Heat waves killed over 60,000 people in Europe during the summer of 2022, making extreme heat the deadliest climate hazard on the continent according to the World Health Organization. Globally, heat-related mortality is rising faster than any other weather-related cause of death, and climate projections show extreme heat events becoming more frequent, intense, and prolonged.
Heat stress occurs when the human body cannot adequately cool itself, leading to a cascade of physiological responses that range from discomfort to organ failure and death. As one of the 12 physical risk hazards in climate risk assessment, heat stress is measured by the number of heat wave days per year at a location. Understanding heat stress causes, health effects, and risk trends is essential for protecting workers, communities, and economic productivity.
What Is Heat Stress?
Heat stress is the physiological condition that occurs when the body absorbs more heat than it can dissipate. The human body maintains a core temperature of approximately 37°C through sweating, blood vessel dilation, and behavioral responses. When ambient temperature, humidity, and physical exertion overwhelm these cooling mechanisms, heat stress develops.
Heat stress severity depends on both temperature and humidity. The wet bulb temperature, which accounts for the cooling effect of evaporation, is a better indicator of heat stress risk than air temperature alone. A wet bulb temperature of 35°C represents the theoretical upper limit of human survivability, where even healthy individuals in shade cannot cool themselves through sweating.
In climate risk assessment, heat stress is tracked as the number of days per year when maximum temperatures exceed a location-specific heat wave threshold. A location adapted to mild temperatures faces heat stress at lower absolute temperatures than a location in the tropics, because infrastructure, behavior, and physiology are calibrated to local norms.
What Causes Heat Stress?
Heat stress results from the interaction of climate conditions, built environment factors, and individual vulnerability:
Extreme heat events. Heat waves are the primary driver of heat stress. Sustained periods of temperatures exceeding the local norm by 5°C or more create conditions where overnight cooling is insufficient for recovery, and cumulative heat stress builds over multiple days.
Urban heat islands. Cities are typically 2-5°C warmer than surrounding rural areas due to heat-absorbing surfaces (concrete, asphalt), reduced vegetation, waste heat from buildings and vehicles, and altered airflow patterns. Urban populations face amplified heat stress compared to rural residents at the same latitude.
Climate change. Rising global temperatures are the fundamental driver of increasing heat stress. Each degree of warming translates to approximately 18 additional heat wave days per year at a given location. The temperature anomaly at a location directly determines how much additional heat stress it will experience.
Humidity. High humidity reduces the effectiveness of sweating, the body’s primary cooling mechanism. Tropical and subtropical regions face the highest heat stress risk because both temperature and humidity are elevated. The combination of 40°C temperature with 60% humidity is far more dangerous than 45°C in dry desert conditions.
Heat Stress Health Effects
Heat stress produces a progression of health effects that escalate from mild to life-threatening:
Heat exhaustion. The initial stage of significant heat stress, characterized by heavy sweating, weakness, nausea, dizziness, and elevated heart rate. Heat exhaustion is reversible with cooling and hydration but signals that the body’s thermoregulation is struggling.
Heat stroke. A medical emergency where core body temperature exceeds 40°C and the sweating mechanism fails. Heat stroke causes confusion, loss of consciousness, organ damage, and death without immediate medical intervention. Mortality rates for untreated heat stroke exceed 50%.
Vulnerable populations. The elderly, children, outdoor workers, people with chronic conditions, and those without access to air conditioning face disproportionate heat stress health effects. During the 2022 European heat waves, over 90% of fatalities were among people aged 65 and older.
Chronic exposure effects. Repeated heat stress exposure damages kidneys, cardiovascular systems, and cognitive function. Agricultural workers in tropical regions show elevated rates of chronic kidney disease linked to sustained occupational heat stress.
How Is Heat Wave Risk Assessed?
Heat stress assessment uses the daily maximum temperature variable (tasmax) from NASA NEX-GDDP-CMIP6 climate projections. The methodology defines a location-specific heat wave threshold and projects how heat wave frequency changes under warming scenarios.
The heat wave threshold is set at the baseline mean maximum temperature plus 5°C. A location with a baseline mean tasmax of 30°C has a heat threshold of 35°C. Each degree of warming above baseline adds approximately 18 heat wave days per year.
| Risk Rating | Heat Wave Days/Year | Heat Stress Level |
|---|---|---|
| Low | Fewer than 5 | Rare extreme heat events |
| Moderate | 5-15 | Seasonal heat exposure |
| High | 15-30 | Frequent dangerous heat |
| Severe | 30-50 | Extended heat emergencies |
| Extreme | More than 50 | Chronic heat crisis |
Climate models show high confidence that heat wave frequency and intensity will increase under all emissions scenarios. Locations currently in the Moderate heat stress category may shift to High or Severe by 2050, particularly in tropical and subtropical regions. Platforms like Continuuiti assess heat stress alongside 11 other climate hazards for a complete physical risk profile.
Heat Stress in the Workplace
Occupational heat stress affects workers in agriculture, construction, manufacturing, and outdoor services. The International Labour Organization (ILO) estimates that heat stress will reduce global working hours by 2.2% by 2030, equivalent to 80 million full-time jobs in lost productivity.
Productivity loss. Worker output drops measurably above 26°C and declines sharply above 33°C. Physical labor in extreme heat requires mandatory rest breaks, reducing effective working hours. Translating that lost output into a defensible dollar figure is the subject of our guide to heat business interruption. Heat stress is projected to cost the global economy $2.4 trillion annually by 2030.
Safety risk. Heat stress impairs cognitive function, coordination, and reaction time, increasing workplace accident rates. Construction and manufacturing sectors see elevated injury rates during heat wave periods.
Regulatory response. An increasing number of jurisdictions are implementing occupational heat stress standards requiring shade, hydration, acclimatization periods, and mandatory rest breaks when temperatures exceed defined thresholds.
Frequently Asked Questions
What is heat stress?
Heat stress occurs when the body absorbs more heat than it can dissipate through sweating and blood vessel dilation. It ranges from mild discomfort to life-threatening heat stroke when core temperature exceeds 40°C.
What are the health effects of heat stress?
Heat stress progresses from heat exhaustion (sweating, weakness, nausea) to heat stroke (organ damage, potential death). The elderly, children, outdoor workers, and people with chronic conditions are most vulnerable. Chronic exposure causes kidney and cardiovascular damage.
How is heat stress measured in climate risk assessment?
Heat stress uses daily maximum temperature (tasmax) from NASA NEX-GDDP-CMIP6 projections. The heat wave threshold is baseline mean tasmax plus 5°C, with each degree of warming adding approximately 18 heat wave days. Risk is rated from Low to Extreme.
How does climate change affect heat stress?
Climate change directly increases heat stress by raising baseline temperatures. Each degree of warming adds approximately 18 heat wave days per year. All climate models project increases in heat wave frequency, intensity, and duration under all emissions scenarios.
What industries are most affected by heat stress?
Agriculture, construction, manufacturing, and outdoor services face the greatest heat stress impacts. The ILO projects heat stress will reduce global working hours by 2.2% by 2030. Productivity drops above 26°C and falls sharply above 33°C.
Heat stress is the climate hazard with the highest confidence of increasing under all warming scenarios. As temperatures rise, locations currently experiencing moderate heat stress will cross into high-risk categories, affecting worker productivity, public health, and energy demand. Screening heat stress exposure across time horizons identifies where cooling infrastructure and heat action plans need to be in place before extreme heat becomes the norm.
