Heat is a serious threat to human health. The number of people exposed to extreme heat is growing exponentially due to climate change in all world regions.
Heat stress is the leading cause of weather-related death and can exacerbate underlying illnesses including cardiovascular disease, diabetes, psychological distress, asthma, and increase the risk of accidents and infectious disease.
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The Lancet Countdown: Heat Stress
The global proportion of people at risk to heat stress is increasing. The Lancet Countdown is tracking our exposure and vulnerability to changes in heat caused by climate change.
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.
Sensitivity
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.
Why are some people more vulnerable than others?
How Does Heat Affect Health?
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.
How Heat Affects Our Bodies
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Source: Killer Heat in the United States, UCS, 2019, p.9
Organs Damaged by Heat Exposure
Organs Mechanisms
Ischemia (limited blood flow) Heat Cytotoxicity (cell toxicity) Inflammatory Response Disseminated Intravascular Coagulation Rhabdomyolysis (muscle tissue breakdown)
Brain
Heart
Intestines
Kidneys
Liver
Lungs
Pancreas
Organs | Mechanisms | ||||
---|---|---|---|---|---|
Ischemia (limited blood flow) | Heat Cytotoxicity (cell toxicity) | Inflammatory Response | Disseminated Intravascular Coagulation | Rhabdomyolysis (muscle tissue breakdown) | |
Brain | |||||
Heart | |||||
Intestines | |||||
Kidneys | |||||
Liver | |||||
Lungs | |||||
Pancreas |
How Does Heat Affect Health Systems?
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).
Monitoring and Measuring Impacts of Heat on Health
There are a range of indicators available to help governments monitor and measure the impact that high temperatures have on public health. Below are common indicators used to measure the impact of health and wellbeing. Tool tip Learn more about network priorities for Vulnerability Science (Pillar 2).
Indicators for Monitoring and Measuring Impacts of Heat on Health
Metric Good Practice Indicators Examples
Excess all-cause mortality
- Estimated daily excess all-cause mortality by age group and region (1-64)(Over 65)
UK Heatwave Mortality Monitor
European Monitoring of Excess Mortality
Heat-related mortality
- Number of summertime heat-related deaths by year;
- Annual rate for deaths classified by medical professionals as “heat-related” based on death certificate record
US-CDC Environmental Public Health Tracking
US EPA Climate Change Indicators: Heat-Related Deaths
Heat-related illnesses
- Hospitalization rates for “heat-related” illnesses such as heat exhaustion, heat cramps, mild heat edema (swelling in the legs and hands), heat syncope (fainting), and heat stroke, based on hospital discharge records for patients who are admitted to the hospital for 23 hours or more;
- Age-adjusted rate of hospitalization for heat stress per 100,000 population;
- Crude rate of hospitalization for heat stress per 100,000 population;
- Number of hospitalizations for heat stress;
- Heat Stress Emergency Department Visits
US EPA Climate Change Indicators: Heat-Related Illnesses
US-CDC Environmental Public Health Tracking
US-CDC Heat Stress Illness Indicators
Emergency department visits for heat stress
- Annual age-adjusted rate of emergency department visits for heat stress per 100,000 population;
- Annual crude rate of emergency department visits for heat stress per 100,000 population;
- Annual number of emergency department visits for heat stress.
US-CDC – Environmental Public Health Tracking
US-CDC Indicators and Data
NIHHIS Experimental Heat Health Monitor
Energy demand
- Heating and cooling degree days
US-EPA Climate Change Indicators
Productivity loss
- Working hours lost to heat stress;
- Percentage of GDP lost to heat stress
Working on a warmer planet: The impact of heat stress on labour productivity and decent work (ILO)
Exposure measures
- Number of Extreme Heat Days;
- Dates of Extreme Heat Days;
- Number of Extreme Heat Events;
- Dates of Extreme Heat Events
US CDC Indicator: Historical Extreme Heat Days and Events
Temperature distribution
- Daily estimates of maximum temperature for summer months (May-September);
- Daily estimates of maximum heat index for summer months (May-September)
US-CDC Indicator: Temperature Distribution
Metric | Good Practice Indicators | Examples |
---|---|---|
Excess all-cause mortality |
| UK Heatwave Mortality Monitor European Monitoring of Excess Mortality |
Heat-related mortality |
| US-CDC Environmental Public Health Tracking US EPA Climate Change Indicators: Heat-Related Deaths |
Heat-related illnesses |
| US EPA Climate Change Indicators: Heat-Related Illnesses US-CDC Environmental Public Health Tracking US-CDC Heat Stress Illness Indicators |
Emergency department visits for heat stress |
| US-CDC – Environmental Public Health Tracking US-CDC Indicators and Data NIHHIS Experimental Heat Health Monitor |
Energy demand |
| US-EPA Climate Change Indicators |
Productivity loss |
| Working on a warmer planet: The impact of heat stress on labour productivity and decent work (ILO) |
Exposure measures |
| US CDC Indicator: Historical Extreme Heat Days and Events |
Temperature distribution |
| US-CDC Indicator: Temperature Distribution |