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Summer temperatures continue to <a href="https://www.washingtonpost.com/climate-environment/2023/07/05/hottest-day-ever-recorded/" target=_blank>break records</a> around the world, reaching the <a href="https://www.washingtonpost.com/weather/2023/07/18/extreme-heat-record-limits-human-survival/" target=_blank>upper limits</a> of what humans can survive.

While much of the focus on extreme heat is the immediate risk to human health, chronic heat can cause a range of chronic health problems. Days and years of exposure to persistent heat can tax the heart and kidneys, disrupt sleep and challenge our mental health.

And while it’s important to protect yourself from the heat, some people make the mistake of never getting acclimatized to hot weather.

“If people live in air-conditioned environments from — well, some places do it 24/7 — then people’s bodies never really acclimatize to warmer temperatures,” said <a href="https://globalhealth.washington.edu/faculty/kristie-ebi" target=_blank>Kristie Ebi</a>, professor of global health at the University of Washington Center for Health and the Global Environment. “And that is going to be increasingly important as we try to prepare better for a warmer future.”

To understand why chronic heat can be dangerous for our health, it is important to understand what our body does to try to defend its core temperature. In healthy humans, a body temperature outside of a narrow band between 98 and 99.5 degrees Fahrenheit risks damage to our cells, tissues and organs.

Our ability to thermoregulate in the face of heat relies primarily on our cardiovascular system and kidneys; as a result, chronic heat predominantly strains these two systems, sometimes to the breaking point.

“It’s that physiological vulnerability that ultimately is responsible for the negative health outcomes,” said <a href="https://www.sydney.edu.au/medicine-health/about/our-people/academic-staff/ollie-jay.html" target=_blank>Ollie Jay</a>, professor of heat and health at the University of Sydney and director of the Heat and Health Research Incubator.

When our <a href="https://www.washingtonpost.com/wellness/2023/07/06/heatstroke-exercise-brain-hot-weather/" target=_blank>central nervous system</a> detects the sweltering heat, it activates our body’s temperature regulatory systems. The blood vessels in our skin dilate and warm blood is directed from our body’s core to the surface for it to cool off. But this can also cause blood pressure to drop, which can be detrimental to those who already have low blood pressure.

To compensate for this drop in blood pressure, our heart increases the rate it circulates the blood by increasing our heart rate, straining our cardiovascular system. For people with an underlying heart condition, the increased strain of heat “increases the likelihood of a catastrophic cardiovascular event,” Jay said.

About half of the excess deaths in heat waves are caused by cardiovascular events, such as heart attacks, said Ebi who, with Jay, co-wrote a paper on the <a href="https://www.thelancet.com/article/S0140-6736(21)01208-3/fulltext" target=_blank>health risks of hot weather</a> for The Lancet. But longer-term exposure to higher temperatures could also have adverse consequences.

Heat and dehydration can take a toll on our kidneys

Sweating is the primary way our body can cool itself. When secreted from our sweat glands, the sweat evaporates and cools our skin — except when the humidity is too high for it to evaporate.

Sweating dehydrates us if the fluids are not replenished. As dehydration develops and progresses, it places more and more strain on our kidneys, which normally filter out waste and excess fluid before removing it from our body.

Over time, chronic dehydration and heat stress could lead to kidney damage and <a href="https://pubmed.ncbi.nlm.nih.gov/26088040/" target=_blank>chronic kidney disease</a>.

One meta-analysis published in <a href="https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(18)30237-7/fulltext" target=_blank>The Lancet Planetary Health</a> reported that 15 percent of over 21,000 individuals who frequently worked under heat stress experienced kidney disease or injury. Workers toiling in <a href="https://academic.oup.com/pmj/article/89/1049/123/6987462" target=_blank>Central America</a>, Sri Lanka and <a href="https://www.washingtonpost.com/world/2023/01/06/climate-change-heat-kidney-disease/" target=_blank>Nepal</a> have been found to develop chronic kidney disease at relatively young ages.

In addition, dehydration can aggravate heatstroke and cardiovascular stress by causing us to sweat less and decreasing our overall blood volume, thereby increasing how hard our heart needs to work.

“Many of the negative impacts of heat stress, they are coupled with dehydration,” said <a href="https://hhp.ufl.edu/about/faculty-staff/laitano_orlando/" target=_blank>Orlando Laitano</a>, assistant professor of applied physiology and kinesiology at the University of Florida.

Persistent heat can negatively <a href="https://www.washingtonpost.com/wellness/2023/07/06/heatstroke-exercise-brain-hot-weather/" target=_blank>impact our brain</a> and mental health as well.

A <a href="https://jamanetwork.com/journals/jamapsychiatry/fullarticle/2789481" target=_blank>2021 study</a> in JAMA Psychiatry reported that warmer temperatures are associated with higher rates of mental health emergency department visits. In a data set of over 2.2 million American adults, more extreme heat was correlated with more hospitalizations for specific mental health conditions, including substance use disorders, mood disorders and self-harm.

Hotter nights are also eroding our sleep quality, which impacts all aspects of our biology and health. A <a href="https://www.cell.com/one-earth/fulltext/S2590-3322(22)00209-3" target=_blank>recent study</a> estimated that we are already losing an average of <a href="https://www.washingtonpost.com/climate-environment/2023/05/21/climate-change-sleep-hotter-nights/" target=_blank>44 hours of sleep each year</a> because of a warming world, and we are likely to lose more shut-eye in the years to come.

Reduced sleep is replete with negative health consequences, including weakening our <a href="https://www.nature.com/articles/nri1369" target=_blank>immune system</a> and cardiovascular system, and increasing our susceptibility to inflammation and <a href="https://jamanetwork.com/journals/jamainternalmedicine/article-abstract/486518" target=_blank>chronic illnesses</a>.

Multiple nights of elevated heat and humidity also elevate the risk for heatstroke because it does not give our bodies the time to recover. <a href="https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(22)00139-5/fulltext" target=_blank>Another study</a> found that nighttime heat is also especially dangerous, and hot nights may increase the risk of mortality by 50 percent.

Learn how to sleep better when it's hot outside

Our bodies can adapt to the heat — up to a point

Our ability to tolerate the heat is not fixed and can be improved, in a process known as heat acclimatization, which takes at least seven days.

Just “like you can train your muscles to tolerate more weight, you can also train your cardiovascular system to tolerate more heat,” Laitano said.

One of the faster responses of adaptation is a blood plasma volume expansion, which increases the portion of our blood made of water and our ability to handle higher cardiac demands (though how this occurs is not quite known),  Laitano said.

Acclimatization also speeds up and increases our <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/sms.12408?casa_token=v8PFAesk_bEAAAAA%3AcHMqPOjMuzXyl4kTAJIF-k5iUPXWihe8pAMjFfVacVR44E5KeD8Ux20CXKDsBtj2UWnhAv6If7XMsQ" target=_blank>sweating</a>.

The more physically fit you are, the more likely you are to withstand the heat. This is because exercise heats up our skeletal muscles and induces partial heat acclimatization.

There is also evidence that our body remembers how to adapt to the heat if it has done it before. <a href="https://link.springer.com/article/10.1007/s40279-017-0808-x" target=_blank>Research</a> suggests that when someone is acclimated to the heat, that adaptation decays when taken out of the heat. But the re-adapting to the heat occurs much faster than the initial adaptation.

Exposure to heat changes how our genes are expressed. However, there is a kind of <a href="https://physoc.onlinelibrary.wiley.com/doi/abs/10.1113/EP090143?casa_token=PS1A25xnBpoAAAAA%3ACvWU4ef2-GIGLCgeSBuXissLI3h1IYThQ25Y7raIYRTaCtXuoiMTLSJF-JenRbmDByzju2V_8YzgJbY" target=_blank>sweet spot</a> for how much heat exposure is beneficial.

Low heat, over longer periods of time, seems to increase our resilience to future heat exposure. But experiencing more extreme heat and suffering heat illnesses such as heatstroke can cause <a href="https://pubmed.ncbi.nlm.nih.gov/33037634/" target=_blank>genetic expression changes</a> that increase the odds of future health complications.

To safely acclimatize to the heat, we need to gradually increase the heat or the level of physical exercise, and make sure not to overdo it, researchers say.

One way is “having people spend some time outdoors every day at kind of the same time so that you can get more adjusted to the higher temperatures,” Ebi said.

“You should start with low exposure in terms of duration, and then you start prolonging the exposure,” Laitano said. “Of course, it’s very important to engage the intensity as well.”

“Enjoy the summer, enjoy the longer days, enjoy the warmer temperatures. But you also have to understand that heat is a risk,” Ebi said. “Heat kills and kills unnecessarily.”

How a hot bath can help you adjust to hot weather

Do you have a question about human behavior or neuroscience? Email <a href="mailto:brainmatters@washpost.com">BrainMatters@washpost.com</a> and we may answer it in a future column.

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Johns Hopkins University, PhD in Neuroscience

Harvard University, BA in Neurobiology 

Richard Sima is a neuroscientist turned award-winning science journalist who writes the "Brain Matters" column for The Washington Post's Well+Being desk. 

Richard Sima

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