk.

In 2023 alone, infants and adults over 65 experienced over 13 billion days exposed to heatwaves, an increase of over 20% compared to 2022. On average, each infant faced over 8 extra days of intense heat, while older adults experienced an additional 9 days compared to levels from 1986–2005. Source 2024 Report of the Lancet Countdown

Heat-related health impacts are also reflected in mortality rates. Between 1990–99 and 2014–23, heat-related deaths among adults over 65 have more than doubled, underscoring the growing toll of extreme heat on vulnerable populations. Source 2024 Report of the Lancet Countdown

The impacts extend beyond health to daily life. In 2023, heat stress reached record highs, resulting in lost sleep, restricted physical activity, and a staggering $835 billion in lost labor productivity worldwide. Source 2024 Report of the Lancet Countdown

Developing countries face the heaviest burden, amplifying global inequalities as they struggle with both health and economic losses tied to climate change. Source 2024 Report of the Lancet Countdown

The good news is that every heat death is preventable. Saving lives demands immediate action in the form of early warning systems, multi-sectoral public policy, public health initiatives, infrastructure improvements, and mitigation of climate change to safeguard human lives.

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, 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).