How does extreme heat affect your body? - Carolyn Beans

The year is 2050 and your morning is not off to a good start. School is closed for yet another heat day, meaning the kids need to stay home and the AC needs to stay on. Your usual babysitter can’t come help because the rails for their commuter train were warped by the heat. And to make matters worse, your dog is desperate for a walk, but the pavement is hot enough to give third degree burns to any paw or person that touches it.

In many parts of the world, this sweltering future is already here. On average, heat waves are happening more often with greater intensity and for longer durations. But according to a 2022 projection, by 2050, Earth’s mid-latitudes could be experiencing extreme heat between 90 and 180 days a year, with tropical regions enduring even more. So, how hot is too hot, and what can people do to handle the heat?

While human bodies are decent at managing temperature, our cooling mechanisms only work under the right conditions. When air temperatures climb, the hypothalamus tells blood vessels near the skin to widen, allowing more blood to flow near the body's surface and release heat. This hormonal cascade also turns on our sweat glands. As sweat evaporates, it pulls the heat from our skin. But if humidity is high, the rate of evaporation slows and eventually stops.

Scientists use this principle to track humidity with a metric called wet-bulb temperature, in which they wrap a wet, room temperature cloth around a thermometer to see if evaporation will lower the reading. If it doesn't, it's too humid for sweat to cool us off. A wet-bulb temperature of roughly 35°C is generally considered the limit of human survival, though current temperatures rarely reach this threshold. The US National Weather Service uses the relationship between humidity and air temperature as the basis for their heat index.

As those two metrics rise, so too does the heat index; and heat is considered dangerous if the index climbs above 39.4°C. That’s 103°F. But even a lower heat index can be hazardous over multiple days. A heat wave is a streak of two or more days of unusually hot weather for a place and season. For example, a string of 32°C days in Houston, Texas, is standard in the summer, but would constitute a heat wave in March.

And the impact of these events touches nearly every aspect of daily life. Imagine a June heat wave strikes a tropical city. The first to experience effects are outdoor workers. Their excessive sweating leads to dehydration and muscle pain known as heat cramps. If they push on, their conditions could worsen to heat exhaustion and even heat stroke— a life-threatening ailment that occurs when a body’s temperature exceeds 40°C.

Medical emergency calls spike across the city, often for children and people who are pregnant or elderly. The heat also increases hospital visits for heart, kidney, and lung-related conditions, creating an influx of patients that threatens to overwhelm medical providers. Over the following week, the city slows to a crawl. Schools and construction sites close. Airplanes need to reduce their weight limits to take off, bumping countless travelers from their flights.

Restaurants shut down as overheated kitchens become unbearable. Residents who remain inside with air conditioners stay safe. But blasting AC isn’t cheap, and many families have to choose between keeping cool and staying fed. Either way, if the heat continues, the stress of these air conditioners could overwhelm the power grid, potentially leading to city-wide outages.

These consequences are all very real. Each year, close to 500,000 people die due to excessive heat, and these extreme conditions are only growing more common. We can limit medical impacts by seeking help for heat-related illnesses, staying hydrated, and keeping people cool through public access to water and AC. But don’t let anyone tell you 1 to 2 degrees doesn’t matter. It will change our very way of life.