Excessive outdoor heat presents serious direct and indirect health risks.

  • High temperatures can amplify the risks of drought, wildfires, air pollution, contaminated or insufficient water supplies, cyclones, infectious disease and UV radiation.
  • These combined hazards present significant indirect health effects due to altered human behaviour, disease transmission, compromised energy, transport, water, and health service infrastructure, and hazardous air and water quality.
  • Multi-hazard risk management across a range of actors and sectors is critical.

Cascading Risks

Drought

Warmer temperatures can enhance evaporation from soil, making periods with low precipitation drier than they would be in cooler conditions. Droughts can persist when very dry soils and diminished plant cover can further suppress rainfall in an already dry area. Severe droughts impact the agriculture, transportation and energy sectors, and can lead to wildfires (source).

Relatively short but severe periods of warm air temperature combined with anomalously low and rapidly decreasing soil moisture can result in what’s known as a heatwave flash drought, impacting agricultural production and food security.


Water scarcity

As global temperatures continue to rise, rainfall will increasingly be experienced in extremes: with long dry spells, dangerous floods – and in some places, intense water shortages. As early as 2025, the World Health Organization estimates that half of the world’s population will be living in water-stressed areas (source). Management of all water resources will need to be improved to ensure water security for all.

Wildfires

While fire is a natural and essential part of ecosystems, warming temperatures and drying soils – both tied to climate change – have contributed to observed increases in wildfire activity in many parts of the world. Earlier snowmelt and higher temperatures, and resulting drier soils from increased evaporation in addition to greater water loss from vegetation, have contributed to lengthening the fire seasons in some areas. As the climate continues to warm, wildfires are likely to remain a risk over the next few decades. We must take action to improve forest and fire management practices and reduce our reliance on fossil fuels to limit the risks of worsening wildfires (source).

Air quality

Extreme heat can interact with and compound the impacts of air pollution. Heat and sunshine intensify ground-level pollution by mixing with nitrous oxide gases (from sources like car exhausts) to create ozone, a pollutant. This is the reason why hot days are often also hazy, which can be detrimental to health, particularly for those with lung conditions like asthma and cardiovascular disease. The combination of heat and air pollution leads to higher death rates than either factor working independently. When wildfires occur during a heatwave, smoke can significantly increase the risk of death.

Cyclones

Increasingly destructive storms are putting a growing number of people and structures at risk in coastal regions worldwide. In some ocean basins, the intensification of hurricanes over time has been linked to rising ocean temperatures (source).  Heatwaves are sometimes observed to occur following tropical storms and cyclones. Communities may find themselves devastated by the storm and often without electricity, only to experience a heatwave.

UV radiation

Human exposure to solar ultraviolet radiation is associated with skin cancer and malignant melanoma among the most severe health effects, and a series of other health effects have been identified.

The sun is by far the strongest source of UV radiation in our environment, and sunburn can significantly impact the body’s ability to get rid of heat. Overexposure to sunlight is also widely accepted as the underlying cause for harmful effects on the skin, eye and immune system. Experts believe that four out of five cases of skin cancer could be prevented, as UV damage is mostly avoidable.

Epidemics

Some of the most virulent infectious diseases are highly sensitive to climate conditions. For example, temperature, precipitation and humidity have a strong influence on the reproduction, survival and biting rates of the mosquitoes that transmit malaria and dengue fever, and temperature affects the life-cycles of the infectious agents themselves. The same meteorological factors also influence the transmission of water and food-borne diseases such as cholera, and other forms of diarrhoeal disease. Hot, dry conditions favour meningococcal meningitis – a major cause of disease across much of Africa. All of these diseases are major health problems. Diarrhoea kills over two million people annually, and malaria almost one million. Meningitis kills thousands, blights lives and hampers economic development in the poorest countries. Some 50 million people around the world suffer from dengue fever each year (source).

While temperature and heat play a limited role in determining where and when many other infectious diseases occur, hotter weather can compound impacts and worsen outcomes for those who are already ill, and can increase transmission rates as people congregate outdoors and in public spaces. An additional caseload of heat stress patients can place strain on healthcare and other critical systems at the same time as infectious disease outbreaks, pushing them beyond capacity.

Hotter conditions are also expected to impact health seeking behaviours. Some people experiencing heat illness may be afraid to seek healthcare during outbreaks of certain diseases for fear of infection in a hospital; or those experiencing symptoms of heat stress may seek care for fear of having an infection.

Built Environment

Infrastructure 

Extreme heat can leading to damage to important infrastructure, including roads, runways, and railroad tracks.

Transport 

Transport infrastructure is vulnerable to extreme heat.  Railway lines, airport runways, bridges and road systems not built for extreme thermal conditions can all be damaged and disrupted by extreme heat.

Energy

Electrical grids are highly vulnerable to failure and reduced efficiency during hot weather. Hot weather almost always comes with a surge of increased energy demand for air conditioning and cooling. Increased demand can physically stress power plants and transmission grids in multiple ways, resulting in failures or managed “brownouts” to minimize potential damage.  The loss of electricity for cooling when it is needed most, can dramatically contribute to exposures to dangerous indoor temperatures.

Management and Adaptation Solutions

Detrimental impacts of extreme heat in the environment require serious action on climate change, as well as comprehensive preparedness and risk management.

Climate Action

Mitigating climate change by reducing greenhouse gas emissions is imperative and urgent. Heat-related problems are destined to increase for decades to come, due to greenhouse gases already in the atmosphere that are rapidly warming the earth’s climate. However, the degree and rate of future warming and impacts beyond 2100 will depend on the success of climate change mitigation efforts. Meeting the targets of the UNFCCC Paris Agreement is critical for us all.

Preparedness

Seasonal and sub-seasonal preparedness, complemented by short-term early warning systems, are key components of heat action plans, health interventions and emergency response actions. All well-functioning action and alert systems rely on strong cross-disciplinary and multi-agency collaboration, with effective communication between stakeholders including national and local governments, universities, media, healthcare and social protection systems, NGOs and humanitarian actors, as well as affected populations.

Location and context specific risk management

Epidemiological studies, social science, risk assessment and heat forecasting capabilities are fundamental to incorporate the differentiated needs of vulnerable groups into risk planning, and to inform appropriate and effective responses. Multi-disciplinary understandings of the risk context and perceptions are critical to effective intervention design.

Learn More

For information on personal cooling and heat illness detection and management, visit our section on managing and adapting to heat in the body.