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This past week, extremely high temperatures blanketed much of the South and Western United States. But figuring out whether conditions are too hot for people to safely conduct their normal outdoor activities is trickier than it might seem.
That’s why the U.S. Centers for Disease Control and Prevention and the National Oceanic and Atmospheric Administration’s National Weather Service launched a new website in April to forecast when people are especially at risk.
Thermometer readings and accompanying humidity levels aren’t the only factors determining safety when it comes to heat, says Aaron Bernstein, director of the National Center for Environmental Health at the CDC. Other important elements include elevated nighttime temperatures, which prevent the body from cooling down; whether hot days are outside of traditionally warm seasons; the duration of the heat (many days versus just one or two); and whether temperatures breach the top 5 percent of the record-high days for a given location.
“A hundred degrees in Boston is not the same as 100 degrees in Houston, when it comes to how heat affects health,” Bernstein says. The disparity flows in part because New England residents are not as accustomed to avoiding the outdoors during the hottest part of the day and because fewer likely have air conditioning in their homes, among other factors. The new forecast tool, called HeatRisk, takes all this into consideration when grading health impacts from heat across the U.S. The tool uses a five-color code ranging from green (no risk) to magenta (extreme risk).
Another important feature is it forecasts seven days. Currently, heat advisories from the National Weather Service are issued just 12 hours before dangerous temperature conditions emerge.
“If you have an event planned six days from now for the middle of the afternoon on a day when the heat risk level is red, if you can move it to a time of day that is cooler or to another day, you have the opportunity to do that,” Bernstein says.
Heat kills thousands of Americans each year
Although other phenomena like floods, tornados, hurricanes, and even cold are often more dramatic, heat waves are the most deadly weather events in the U.S., killing more than twice as many people as any other. In 2023, 2,300 Americans died from heat-related illnesses, according to preliminary data. It’s a number that has been rising for several years.
Moreover, the official fatality figure is a severe undercount, says Jane Gilbert, the chief heat officer for Florida’s Miami-Dade County. Deaths on hot days due to heart attacks or kidney disease, which are exacerbated by temperature, or accidents occurring after someone gets woozy from heat, aren’t generally included.
Experts have linked extreme heat to increased hospital visits by people with diabetes, kidney disease, and mental-health issues including anxiety and mood disorders, as well as to pregnancy complications.
Heat also boosts the risk of dehydration and muscle cramps, as well as the nausea, fatigue, and dizziness caused by heat exhaustion. At this point, if respite from the heat is not quickly achieved, heatstroke can follow. This dangerous condition, where the body’s internal temperature rises to 104-degrees Fahrenheit, results in confusion, vomiting, loss of consciousness, and potentially permanent brain or organ damage, as well as death.
(Here’s what extreme heat does to the body.)
More hot days. Longer heat waves.
With climate change causing more frequent and longer heat waves worldwide, recognizing when heat can be deadly is essential, says Kurt Shickman, formerly the director of the extreme heat initiative at the nonprofit Adrienne Arsht-Rockefeller Foundation Resilience Center in Washington, D.C.
Experts say climate change exacerbated the African heat wave in April that killed 102 people during a four-day period, as well as last July’s blazing temperatures in Phoenix, where thermometers soared above 100 degrees every day of the month.
Climate change is also responsible for more hot days outside of waves, which carries its own perils. One analysis found numerous excess deaths occur during regular hot days in the U.S. when temperatures reach the 80s or 90s.
(How heat can make you sick—and kill you)
Colored alerts separate risk levels
The HeatRisk color-coding system identifies who is most at risk for heat related health issues under specific conditions.
A yellow alert indicates that most people in a particular location can safely be outdoors all day. But the same isn’t true for sensitive people who develop headaches or heat-triggered symptoms when temperatures rise.
Orange alerts indicate danger for older people and children, pregnant women, those with chronic medical conditions including heart disease or poor circulation, and people taking medications that interfere with internal heat regulation. This includes diuretics and certain antipsychotics, antidepressants, and antihypertensives.
When the alert blinks red or magenta, everyone in that geographic area is potentially in peril. More than one in five heat-related deaths in the U.S. have occurred in people considered in the prime health years, ages 15 to 44.
Outdoor workers are an especially vulnerable population, Shickman says. “There is a need for a more concerted approach to outdoor workplace safety,” with employer-provided shade, rest, and water breaks becoming standard business practice, he says.
When heat is deadly
When a given area is under an orange heat alert, the CDC advises people at risk to stay hydrated, avoid the sun during the hottest parts of the day, and use air conditioners or open windows and turn fans on at night to cool down.
On red-alert days, which the National Weather Service notes are common in the southern U.S., everyone is advised to stay hydrated, remain indoors, ideally shift outdoor plans to another day or reschedule to a cooler time, and to find a location with air conditioning for at least a few hours.
On magenta days, people should “strongly consider” cancelling outdoor activities and check on neighbors who may need extra assistance.
Air quality levels additionally influence health on hot days, Bernstein notes. This is partly because people without air conditioning tend to open windows more and increase their exposure to toxins in the air, including smoke from area wildfires. Higher temperatures also often coincide with stagnant air that traps smog or ozone pollutants.
(Ground-level ozone is getting worse. Here’s what it means.)
Because of this link between heat and air quality, the CDC recently released a HeatRisk dashboard that allows people to enter their zip code and view both metrics for their geographic area.
Bernstein hopes the online tools protect the growing number of Americans at risk from heat. “Our goal is to make sure we have new pathways to continually keep people safe,” he says.
“We see a real need to bring attention in places where heat has not been a challenge historically,” he says, as well as to areas traditionally prone that may not realize hot days are more persistent now.
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