A heat dome is baking the United States. Here’s why that’s so dangerous

June is the new July. Or maybe even August. At least it feels that way, as summer heat has already soared to record highs.

In the United States, West Coast residents sweltered earlier in the month as a high-pressure weather system called a heat dome trapped record-breaking high temperatures over the region (SN: 7/19/23). Now, another heat dome is bringing another wave of extreme heat to swaths of the Midwest and East Coast, with temperatures forecasted to reach close to 38° Celsius (100° Fahrenheit) in many cities.

It is early for this kind of heat, although probably not unprecedented, says Benjamin Zaitchik, a climate scientist at Johns Hopkins University. “We’re seeing temperatures that have been more common for late July, early August. We’re seeing them this year in June, which is unusual. Weather can do that.” With heat waves increasing in frequency and intensity around the globe, it’s part of the pattern of climate change.

Such early heat waves can be more dangerous than ones that happen later in the summer, Zaitchik says. That’s in part because our bodies have adjusted to the heat by late summer, a bit like acclimatizing to a hot water bath. Such sudden hot temperatures can also catch people by surprise. Outdoor activities that would normally be perfect for June weather in temperate locations can turn risky.

In addition, across whole regions, overnight temperatures aren’t dropping enough to provide relief. Those regions light up magenta on a new online tool called HeatRisk, developed by U.S. public health and climate experts to track dangerous heat up to a week out (SN: 4/22/24).

As temperatures across the country sizzle, here’s what to know about heat and health.

Extreme heat is hard on our bodies

Heat can be deadly. In the United States, heat causes more fatalities than any other natural disaster, including floods, tornados and hurricanes.

Our bodies normally have ways to handle the heat. As blood circulates just below the skin’s surface, it releases heat. Evaporating sweat helps to cool our skin. But these methods work only up to a certain point (SN: 7/27/22). And as temperatures and humidity climb ever higher during heat waves — dampening our ability to cool down with dispersal or evaporation — we become more vulnerable to unrelenting heat. 

Several body systems become stressed under prolonged hot temperatures (SN: 8/6/23). Pushing blood toward the skin in an effort to cool the body can deprive vital organs like the heart and lungs of oxygen. Excess sweating increases the risk of dehydration, putting additional pressure on the heart to pump blood thickened from lack of water. The kidneys, which need water to regulate fluid levels and filter toxins from blood, also suffer.

Body temperatures that climb too high can lead to faintness, headache and dizziness, hallmarks of heat exhaustion. Heat exhaustion can progress to heat stroke if the body doesn’t cool down, causing overheated, oxygen-deprived organs to fail. People with heat stroke may become delirious or have seizures, suffer from permanent organ damage or even die.

Some people are at higher risk

Staying hydrated, limiting activity and camping out in air-conditioned buildings can help people (and their pets) stay safe during heat waves.

But for some, following that guidance is easier said than done. Unhoused people or workers whose jobs require them to be outdoors in high heat might not have the option to remain in the cool indoors. Those living in urban areas with few trees or in humid climates can also be at higher risk than people living in other, naturally cooler places (SN: 4/3/18).

Others may have health conditions that make staying cool more important because those conditions put individuals at higher risk for heat-related symptoms. For example, extreme temperatures can put added stress on the hearts of pregnant people, who are already under tremendous pressure to pump more blood to two bodies. Heat exposure has been linked with preterm births and low birthweights, according to the U.S. Centers for Disease Control and Prevention.

Adults over 65 are also at higher risk, in part because older people can’t cool their bodies as efficiently as they could when they were younger. Individuals in this age group are also more likely to have chronic diseases such as diabetes or heart problems that heat can make worse. Older people with coronary artery disease who were exposed to increasing temperatures had decreased blood flow to the heart compared with people the same age without the disease, researchers report June 11 in Annals of Internal Medicine.

Research suggests that by 2050, nearly a quarter of the global population of people age 69 and older could experience temperature extremes beyond 37.5° Celsius (SN: 5/14/24).

Heat is dangerous for younger people with chronic conditions, too. Hot weather can cause air pollution to spike, triggering asthma attacks. And some medications such as metformin for diabetes can cause dehydration; antipsychotic drugs for conditions like schizophrenia can dampen the sweat response.   

Communities must adapt as late summer heat hits earlier

Finding ways to beat the heat isn’t easy.

As everything gets warmer under climate change, scientists have seen a shift toward hotter temperatures happening earlier. Changing atmospheric circulation and climate feedback loops can lead to a higher number of days hitting those extremes, which can mean more heat waves, Zaitchik says (SN: 11/9/23). “This is potentially leading to extremes that are intensifying at a rate that’s even faster or even more intense than what we’re seeing on average.”

With more hot days than in the past, researchers are scrutinizing ways to protect the most vulnerable. Checking in on high-risk people who are largely solitary to make sure they’re in a cool space or have water can save lives. Cities can make sure that bus stops are out of direct sunlight or that buses are running on time and scheduled so that people taking public transportation don’t have to wait half an hour in the heat.

But it’s important to work with communities to ensure people are getting what they want as well as what they need, Zaitchik says. For instance, air conditioners can help only if people can afford to pay the electricity bill. “We have to move quickly, but also robustly and together so that we don’t have false starts. We don’t have a lot of time to fail and try again.”