Going outside in this heat? Follow these guidelines to stay safe

People cool off at Washington Square Park during hot weather in New York City on Monday. Selçuk Acar/Anadolu/Getty Images hide caption

As the holiday weekend approaches, large swaths of the Midwest and the Eastern U.S. are under extreme heat warnings or heat advisories, according to the National Weather Service .

Heat is a serious health threat. Hundreds of people die of heat-related illness in the U.S. each year. Heatstroke can damage your body , leading to multiple organ failure or kidney failure, and it can trigger a heart attack.

If you plan to spend time outdoors this week, here's a roundup of advice from NPR reporting on how to protect yourself and your loved ones.

For the latest stories on the science of healthy living, subscribe to NPR's Health newsletter .

COMIC: How excessive heat kills and how to stay safe

1. Assess the heat risk before you go outside

There's not an absolute temperature at which extreme heat turns dangerous. Rather, it depends on your individual circumstances, how acclimated to heat you are and how long you're exposed to the heat, Lewis Halsey, a professor of environmental physiology at the University of Roehampton in the U.K., told NPR.

Humidity is also a key factor, he says. Sweating works better to cool you off in dry heat than in humidity. Wind flow can also affect the risk of overheating.

"If it's a very humid day and there's no wind speed, then you are more at risk, even if the air temperature is lower," Ashley Ward , director of Duke University's Heat Policy Innovation Hub, told NPR's Life Kit .

Keep an eye out for heat advisories or warnings in your area issued by the National Weather Service. These typically take into account the local heat index, which includes humidity as well as the air temperature, to get a more accurate read on how hot it will feel outside.

But they are not foolproof, says Nick Staub, incident commander for extreme heat response in Maricopa County, Ariz. "We see a significant number of heat-related deaths on days that are not an extreme heat warning," Staub said, speaking on NPR's Short Wave.

Goats and Soda

How to stay cool in extreme heat without air conditioning

One reason for this is that the heat index values are calibrated for shady locations with light wind. If you're in direct sunlight, it can be 15 degrees hotter than in the shade.

Instead of relying on heat indices alone, check the National Weather Service's HeatRisk tool, which takes several factors into account, including: how far above normal the current temperature is for your location and time of year; heat duration; and humidity forecasts.

Carrying your own shade is one way to stay cool. Experts advise avoiding the midday sun altogether. Alex Wroblewski/AFP via Getty Images hide caption

2. Know if you're high risk

Some people are at higher risk for heat-related illness, as NPR has reported , and they need to take extra care to stay cool.

This includes people who aren't acclimatized to the heat because they are from cooler regions or spend little time outdoors. Infants and small children can be particularly at risk, partly because of their small body size. Kids also might not notice how they're feeling, or not communicate it clearly, so it's important to watch for changes in their behavior that might suggest they're overheating.

People in their 60s and older may have a reduced ability to sweat and thus cool off. Pregnant women's bodies also may not cool off well, and heat can raise the risk of pregnancy-related health problems. People with certain chronic health conditions or on certain medications may have trouble too.

And finally, outdoor workers or others who do strenuous outdoor activity and those without access to air conditioning are at high risk.

3. Check prescriptions — heat and some medicines don't mix

Certain medications can make people more vulnerable to heat. For instance, blood pressure drugs , which millions of people take, are diuretic. "Those medications are trying to get rid of fluid from your body," Wafi Momin, a cardiologist at Memorial Hermann Health System in Katy, Texas, told NPR . And if you then add heat, "that can cause a very dangerous situation" of dehydration.

Other types of medications can impair your body's ability to cool off, including anticholinergic medications (a category that includes Benadryl) and stimulants, like ADHD drugs. ACE inhibitors, commonly prescribed for heart issues, make it harder to notice you're thirsty, and beta blockers — another heart medication — can decrease sweating.

"When you're taking these medications and you're exposed to heat, it's like asking a car to drive up a mountain in the summer with the air conditioner broken," David Eisenman, a physician and researcher at the University of California, Los Angeles, told NPR. "Overheating becomes much more likely."

Psychiatric medications can also interfere with your body's thermoregulation. Antipsychotics and some antidepressants can reduce how much you sweat.

Sedative medications like benzodiazepines may "reduce alertness and perception of heat, says Dr. Ashwini Nadkarni, a psychiatrist and medical director of Brigham General Adult Psychiatry at Mass General Brigham in Boston.

Patients should still take their prescribed medications, Eisenman stresses. But he urges them to take heat risks seriously and make a plan to keep cool.

Ask your doctor whether any of your medicines could increase your risk of heat-related illness. And you can explore a list of medicines that contribute to heat sensitivity from the Centers for Disease Control and Prevention.

4 . Know the difference between heat exhaustion and heat stroke and when to seek emergency care

The warning signs of heat exhaustion include fatigue, extreme thirst, nausea, headache, shortness of breath, rapid breathing, muscle cramping and feeling dizzy or lightheaded.

Heatstroke is a much more serious emergency. The warning signs include all the above but also may include a strong, rapid heartbeat, confusion, vomiting, seizures, slurred speech, or passing out. Watch for a body temperature that exceeds 103 degrees F.

The first step to help anyone who exhibits symptoms of heat exhaustion or heatstroke is to get them to immediately stop what they are doing and rest somewhere out of the sun, Dr. Matt Leonard , attending emergency physician at Suburban Hospital, Johns Hopkins School of Medicine, told NPR .

Take steps immediately to cool the person who's overheated: Get them into air conditioning if possible, or at least shade with a fan or breeze. Mist them with a spray bottle, for example, or pour water over their head. The most important places to cool are the head and face, the armpits and the groin, he says.

If symptoms point to heatstroke (watch especially for neurological symptoms like confusion), get them to an emergency room right away.

Staying hydrated and letting your skin aerate can help you stay cool. Spencer Platt/Getty Images hide caption

5. Keep cool: Avoid midday sun and choose protective clothing

One of the simplest ways to keep your body temperature at a reasonable level on a hot summer day is to only go outside in the morning or evening when it's cooler. And stay in the shade, especially if you're doing anything active.

"That midday sun can add as much as 20 degrees or more to the actual air temperature," Carol Ewing Garber , professor of movement science at Columbia University, told NPR . "You really have to be careful about that."

And it's important to protect your skin, says Leonard. Your skin is an "amazing self-cooling apparatus," he says. It's designed "to move warm blood away from your core to keep your vital organs cool."

Consider wearing lightweight clothing that blocks the sun , and include a hat. Leonard recommends wicking athletic fabrics that allow you to sweat.

Choose loose-fitting silhouettes, recommends Dr. Renee Salas , assistant professor of emergency medicine at Harvard Medical School. If your clothes are too tight, the sweat on your skin can't evaporate, which is key to keeping your body cool. "I'm sure people don't love sweating profusely, but sweating is actually our friend when it comes to heat," Salas told Life Kit .

6. Prehydrate — and rehydrate often

If you're outside or in a hot place, dehydration can happen fast. So prepare by hydrating in advance, says Momin, of Memorial Hermann Health System. "Grab a glass of water or a sports drink before you head out to the outdoors," he says.

Bring plenty of water with you, and don't wait until you're thirsty to start drinking during an outdoor activity. "The moment you begin to feel thirsty, you're likely anywhere between 10 to 25% dehydrated already," says Neil Gandhi , a physician at Houston Methodist Hospital.

Water is the best way to hydrate. Adding electrolytes is not necessary under normal circumstances. But they can be helpful if you've gotten overheated or you're doing something very physically active and sweating a lot. In that case, it can be a good idea to replace those electrolytes promptly, Asher Rosinger , an associate professor and director of the Water, Health, and Nutrition Lab at Penn State University, told NPR .

7. If you struggle with mental health, take extra care

Higher temperatures are known to have an impact on mental health. Studies show that higher temperatures are linked to higher rates of visits to the Emergency Department for mental health conditions and addiction.

And suicide risk goes up too, a recent study found . "We think that every summer roughly an additional hundred young adults are dying by suicide due to increasing temperatures," says study author and psychiatrist Dr. Joshua Wortzel at the Hartford HealthCare Institute of Living, who runs the institute's Heat Mind Lab.