A body adjusted to the heat knows how to sweat.
To keep the internal organs cool, blood flows to the skin at a higher rate. There’s more sweat, and it’s diluted more to reduce electrolyte loss (a key problem in dehydration). The body slows down its metabolic rate and heart rate for a lower core temperature, basically consuming less oxygen.
But it takes weeks of consistent exposure to heat to build up all this tolerance. We’re at our best when the heat doesn’t catch us off guard. A summer athlete might be familiar with this process, called acclimatization: The key is taking it slow, all while hydrating and taking breaks to cool down.
Climate change is making a safe, slow adjustment to heat much harder by upending what we’d typically expect as seasons change. Summers are getting longer and more intense, encroaching on winter and extending long into the fall.
Large parts of Asia have been hit particularly hard the past two weeks. Axios reported how heat records have fallen throughout China, India, Bangladesh, and Thailand, as areas have surpassed 100 degrees Fahrenheit (40 degrees Celsius). Thirteen people have died of heat stroke so far in India. Thailand saw a new all-time temperature record for the month. And more than a dozen Chinese provinces broke new heat records.
It’s been exactly a year since Pakistan and India last experienced similarly shocking temperatures; last May, half of the US population faced a historic heat wave 20 to 30 degrees Fahrenheit above average. Temperatures this spring are already hitting historic highs in the 90s throughout the US.
Although late summer can bring more extreme temperatures, these early heat waves take a particularly dangerous toll. “It’s well documented that there’s greater mortality earlier in the season because people are not acclimatized to the higher temperature,” said University of Washington professor of global health Kristie Ebi, an author for the Intergovernmental Panel on Climate Change’s (IPCC) sixth assessment.
Another public health researcher, Boston University professor Patrick Kinney, explained that acclimatization can also help in understanding why heat deaths are so uneven across regions.
“In the northern part of the United States, like Boston or New York or Chicago, it doesn’t have to get as hot before you start seeing people dying in the summer on hot days. Whereas if you go down to like Atlanta or Houston, it has to get much hotter before you start to see the mortality go up,” he said.
Being unprepared for the heat can be worse than the heat itself. Acclimatization offers us one clue as to why it is so dangerous. But there is a lot more that’s in our control to ensure we’re helping the most vulnerable adapt and adjust.
Older people, outdoor workers, and people taking certain medications are threatened by early-season heat
A large body of research tends to find that early summer heat waves can have higher mortality compared to later in the season. It changes how we think about heat to consider its year-round effects and not just the longest and most extreme exposures.
The same earlier-is-worse trend can be true of a particular event: Even in a prolonged heat wave, there is more mortality at its start than its end.
There are a few explanations at play. One theory is that the most vulnerable populations (older adults and people with chronic conditions) will succumb to the higher temperatures at the start of the season, so there are fewer vulnerable people in the more intense heat waves a few months later.
A second explanation is the acclimatization effect, since people need time to adjust to heat. How much time a person needs to physiologically adapt depends. “People acclimatize at different rates, depending on your age, physiology, or the previous exposures you’ve had,” Ebi said.
A long list of populations is especially vulnerable to heat, including adults older than 65, children under the age of 1, people who take certain prescription medications, people with certain chronic diseases, outdoor and agricultural workers, houseless people, and pregnant people. Dehydration, heat illness, and death are all risks for the more vulnerable, and studies have linked exposure to high heat in the last trimester to lower birth weight in babies.
Climate change is making it harder for us to adjust
Climate change is throwing out the usual expectations of when heat becomes too much.
One of the issues is that nights are a lot hotter than they used to be. The ability to cool down at night is one of the most important factors to preventing heat illness. But hot nights disrupt sleep and strain the body.
“In general, since records began in 1895, summer overnight low temperatures are warming at a rate nearly twice as fast as afternoon high temperatures for the U.S. and the 10 warmest summer minimum temperatures have all occurred since 2002,” according to the National Oceanic and Atmospheric Administration.
There’s also a difference between humid and dry heat. Humidity makes it feel hotter, so areas that are used to coping with dryer heat may be ill-adapted for different conditions.
Places typically used to dry heat, like Southern California, are expected to face more humidity. Warmer air is capable of holding more moisture — approximately 7 percent more for every degree Celsius —and the higher surface temperatures are also causing more water to evaporate. This not only lends itself to big precipitation events, but more uncomfortable temperatures overall.
Finally, the world is experiencing more weather whiplash, including wild fluctuations in temperature. The relationship between weather whiplash and climate change is trickier to establish, but some studies suggest that it may be shifting how air moves around the North Pole, which is causing the polar vortex to wobble and spill more cold air to the south (toward the US). It helps explain why parts of the US experienced unusually cold winter storms.
Climate change has thrown out any sense that heat vulnerability follows a pattern limited to July and August. Experts like Ebi argue it requires a shift in thinking about heat risks year round so we can better prepare for the heat and help the most vulnerable at the times it’s most needed.
Early heat is more dangerous in areas with ill-adapted infrastructure
The way we adapt to early-season heat isn’t just about physiological changes in our bodies. Behavior makes a difference too, as do policies and infrastructure that help people cope.
Those of us not accustomed to the heat may compensate in other ways, such as running the AC earlier or keeping cool by water or in the shade. Green landscaping, white surfaces, and breathable clothing can also help us adjust.
But access to all these resources is another matter, and we see huge disparities based on income, race, geography, and politics.
“A heat wave in some part is a natural phenomenon, but there are political choices that have been made over decades and centuries that have withheld vital resources, infrastructure, disaster preparedness, green amenities from low-income communities of color,” said Michael Méndez, an environmental policy and planning assistant professor at the University of California Irvine. “And it’s no surprise that when a disaster strikes, the communities that are the hardest hit are least prepared.”
Trees are a prime example of this disparity. Historically redlined communities still have significantly less tree cover (estimated at 21 percentage points less in one paper) than areas that didn’t face racial discrimination. Green spaces temper the effects of heat waves: In addition to providing shade, they lower the temperature, especially compared to the radiated heat that comes from concrete.
Schools are also unevenly equipped for the heat. Across 58 countries, every additional day of temperatures over 80 degrees Fahrenheit lowered kids’ test scores, a paper published in the journal Nature Human Behavior found. When researchers broke down the data for the US, they found a stark impact of heat on Black and Hispanic students, noting that it’s likely they have less access to air conditioning at home and school.
The US still has few policies that help protect people from the most extreme summertime heat. Federal and state programs that help people with their energy bills focus more on winter months than the summer, so some low-income customers wind up not turning on the AC to keep their bills affordable.
Many states also lack policies that prevent utilities from cutting off the electricity when there is an unpaid bill in summer months. Outdoor workers also have poorly enforced protections: Generally, the Centers for Disease Control and Prevention (CDC) recommends outdoor workers only work a small portion of the day and take frequent breaks and drink plenty of water as they build up tolerance over a period of two weeks. Some states like California have standards for training, water, and shade, but abuses abound, especially in the agricultural sector.
“Workers in the agricultural industry have 35 times higher mortality rate than nonagricultural workers, and we also see this particularly with Latino agricultural workers and migrant farm workers that are the ones in conditions with limited occupational health and safety,” Méndez said.
The Labor Department’s Occupational Safety and Health Administration (OSHA) is making slow progress on draft heat standards to protect outdoor workers, which are still a few years away from taking effect.
Even the protections that exist around heat, like cooling centers, energy bill assistance, and preventing utility cutoffs, don’t focus on the vital early season, when people are least acclimated to the hot temperatures. These kinds of initiatives, where they do exist, don’t typically kick in until after a certain date or when temperatures are above 95 degrees Fahrenheit, which is far past the threshold of what some vulnerable populations can handle. Tackling heat risk requires far more invested in heat preparedness much earlier in the year.
“We barely have enough resources to deal with what’s occurring right now,” said University of Arizona professor of urban planning Ladd Keith. “With climate change, we’re seeing increasing frequency, intensity, and duration of heat waves, so we need to adjust our response.”
Update, April 19, 11:40 am: This story was originally published on April 14 and has been updated to include information about the heat waves in China, India, Bangladesh, and Thailand.