In a year marked by record-breaking heat, experts are raising alarms about heat stress, a condition that claims more lives than any other climate-related extreme. Heat stress occurs when the body's natural cooling mechanisms are overwhelmed, leading to symptoms such as dizziness, headaches, organ failure, and even death. It is caused by prolonged exposure to high temperatures and other environmental factors that disrupt the body's ability to regulate temperature.

Alejandro Saez Reale of the World Meteorological Organization (WMO) describes heat as a "silent killer" because its symptoms are often not immediately obvious. Vulnerable groups include infants, the elderly, those with health issues, and outdoor workers. Urban residents are also at higher risk due to heat-absorbing materials like concrete and brick. The WMO estimates that heat kills around half a million people annually, but the true toll could be up to 30 times higher.

Heat Stress
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As climate change intensifies, heatwaves are becoming longer, stronger, and more frequent, exposing more people to dangerous conditions. However, maximum temperature readings alone do not fully capture the threat. Factors such as humidity, wind speed, clothing, direct sunlight, and the presence of concrete or greenery all influence how the body perceives and responds to heat.

Various methods exist to measure heat stress, each combining different environmental factors into a single index. The wet-bulb temperature method, for instance, considers both temperature and humidity. Exposure to 35 degrees Celsius with 100% humidity for six hours can be lethal, as the body cannot cool itself through sweating.

The Universal Thermal Climate Index (UTCI), used by the EU's climate monitor Copernicus, accounts for temperature, humidity, wind, sunshine, and radiated heat to classify heat stress levels. Extreme heat stress, with a "feels like" temperature of 46 degrees Celsius and above, requires immediate action to prevent health risks.

In the US, the Heat Index provides an "apparent temperature" based on heat and humidity in the shade, while Canada uses the Humidex rating to reflect perceived temperature and associated risks. Other indices include the Tropical Summer Index and Predicted Heat Strain, but their effectiveness varies by climate.

Heatwave expert John Nairn notes that some measures work better in certain climates than others. For instance, the UTCI is effective in Germany but less so in global south countries, which might benefit more from wet-bulb temperature readings. These indices also often overlook the impact of heat on infrastructure, which can further endanger human health when systems fail.

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