A dangerous mix of extreme heat and humidity settled over parts of the eastern U.S. during the Fourth of July, disrupting holiday traditions like parades, cookouts, and fireworks.
These conditions can quickly turn dangerous because the body's cooling response comes at a cost: as sweating ramps up, blood volume falls, the heart is pushed harder, and the odds of a serious medical emergency rise fast.
What's happening?
As Gizmodo reported, doctors and heat experts are warning that major heat waves can strain nearly every organ in the body, especially when high temperatures are paired with heavy humidity.
Europe is currently enduring an extraordinary heat wave, with France hitting a record temperature on June 23. Officials there said heat-related deaths topped 1,000 between June 24 and 27, while one preliminary estimate suggested the total death toll across the continent could exceed 20,000.
Orlando Laitano, a University of Florida exercise physiology professor and lead investigator of the Laboratory of Muscle and Environmental Physiology, said the body's main cooling responses are widening blood vessels near the skin and producing sweat.
Humidity makes that cooling system much less effective. As Laitano put it, "the more you sweat, the less blood volume you have available."
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As that fluid loss builds, the cardiovascular system has to compensate by making the heart pump faster and harder so vital organs keep getting blood.
Why does it matter?
Craig Crandall, a UT Southwestern internal medicine professor who leads the Thermal and Vascular Physiology Lab at Texas Health Presbyterian Hospital in Dallas, said this added workload on the cardiovascular system can increase the chances of major health events such as heart attack and stroke.
Heat stress also affects much more than the heart. Less blood reaches the intestines and kidneys, which can damage the gut barrier, while the kidneys have to work harder to conserve water.
"The brain is not functioning properly," Crandall said, helping explain why overheated people may have trouble walking, speaking, or thinking clearly.
Some groups face greater risk than others, including older adults and young children, along with pregnant people, athletes, outdoor workers, those with chronic conditions, and people without air conditioning.
Still, Riana Pryor, a University at Buffalo associate professor of exercise and nutrition sciences who directs the Hydration, Exercise and Thermoregulation Lab, said everyone is vulnerable during a major heat wave, especially over holiday weekends, when people often spend extra time outside and consume alcohol.
What can I do?
Pryor said, "If you can adjust your schedule, that's the first thing you'll want to do."
That can mean moving outdoor activities to cooler parts of the day or taking frequent breaks indoors in air-conditioned spaces.
Hydration is also essential, and waiting until you feel thirsty may already be too late.
"If you are feeling thirsty, you are already dehydrated," Pryor said. "Once you get behind with consuming fluids, it's really hard to catch up."
Troublingly, older adults may not notice thirst as quickly. If someone shows signs of heat illness, such as fatigue, nausea, confusion, headache, or a sudden change in behavior, cooling them down quickly is critical. Move them into shade, or preferably into air conditioning, and begin rehydration once they're in a cooler place.
If heat stroke is possible, seek medical attention immediately. Pryor said cold water works best, whether through a cool bath, shower, or cold, wet towels.
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