With more than 160 million people under extreme heat advisories as the Fourth of July weekend approaches, warnings about the dangers are clear. Heat is the leading cause of weather-related deaths in the United States. It can also make you flushed, irritable and worse at math.
A growing body of research has shown that our brains work differently when temperatures spike. Test scores fall and drivers honk more often. The good news is that we’re learning a lot more about how heat affects the brain. In fact, hundreds of peer-reviewed papers on the connection between heat and health have been published in recent years.
But there are longer-term effects of exposure to heat to consider, too. Recent research has found that hotter weather may make the brain more vulnerable to air pollution, increasing the risk for dementia and Parkinson’s disease.
This year, temperatures have already broken records in Europe and surpassed 120 degrees Fahrenheit in Australia. The temperature in hundreds of cities across the United States have reached record highs over the past few days and are expected to remain elevated into the holiday weekend. That means people are increasingly being exposed to heat stress and its accompanying health concerns.
Today, we’re going to explain what we know about the effects extreme heat can have on the human brain, and what science says about preventing the worst outcomes.
A hot day and the brain
“Our brains work best under certain optimal temperatures and environments,” Burcin Ikiz, a neuroscientist and the director of the research group EcoNeuro, said at a recent panel at the National Academies.
“Anything that deviates from that, even a couple of degrees centigrade or a different environment, can cause stress on our brains,” she added.
The reaction can make people feel slower, angrier and less able to focus.
Several studies have found higher temperatures correlate with lower test scores. A 2025 analysis of three million tests taken by students from third to eighth grade found that math test scores worsened on hot days, with the effect being particularly pronounced at schools with higher poverty rates. The researchers speculated that the higher-poverty schools with the bigger drops may have had worse air-conditioning.
High temperatures are also associated with increased accident rates at workplaces and upticks in violent crime. Researchers studying “reactive aggression” have found that people playing video games in a hot room acted more spitefully than those in a cool room.
Air pollution and heat, a potent combination
Last year, the State of Global Air report, an annual study put together by the Health Effects Institute and the Institute for Health Metrics and Evaluation, which tracks air quality and research, included a section on brain health for the first time in its history.
The authors wrote that recent developments in research that “strengthened the evidence linking air pollution to neurodegeneration” necessitated the change. Air pollution killed nearly eight million people globally in 2023, the report found.
Exposure to particulate air pollution, known as PM 2.5, is associated with Alzheimer’s disease and dementia in older adults, they wrote. It can also affect brain development in young people, increasing the risk of anxiety and depression. The report cited an estimate that one in four dementia deaths is attributable to air pollution.
Dr. Ikiz said that heat might make the blood-brain barrier more permeable, allowing more pollutants into the brain. The effect is most pronounced in children and older adults, she added.
Other recent research has found that improving air quality can have unexpected effects. When China began aggressive efforts to reduce air pollution in 2013, suicide rates decreased, according to a 2024 study by researchers at the University of California, Santa Barbara, and the Chinese University of Hong Kong.
What can be done?
At the individual level, the advice for extreme heat is straightforward: Try to stay cool. If air-conditioning isn’t available, close the curtains, drink cold fluids and avoid using the oven or stove. To remain protected against air pollution, stay indoors with the windows shut, keep an eye on the air quality index and avoid outdoor exercise when advised.
City officials are doing their part by opening cooling centers, repainting dark surfaces, investing in shade and converting air-raid shelters into heat relief zones.
A person cannot do much to control local air quality overnight. But the State of Global Air report also notes that it improved across much of the world between 2010 and 2023, and the richest countries tend to enjoy the cleanest air.
Many governments continue to improve air conditions by imposing limits on pollution from coal power plants and vehicle exhaust, though wildfire smoke remains a growing concern.
There can be trade-offs for better air quality. The particulate matter that causes health problems in humans actually cools the climate by blocking the sun. Removing it from the atmosphere has been shown to worsen extreme heat.
A wildlife regulator faces questions about benefiting from her position
Last summer, federal officials were considering the fate of more than 100 ruffed lemurs, a critically endangered species known for the fluffy fur around its neck.
4J Conservation Center, an animal breeding facility based in Florida, had applied to renew its permit to raise the lemurs in captivity. Without the permit, the business would be forced to pause operations or shut down.
The application landed on the desk of Jenifer Chatfield, a top wildlife regulator at the Interior Department. Dr. Chatfield knew the business well: Her family owns 4J Conservation Center, and its application listed her as one of its veterinarians.
Within a day, Dr. Chatfield advanced the application through the agency’s complex permitting process.
It was not the only time that Dr. Chatfield’s government role intersected with her family’s business, according to hundreds of pages of documents reviewed by The New York Times. — Maxine Joselow
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