How Hot Is Too Hot?

 

 It's 2050, and your morning is off to a rough start. School is closed due to another extreme heat day, leaving your kids at home while the AC runs non-stop. Your regular babysitter can’t help because their commuter train rails melted in the heat. Meanwhile, your dog desperately needs a walk, but the pavement is too hot to step on without risking third-degree burns. This scorching future is becoming a reality in many parts of the world. Heatwaves are more frequent, intense, and longer-lasting, and by 2050, projections suggest that Earth’s mid-latitudes could face extreme heat for up to 180 days a year, with tropical regions experiencing even more.

So, how hot is too hot, and what can we do to cope? Our bodies can regulate temperature to some extent, but our cooling mechanisms work best under specific conditions. When air temperature rises, our hypothalamus directs blood vessels to expand, increasing blood flow near the skin’s surface to release heat. Sweat is produced to cool the body as it evaporates, but if the humidity is high, evaporation slows or stops. This principle is tracked using the wet-bulb temperature, a metric that helps determine when sweat no longer cools us. A wet-bulb temperature around 35°C is the upper limit for human survival, though such temperatures are rare today. The US National Weather Service uses the heat index, which combines temperature and humidity, to gauge danger, with anything above 39.4°C (103°F) deemed hazardous. Even lower levels of heat can be dangerous when they persist over several days.

A heatwave is defined as a period of unusually high temperatures lasting two or more days. In a tropical city, a June heatwave can bring devastating effects. Outdoor workers are the first to suffer, experiencing heat cramps, dehydration, and muscle pain. If they push through, they risk heat exhaustion and, potentially, heatstroke—a life-threatening condition. Hospitals fill with patients, particularly children, pregnant people, and the elderly. As the heatwave continues, schools and construction sites close, airlines reduce weight limits for takeoffs, and restaurants shut down due to unbearable kitchen temperatures. Residents who stay inside with air conditioners are protected, but using AC is costly, and many families face the dilemma of choosing between staying cool and affording food. Extended heat can overwhelm the power grid, resulting in widespread outages.

Heat-related deaths currently number close to 500,000 annually, and these extreme events are only expected to increase. To mitigate the impacts, we must ensure access to cooling resources, stay hydrated, and provide public support for those most at risk. Every degree matters—it will profoundly affect our lives, our cities, and the future of the planet.