Extreme heat is sending unprecedented numbers of Americans to emergency departments—and causing untimely deaths. Scientists, clinicians, and public health officials emphasize the importance of staying hydrated to prevent heat-related illnesses. However, determining the right amount of liquid for proper hydration can be challenging, as needs vary based on individual circumstances and body types. Scientific American consulted with physicians and researchers to provide guidance on how to stay hydrated during heat waves and other weather conditions.
Why Is Hydration Especially Important in Extreme Heat?
Heat-related illnesses can occur whenever people are exposed to high temperatures, especially those above 90 degrees Fahrenheit (32 degrees Celsius). Humid climates exacerbate this risk by making sweating less effective, says Stavros Kavouras, director of the Hydration Science Lab at Arizona State University.
High temperatures cause the body to lose water, reducing blood volume and forcing the heart to work harder to support vital organs. Dehydration limits sweating, which can lead to overheating. Initially, this can result in painful muscle cramps for those exercising outdoors. However, if left unchecked, it can escalate to heat exhaustion, and eventually to the potentially fatal condition of heat stroke.
Recognizing the signs of heat-related illness is crucial. According to Sheri Belafsky, an occupational and environmental medicine physician at UC Davis Health, “When I’ve seen people progress into heat stroke, it was because they weren’t aware of what was happening to them, and the people around them didn’t know they were unwell.” Common symptoms of heat exhaustion include profuse sweating, fatigue, and nausea. While not immediately fatal, repeated heat exhaustion can lead to chronic health issues like kidney disease. Heat stroke, characterized by body temperatures of 104 degrees F (40 degrees C) or higher, can cause confusion, unconsciousness, and seizures, leading to major organ failure without emergency medical attention.
What Heightens the Risk of Dehydration?
Certain groups are at higher risk for dehydration, including those who spend extended periods outdoors or engage in physical activity, such as outdoor laborers and athletes. Other factors include age and medical status. Older adults tend to sweat less efficiently, and the bodies of pregnant people and children struggle more with temperature regulation. Many medications, including beta-blockers, stimulants, antipsychotics, and antidepressants, can impair the body’s ability to detect or combat dehydration. Additionally, individuals with heart or kidney conditions may face exacerbated health risks.
How Can You Tell If You’re Properly Hydrated?
Thirst is not always a reliable indicator of dehydration, explains Kavouras. Our ability to recognize thirst diminishes with age, and certain medications, like antidepressants and weight-loss drugs, can further dull this signal. Experts recommend keeping a water bottle handy and tracking daily intake. Health guidelines suggest that women consume 1.6 to 2.1 liters of water per day, while men should aim for two to three liters. These recommendations increase in hot weather. A well-hydrated person should urinate every two to three hours, with urine generally appearing pale yellow (though diet and medication can influence its color).
For those spending time outdoors in extreme heat, Brandee Waite, a sports medicine doctor at UC Davis Health, advises drinking a glass of water every half hour. While overhydration is rare, it can occur if someone drinks more than a liter of fluid per hour.
Does Only Water Count for Hydration?
Not at all! Many foods, especially fruits and vegetables, contain water. In addition to meals, people need to consume liquids to meet health guidelines. Beverages like milk and some fruit juices are primarily water and contribute to overall hydration.
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