Heat kills more Americans each year than any other weather-related event. In 2025, 430 people in the Valley died from heat-related causes, according to the Maricopa County Department of Public Health.
And after a record-hot spring, Maricopa County has already confirmed 16 heat-related deaths this year — with 192 more under investigation.
But how does hot weather turn fatal?
The human body can function only within a relatively small temperature range, said Dr. Amelia Gallitano, professor of basic medical sciences at the University of Arizona College of Medicine-Phoenix and co-leader of the university’s Heat and Health Resilience Innovation Consortium.
In hot conditions, the body tries to regulate its temperature — a person who is overheating will sweat as the body tries to create evaporative cooling and they may appear flushed as the body sends more blood to the skin to try to release heat.
But those processes have limits, Gallitano said.
“If you are dehydrated, you no longer have that extra fluid to be able to sweat. So you will stop sweating and then you stop cooling,” Gallitano said.
As the body’s core temperature rises, the heart strains to pump blood. Organs don’t get enough oxygen because blood flow is disrupted. The kidneys, stressed by dehydration, struggle to filter out toxins.
“Everything goes haywire,” Gallitano said. “Ultimately then, what happens is when you start to reach these really high temperatures, then basic cellular function breaks down.”
As cells break down, Gallitano said they release toxins into the body which can cause inflammation and swelling, known as edema.
“And edema in the brain can very rapidly lead to brain dysfunction and death,” Gallitano said.
People who spend longer hours exposed to heat face the highest risk. About half of heat-related deaths in Maricopa County last year were among people experiencing homelessness, in spite of the fact that homeless people make up less than 1% of the Valley’s population.
Certain underlying health conditions can also make the body more vulnerable to the dangerous impacts of heat.
“If you have heart disease that is going to impair your heart's ability to meet that extra demand put on it when your body gets hot,” Gallitano said.
Cardiopulmonary disease contributed to 47% of heat deaths in Maricopa County last year. Diabetes, obesity and kidney disease also factored into some heat-related deaths in the county. And older adults, who are more likely to have those underlying conditions, face disproportionate risk in high heat — 60% of the county’s heat death in 2025 were among people 50 or older.
Substance use can also inhibit the body’s ability to sweat, raise the body’s temperature, or raise the heart rate, exacerbating the effects of heat. Substance use factored into 55% of Maricopa County’s heat-related deaths last year.
But extreme heat can be dangerous for anyone, whether or not they have underlying conditions or use substances.
The Maricopa County Department of Public Health notes even people who are acclimated to hot weather face risks. In Maricopa County last year, 82% of people who died from heat-related causes had lived in Arizona for 10 years or more, while only 3% of the Valley’s heat deaths involved out-of-state residents.
The department recommends everyone take precautions in extreme temperatures:
- Drink water throughout the day, even before you feel thirsty
- Wear lightweight, light-colored clothing
- Stay in air conditioned spaces as much as possible
- Check on vulnerable friends, family or neighbors on hot days
- Call 2-1-1 for help finding cooling locations and other support services
- Follow heat advisories and avoid outdoor activity during peak heat
- Never leave children or pets in cars
- Recognize the signs of heat illness and act quickly if someone appears ill
- Call 911 if someone is confused, collapses, or stops sweating
