A spotty evening shower or thunderstorm, then partly to mostly clear and a bit muggy. .

A spotty evening shower or thunderstorm, then partly to mostly clear and a bit muggy.

Long-term exposure to high heat can become lethal. Mark Wilson/Getty Images

Heat waves are becoming supercharged as the climate changes — lasting longer, becoming more frequent and getting just plain hotter. One question a lot of people are asking is: “When will it get too hot for normal daily activity as we know it, even for young, healthy adults?”

The answer goes beyond the temperature you see on the thermometer. It’s also about humidity. Our research shows the combination of the two can get dangerous faster than scientists previously believed.

Scientists and other observers have become alarmed about the increasing frequency of extreme heat paired with high humidity, measured as “wet-bulb temperature.” During the heat waves that overtook South Asia in May and June 2022, Jacobabad, Pakistan, recorded a maximum wet-bulb temperature of 33.6 C (92.5 F) and Delhi topped that — close to the theorized upper limit of human adaptability to humid heat.

People often point to a study published in 2010 that estimated that a wet-bulb temperature of 35 C — equal to 95 F at 100% humidity, or 115 F at 50% humidity — would be the upper limit of safety, beyond which the human body can no longer cool itself by evaporating sweat from the surface of the body to maintain a stable body core temperature.

It was not until recently that this limit was tested on humans in laboratory settings. The results of these tests show an even greater cause for concern.

To answer the question of “how hot is too hot?” we brought young, healthy men and women into the Noll Laboratory at Penn State University to experience heat stress in a controlled environment.

These experiments provide insight into which combinations of temperature and humidity begin to become harmful for even the healthiest humans.

S. Tony Wolf, a postdoctoral researcher in kinesiology at Penn State and co-author of this article, conducts a heat test in the Noll Laboratory as part of the PSU Human Environmental Age Thresholds project. Patrick Mansell / Penn State, CC BY-NC-ND

Each participant swallowed a small telemetry pill, which monitored their deep body or core temperature. They then sat in an environmental chamber, moving just enough to simulate the minimal activities of daily living, such as cooking and eating. Researchers slowly increased either the temperature in the chamber or the humidity and monitored when the subject’s core temperature started to rise.

That combination of temperature and humidity whereby the person’s core temperature starts to rise is called the “critical environmental limit.” Below those limits, the body is able to maintain a relatively stable core temperature over time. Above those limits, core temperature rises continuously and risk of heat-related illnesses with prolonged exposures is increased.

When the body overheats, the heart has to work harder to pump blood flow to the skin to dissipate the heat, and when you’re also sweating, that decreases body fluids. In the direst case, prolonged exposure can result in heat stroke, a life-threatening problem that requires immediate and rapid cooling and medical treatment.

Our studies on young healthy men and women show that this upper environmental limit is even lower than the theorized 35 C. It’s more like a wet-bulb temperature of 31 C (88 F). That would equal 31 C at 100% humidity or 38 C (100 F) at 60% humidity.

Similar to the National Weather Service’s heat index chart, this chart translates combinations of air temperature and relative humidity into critical environmental limits, above which core body temperature rises. The border between the yellow and red areas represents the average critical environmental limit for young men and women at minimal activity. W. Larry Kenney, CC BY-ND

Current heat waves around the globe are approaching, if not exceeding, these limits.

In hot, dry environments the critical environmental limits aren’t defined by wet-bulb temperatures, because almost all the sweat the body produces evaporates, which cools the body. However, the amount humans can sweat is limited, and we also gain more heat from the higher air temperatures.

Keep in mind that these cutoffs are based solely on keeping your body temperature from rising excessively. Even lower temperatures and humidity can place stress on the heart and other body systems. And while eclipsing these limits does not necessarily present a worst-case scenario, prolonged exposure may become dire for vulnerable populations such as the elderly and those with chronic diseases.

Our experimental focus has now turned to testing older men and women, since even healthy aging makes people less heat tolerant. Adding on the increased prevalence of heart disease, respiratory problems and other health problems, as well as certain medications, can put them at even higher risk of harm. People over the age of 65 comprise some 80%-90% of heat wave casualties.

Staying well hydrated and seeking areas in which to cool down — even for short periods — are important in high heat.

While more cities in the United States are expanding cooling centers to help people escape the heat, there will still be many people who will experience these dangerous conditions with no way to cool themselves.

The lead author of this article, W. Larry Kenney, discusses the impact of heat stress on human health with PBS NewsHour.

Even those with access to air conditioning might not turn it on because of the high cost of energy — a common occurrence in Phoenix, Arizona — or because of large-scale power outages during heat waves or wildfires, as is becoming more common in the western U.S.

A recent study focusing on heat stress in Africa found that future climates will not be conducive to the use of even low-cost cooling systems such as “swamp coolers” as the tropical and coastal parts of Africa become more humid. These devices, which require far less energy than air conditioners, use a fan to recirculate the air across a cool, wet pad to lower the air temperature, but they become ineffective at high wet-bulb temperatures above 21 C (70 F).

All told, the evidence continues to mount that climate change is not just a problem for the future. It is one that humanity is currently facing and must tackle head-on.

W. Larry Kenney receives research grant funding from the National Institutes of Health/National Institute on Aging.

Daniel Vecellio is supported by a training grant from the National Institute on Aging through the Penn State Center for Healthy Aging.

Rachel Cottle is supported by a training grant from the National Institute on Aging through the Penn State Center for Healthy Aging.

S. Tony Wolf is supported by the National Institutes of Health.

This article is republished from The Conversation under a Creative Commons license.

2021 is already shaping up to be a hot year, with warmer than average spring temperatures and hot streaks across much of the continental U.S into June. In May 2021,  there were 947 record warm daily high temperatures recorded across the U.S., according to the National Oceanic and Atmospheric Administration (NOAA). Stacker scoured scientific studies, DIY websites, and even American history to come up with 15 ways to stay cool on warm days without AC.

For anyone not living along the northern edges or mountains of the U.S., the dog days of summer feature weeks—if not months—on end of intolerable heat. Sizzling temperatures have been shown to negatively affect memory, reaction time, productivity, moods, and even suicide rates. One study by Harvard University in 2016 tracked students before and after a heatwave and determined that higher temperatures correlated with decreased cognitive function; another by Stanford University in 2018 studied the links between suicide rates and temperature and ultimately theorized that bumps in temperature from now through the year 2050 may correlate with an added 21,000 deaths from suicide.

For the times we can't be in air conditioning 24/7, there are thankfully many low-cost, energy-efficient ways to cool off during summer heat that work and don't involve sitting in a bath of ice water all day. These tricks can make a difference when your attention is required in a classroom or office setting, or if you're working outside and need relief from the hot sun.

Keep reading to see a simple hack for using skin care products to bring down your core temperature, how hot beverages may actually cool you off, and how to up your use of fans to beat the heat this summer.

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Despite one's best efforts to reapply sunscreen as directed, being out in the sun all day can still lead to sunburn and skin damage. The medicinal use of aloe vera to help treat skin ailments can be tracked all the way back to the 4th century B.C. The antiseptic aloe vera is ultimately a wonder plant: In skin care applications it acts as a cleanser, astringent, and moisturizer, and it can stimulate the growth of new skin tissue, while also calming the nervous system. For an extra cooling effect, mix water and aloe vera in ice cube trays, and pop them into the freezer for use when needed.

Whether your skincare routine involves multiple products or just one, keeping them in the refrigerator can make for a refreshing reprieve from the heat. Many lotions, serums, sprays, and masks will even keep their potency for longer when kept cold.

There is no better way to heat up a home in the summer than by switching on the oven, burners, or toaster oven. Avoid that entirely by grilling outside, eating out, or switching things up with meals that don't require the stove on the hottest days. Salads, cold sandwiches, cold soups like gazpacho, and nori rolls are all great options for hot days.

A well-placed bandana that's been soaked in cold water will work wonders to cool you off. Be sure to tie it around a major pressure point (ankles, wrists, neck, or elbows) and it can provide an instant chill.

Strategic placement of fans throughout the home can completely change an indoor climate. Window fans pulling in cooler outside air should be positioned on the first floor of the shadiest, coolest side of the home (most likely in a north-facing window). A fan blowing out the window, which will suck warmer air from inside, should be placed on the sunniest, hottest side of the second floor (or same floor if it's a one-story home or apartment). Ceiling fans should be rotating counter-clockwise (from your vantage point below it)—don't forget to reverse this direction during cooler months!

Besides keeping your body hydrated, drinking water helps regulate internal temperatures. Keep your hydration levels topped off throughout the day and evening to ensure heat is being managed most effectively by your most powerful weapon. The American Heart Association recommends drinking water in equal measure to how many ounces of sweat you're expelling—for a high school football player, that can be up to five pounds in one summer practice, or 80 ounces.

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Hot water bottles are built to keep people warm on cold nights, but they can also serve the opposite purpose. Fill your hot water bottle (or a regular water bottle, if that's what you have) with tap water, put it in the freezer for a few hours, and then sleep with the bottle at your feet. Your feet are one of the most temperature-sensitive parts of your body, so keeping them cool will help the rest of your body stay cool.

While a west-facing home may be perfect in winter, it makes cooling the house down in the summer a bit more challenging. Stock up on reusable UV window film to keep the sun's harmful rays out and the cool air in (it will also work to keep your home warm in the winter). Tinfoil and space blankets work great for blocking heat from windows, too, and may save you a few bucks.

Cotton is a great fabric for the summer whether you live in the dry heat of Arizona or humid Florida; this light material allows air to circulate without getting trapped in one place, cooling your body naturally. Other fabrics that can help you fight off the sweat are chambray, rayon, linen, and synthetic blends, and lighter colors are generally cooler than darker colors.

Drinking a lot of water is one way to stay hydrated during the summer, but it can also be done through eating foods that have high water content. This includes fruits and vegetables such as melons, cucumbers, lemons, grapes, leafy greens, and berries. Spicy foods prepared with chili peppers are great coolers as well: the chilis increase circulation and cause perspiration. And while ice cream might seem refreshing in the moment, it will only make you feel warmer in the long run—flavored ice is a good alternative.

It seems counterintuitive, but drinking something hot on a warm, dry day can help to cool a body off. A Thermal Ergonomics Lab study found that drinking a hot beverage increases sweat production, which in turn cools the body off. There's just one caveat—the sweat needs to be able to evaporate in short order. This trick will not work if you're wearing a lot of clothing that can trap sweat or if the scorching day is paired with high humidity.

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Strategically opening windows at night when the air is coolest and keeping them closed (with curtains drawn!) during the day is one of the most straightforward ways to keep a home cooler in summer months. This method is most effective if you have adequate insulation in the walls to trap the cooler night air inside. Another window trick is to take advantage of convection: When double-hung windows are open, keep them as such at the top and the bottom. Having the top window cracked allows hot air to escape, while the bottom window being cracked will suck in cooler air from outside.

Plan ahead for hot nights: fold up a top sheet, put it in a bag, and set said bag in the freezer a few hours before bed. When it's time to hit the hay, take the sheet out and lay it on top of you. For a last-minute (slightly messier) iteration of the same technique, before going to bed wet down a top sheet in cold water, wring the sheet out well, and put it over yourself.

In addition to keeping windows closed during the day and open at night, use blackout curtains to trap cool air inside and keep the heat from coming in. They've been said to reduce heat transfer through windows by as much as 24%—that's a lot of degrees when it's sweltering outside.

Pranayama is the practice of breath control—and utilizing it can make you feel noticeably cooler. Two breaths, in particular, that can help are the sitali (“cooling breath”) and sitkari (“sipping” or “hissing”). In the sitali, the mouth is opened into an “O,” and the tongue folded lengthwise into a tube to suck air through. After sticking the curled tongue out almost an inch from the lips, a big breath is drawn in, using the tongue like a drinking straw. This will pull cool air into the diaphragm. After breathing in, the mouth is closed and air slowly exhaled through the nose. The sitkari variation of this breath involves closing the top and bottom teeth together with lips open in a smile. Long, slow breaths are drawn in this position through the teeth and mouth, then the mouth is closed and the breath exhaled slowly through the nose.

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