Don’t underestimate heat. While less visible than other extreme weather events like hurricanes or floods, heat consistently accounts for more U.S. fatalities than any other type of weather disaster. Around the world, extreme heat now kills one person every minute, according to a recent report, a rate that has risen 23 percent since the 1990s. Heat deaths are also chronically undercounted, both in the U.S. and around the world. Outdoor workers are among the most vulnerable, along with older people, children, and those with chronic health conditions. But heat can affect anyone, and our own biases about what constitutes good weather (sunny summer days are glorious) often downplay its risks. Grist has been reporting on the steadily increasing threat of extreme heat for years. So as we contend with continued record-breaking years of temperature highs, we’re rounding up some of that reporting to help you understand why it’s vital to take heat seriously, and how to stay safe. The rising threat of heat A sign posted in Phoenix, Arizona during the city’s 2023 heat wave. Mario Tama / Getty Images To put it simply, hot places are getting hotter, and places that have traditionally not experienced extreme heat and are less prepared to weather it are increasingly subject to dangerous temperatures. The risk of heat is increasing. Any given heat event can’t be solely attributed to climate change, but there is no doubt that heat waves are happening more often and reaching higher highs as a result of climate change. In fact, scientific consensus is now that climate change has played a role in making almost all heat waves more likely or more severe. And yes, even in years that experience extreme cold weather, the average overall temperature is rising. Other factors can amplify the warming effects of climate change, like the naturally occurring El Niño weather pattern. In an El Niño year, global heat is typically exacerbated by high ocean temperatures in the Pacific. The hottest years on record tend to happen during El Niño. The term heat wave generally describes any period of unusually hot weather. But some of the most dangerous heat waves are caused by what’s known as a heat dome. In a heat dome, a high-pressure system traps hot air in one region, feeding off itself to increase heat and humidity the longer it stagnates in one place. Humidity makes heat even more dangerous. Any type of heat can be dangerous when your body can’t pump out sweat fast enough to stay cool. But heat and humidity play together to affect your ability to cool off. Higher humidity makes it harder for sweat to evaporate, reducing your body’s ability to cool itself naturally. That’s why you may hear not just about temperature, but about something called ‘wet bulb temperature,’ a measure that includes both heat and humidity — essentially the temperature we experience after sweat cools us off. When the wet bulb temperature crosses 95 degrees Fahrenheit, our bodies lose the ability to cool down entirely — a threshold that has already been breached more than a dozen times, mostly in Pakistan and the Arabian Peninsula. But stress on the body can start much lower than that. Even healthy young adults can experience health effects at a wet bulb temperature as low as 86 degrees F. Heat affects your whole body A poster at a Los Angeles warns about heat-related illness in 2026. Justin Sullivan / Getty Images Exposure to high heat has immediate health effects. Extreme heat forces the heart to pump two to four times as much blood per minute to cool the body. That can lead to dehydration, which makes blood thicker and harder to circulate, and worsen into heat exhaustion or heatstroke, or lead to heart failure. On particularly hot days, burns from scorching hot surfaces, including pavement, are an added risk. Each year, hundreds of thousands of people in the U.S. end up in the emergency department due to heat illness and related cardiovascular, respiratory, and kidney issues. What to watch for: The first symptom of severe dehydration is typically cramps, which might feel like normal workout cramps. Signs of heat exhaustion include muscle aches or cramps; headache; excessive sweating and thirst; feeling lightheaded or dizzy; nausea or vomiting; and pale, cool, clammy skin, especially on the extremities. To combat heat exhaustion, bring down a person’s body temperature by removing heavy clothes, drinking water, and moving to shade or AC. Laying them down with elevated legs and applying damp cloths or misting with water can also help actively cool the body. Save this quick tip sheet from Grist’s Instagram. Adapted from Extreme Heat 101. Heatstroke occurs when the body loses its ability to cool itself and reaches dangerous internal temperatures. It can result in permanent brain injury or loss of life. Look out for symptoms like confusion, irritability, or unresponsiveness; severe fatigue; a fast pulse; nausea, vomiting, or diarrhea; lack of sweat (though sweating is possible too); and hot and dry skin. Call 911 if you think someone is experiencing heat stroke, then dunk them in cold or iced water, or apply ice on areas near large blood vessels, like the groin, armpits, neck, and core. Save this quick tip sheet from Grist’s Instagram. Adapted from Extreme Heat 101. Heat’s health effects also reach much deeper. Exposure to extreme heat affects us in all sorts of other ways that doctors and scientists are just beginning to understand. For example, heat has been shown to increase rates of violent crime, and has been linked to worsening mental health including suicide. Long-term exposure to heat can result in chronic health issues, like cardiovascular disease. Outdoor workers in some of the hottest parts of the world are developing an unusual form of chronic kidney disease linked to extended dehydration and heat stress. Heat affects all of your body systems. These are a few key ones. Find even more here: How climate change gets under the skin Nighttime heat is a particular driver. Nighttime temperatures are increasing faster than daytime temperatures in much of the world, eliminating a crucial window for cooling down. We also sleep worse when it’s hot, already losing 45 hours of sleep per year due to the heat. Ways to prepare Improvised window coverings in a Paris apartment during the June 2026 heat wave. Sophia Berger / Hans Lucas via AFP / Getty Images Your house makes a big difference. If a heat wave is headed your way, dehumidify your house to get the indoor humidity down. Seal off openings around windows and doors to prevent hot air from entering. Cover windows to prevent the sun from heating up the indoors — blackout shades, blinds, or even DIY solutions work. In a low humidity environment, you can even DIY your own air conditioning. Keep water, electrolytes, and ice on hand, at home and at work, and identify cool places to go, like a public library. This isn’t just useful to ease boredom — heat can place additional strain on the electricity grid as everyone runs their air conditioners, sometimes resulting in power outages. In the long term: Retrofits like reflective roofs, window overhangs to block the sun, and better insulation can provide more relief. And as you’re preparing, don’t forget to check on vulnerable neighbors and community members. Grist has even more advice for safety and preparedness. Go deeper on community readiness and workplace safety here: Extreme heat 101 And more tips on emergency preparedness, including for heat and power outages, can be found here: How to prepare for a disaster Stay safe out there (or, better yet, inside where it’s cool). This story was originally published by Grist with the headline Why heat is so deadly and how to stay safe on Jul 9, 2026.