As climate change continues to raise temperatures around the world, the impact of extreme heat is becoming increasingly felt, including in Europe, which is warming twice as fast as the global average. Heat stress, a condition resulting from the body's struggle to regulate its temperature during prolonged exposure to high temperatures and other factors, becomes a significant problem.

What is heat stress?

Heat stress refers to accumulation of body heat generated either internally (by muscle use) or externally (by the environment). It occurs when the heat that the body absorbs from the environment exceeds the body's ability to dissipate it. It is generally the result of a combination of factors such as high ambient temperatures, humidity, physical activity, and insufficient fluid intake. The ability of the human body to maintain its internal temperature within certain limits is influenced by various factors. The thermal budget of the human body is the result of the balance between the heat produced by the human body and the heat exchanged with the surrounding environment. This balance is controlled by the body's thermoregulatory system (hypothalamus, sweat glands, circulatory system, and skin), physiology (how the body functions), and behavior, which includes things like eating and drinking, activity and rest, clothing, etc. Symptoms of heat stress include increased core body temperature, increased heart rate, rapid breathing, excessive sweating, nausea, dizziness, and in severe cases, heat-related illnesses such as heat exhaustion or heat stroke—a potentially life-threatening medical emergency.

Heat Stress and Heat Load: What's the Difference?

It is important to distinguish between these two concepts. While heat stress is an environmental stress that affects the body, thermal load refers to the physiological and psychological responses of the body to heat stress. It includes measurable changes in body function and performance as a result of exposure to heat. Physiological responses such as changes in heart rate, core body temperature, skin temperature, and sweat rate are considered indicators of heat stress. Monitoring heat stress is essential in work environments, sports, and other activities to prevent heat-related injuries and ensure the well-being of individuals.

Main risk factors for heat stress Several key risk factors contribute to an individual's susceptibility to heat stress.

• Environmental conditions: High temperatures and humidity levels play a key role in increasing risk by reducing the evaporation of sweat, the human body's main cooling mechanism. Higher humidity levels limit evaporative cooling, which can lead to heat stress and illness.
• Intense physical activity: Increases heat production in the body, making individuals more vulnerable.
• Personal factors: Age, health conditions, and medications can also make people more susceptible to heat stress. Older adults and those with existing health problems are at higher risk because their thermoregulatory system may be impaired. Certain medications, such as diuretics and medications to treat hypertension, can impair the body's ability to respond adequately to heat stress.
• Insufficient acclimatization: A hot environment is another risk factor.

How do we measure heat stress?

Heat stress can be measured using a variety of physiological and environmental indicators to assess the impact of heat on the human body. Common metrics include:

• Wet bulb thermometer (WBGT) temperature: Measures heat stress resulting from exposure to temperature, humidity, wind speed, and sunlight.
• Universal Thermal Climate Index (UTCI): It is a human heat stress index that takes into account various environmental factors to assess the “feeling” temperature and the physiological response of the human body to different levels of heat stress. The UTCI considers air temperature, radiant temperature (solar and longwave radiation), wind speed and humidityBy combining these factors, UTCI provides an accurate representation of how the human body perceives and responds to thermal environments. It is particularly useful in assessing heat stress in a variety of contexts, such as work environments, outdoor activities, and urban planning.

UTCI categorizes heat stress into different levels based on perceived temperature and corresponding physiological responses, helping to better understand and manage potential health impacts.

• No heat stress (+9 to +26 °C): Physiological thermoregulation is sufficient to ensure thermal comfort, meaning that individuals can maintain thermal balance without excessive strain on the body.
• Mild heat stress (+26 to +32 °C): Mild stress may occur, but the body's ability to regulate temperature is generally effective. Individuals may experience some discomfort, but can adapt without major physiological strain. Drinking more than 0.25 L/h is recommended.
• Severe heat stress (+32 to +38 °C): Rectal temperature increases after 30 minutes of exposure. Drinking more than 0.25 L/h and temporary reduction of physical activity are recommended.
• Very severe heat stress (+38 to +46 °C): Increased strain on the body's thermoregulatory mechanisms. Individuals may experience discomfort and prolonged exposure may lead to mild heat stress. Temporary use of air conditioning, seeking shade, drinking more than 0.5 L/h and reducing physical activity are recommended.
• Extreme heat stress (above +46°C): Severe thermoregulatory stress. This category is associated with an increased risk of heat-related health problems and measures are needed to prevent heat-related illnesses. Temporary cooling and drinking more than 0.5 L/h are recommended, and no physical activity is permitted.

The role of Copernicus and C3S

The Copernicus Climate Change Service (C3S) generates the ERA5-HEAT dataset, which can be used to assess the impact of heat stress worldwide. C3S also provides datasets for heat-inducing factors such as air temperature, relative humidity and wind speed. These data are freely available and can support policy measures to adapt to extreme heat, especially in urban environments. The European State of the Climate 2023 report states that across Europe the number of days with at least "severe heat stress" is increasing and 2023 saw a record number of days with "extreme heat stress."

Understanding and managing the impacts of heat stress is key to protecting human health and well-being in an increasingly warm world. JRi