How Sweat Supports Survival Amid Climate Change

 

How Sweat Supports Survival Amid Climate Change

Beneath the unforgiving African sun, the cradle of humanity, every living creature devised strategies to combat the searing heat. Lions sought refuge in the shade, termites constructed massive ventilation mounds, and elephants evolved colossal ears that could function as fans. Approximately 2 million years ago, our ancestors honed an even more peculiar survival technique: the ability to transport water from within our bodies to the outside—a remarkable adaptation for enduring scorching temperatures.

While some animals have limited sweating abilities, humans excel in this regard. During intense physical activity in the heat, a person can lose over 2 gallons of water daily, depleting a precious resource at a rapid pace. In an effort to cool down, the body widens blood vessels, redirecting hot blood from the core to the skin's surface. Simultaneously, sweat glands extract water from the bloodstream and release it onto the skin. As these tiny droplets evaporate, they dissipate heat from the body into the surrounding air.

"It is crucial to being human," explained Yana Kamberov, a geneticist studying sweat evolution at the University of Pennsylvania. "It’s something that differentiates us from every other animal on the planet," she added, placing it in the same league as our oversized brains. Humans boast between 2 and 4 million sweat glands on their skin, ten times more densely distributed than in chimpanzees, our closest relatives. Sweat proved to be more advantageous than protective fur; our once-thick coat dwindled to peach fuzz, enhancing water evaporation efficiency.

Now, our biological cooling systems face a stern test. This summer marked not only the hottest three consecutive months on record but also the hottest in 125,000 years. Phoenix endured 31 consecutive days with temperatures soaring above 110 degrees Fahrenheit. Across the Northern Hemisphere, heat records tumbled at an alarming rate, with Morocco and China reaching all-time highs above 120 degrees Fahrenheit. The humid Gulf Coast registered temperatures as high as 115 degrees Fahrenheit, breaking records in cities like Houston and New Orleans. Even during South America's winter, a town in the Chilean Andes recorded temperatures exceeding 100 degrees Fahrenheit—an unprecedented high.

Life is becoming perilous without air conditioning. A widespread power outage in Phoenix during a heatwave, lasting for days, could lead to thousands of deaths and send half the city to the emergency room, as per a recent study. In the sweltering Gulf Coast, the excessive heat and humidity impede sweat evaporation, trapping heat within the body, increasing the risk of internal overheating.

"Dying from a heat wave is like a horror movie with 27 endings that you can choose from," remarked Camilo Mora, a climate scientist at the University of Hawaiʻi at Mānoa, who has cataloged 27 different ways that heat can lead to organ failure and death.

As blood is redirected toward the skin's surface, it strains the heart and deprives the brain and gut of oxygen, leading to heart attacks and other grim consequences arising from widespread inflammation and clotting. Prolonged sweating can also cause dehydration, potentially leading to kidney failure. Among all weather-related disasters, heat is by far the deadliest for Americans. In 2017, Mora and colleagues found that 30 percent of the world's population was already exposed to potentially lethal heat for 20 days or more annually.

Despite the pivotal role of sweat in survival, questions remain unanswered. What is the upper limit of heat tolerance for the human body? How significant is humidity? And why aren't we more appreciative of sweat? Its reputation for causing body odor obscures the fact that it serves as a built-in life preserver during record-breaking heatwaves.

Feeling damp and sticky is preferable to the alternative—death by heatstroke. "I think it’s funny that humans have this enormous taboo about a biological function that’s ultimately going to help us survive climate change," mused Sarah Everts, author of "The Joy of Sweat."

Deadly heatwaves can strike anywhere, often catching people off-guard. Last summer, record-breaking temperatures swept across Europe, resulting in over 61,000 deaths. While the human body can acclimate to heat over weeks, allowing for increased sweating, temperature spikes can occur suddenly. For instance, in February this year, Washington, DC, experienced a temperature increase of nearly 30 degrees in just one day, from 53 to 81 degrees Fahrenheit. Such abrupt changes can be especially deadly during heatwaves in cooler climates.

Even in countries like Pakistan, where people are accustomed to heat, sweltering temperatures are claiming lives. "With climate change, things are just going beyond limits of adaptation," said Fahad Saeed, a scientist with Climate Analytics, based in Islamabad.

A critical metric, the "wet-bulb temperature," factors in heat, humidity, sunlight, and wind speed to calculate the threshold beyond which a healthy human cannot survive. Invented by the US military in the 1950s after recruits suffered heat-related illnesses, this metric suggests that a person can endure a wet-bulb temperature of 35 degrees Celsius (95 degrees Fahrenheit) for a maximum of six hours before succumbing. Such conditions might involve 95 degrees Fahrenheit with 100 percent humidity or 115 degrees Fahrenheit with 50 percent humidity.

Recent data indicates that parts of Pakistan and the Arabian Peninsula have already briefly exceeded this dangerous threshold. Moreover, climate change is elevating the moisture content of the air, particularly in tropical regions.

Newer research suggests that the limit might be even lower, closer to a wet-bulb temperature of 31 degrees Celsius (88 degrees Fahrenheit). Additionally, individual factors like age, illness, and body size influence the equation. People over 60, who account for an estimated 80 percent of the 12,000 heat-related deaths in the United States each year, often have health conditions that make heat more perilous. Furthermore, as people age, their sweat glands deteriorate, impairing their ability to cool down. Certain antipsychotic medications even suppress sweating, potentially contributing to the vulnerability of individuals with schizophrenia to heat-related fatalities.

In reality, most people do not take the necessary precautions during a heatwave, such as seeking shade or staying well-hydrated, further underscoring the need for a more conservative "danger zone" temperature threshold. Recent research by the University of Oxford and the Woodwell Climate Research Center indicates that, under current climate conditions, 8 percent of the Earth's land area will reach a temperature threshold requiring specific cooling measures at least once a decade. This percentage could rise to a quarter if global temperatures increase by 2 degrees Celsius above preindustrial levels, a scenario that could result from existing and planned fossil fuel projects.

Nevertheless, a debate persists regarding the role of humidity in health outcomes. Jane Baldwin, an Earth systems science professor at the University of California, Irvine, notes that humidity does not emerge as a primary factor in death rates in real-world epidemiological data. This discrepancy might arise because epidemiological data primarily come from cooler regions like Europe and the United States, where the humidity-death link may not be as pronounced. Clarifying this issue would enable more accurate predictions of how climate change will impact health.

Conversely, dry air poses its own challenges. In arid conditions, sweat evaporates rapidly, which is effective for cooling but has its limits. At rest, a person can sweat about a liter per hour, but during exercise, this can increase to around 3 liters per hour.