WHAT'S THE THRESHOLD?
This remains a subject of ongoing investigation among researchers. A groundbreaking study, co-authored by Huber in 2010, revealed that a wet-bulb temperature of 35C (95F) persisting for more than six hours could induce hyperthermia in individuals, leading to severe health repercussions or even death.
While Huber's research assumed a somewhat ideal scenario — perhaps a person staying hydrated in the shade — real-life situations might involve individuals exercising under direct sunlight.
A study published in the Journal of Applied Physiology in 2022 proposed that this limit could be lower.
Researchers placed young, healthy adults in chambers replicating high wet-bulb temperatures and had them engage in activities mirroring everyday tasks. They found the threshold could be significantly lower — ranging between 30C (86F) and 31C (88F).
"If that's indeed the case, it's quite a game changer," remarked Huber.
WHAT AREAS ARE AT HIGHEST RISK?
Tropical regions characterized by high humidity, particularly those along the monsoon belt, are generally deemed most susceptible to encountering lethal wet-bulb temperatures.
China, India, Bangladesh, Pakistan, and Africa's Sahel region are all recognized as primary risk zones.
HOW DOES CLIMATE CHANGE AFFECT WET-BULB TEMPERATURES?
Climate change is anticipated to impact not only the peak wet-bulb temperatures but also their duration. The upper limit of wet-bulb temperatures correlates directly with the rising global average temperature.
"If the world warms by approximately 1C, then the maximum wet-bulb temperature experienced across much of the world could increase by about one degree," explained Huber.
The current trajectory suggests a global warming of 2.7C (4.9F) by 2100 under existing government policies, according to the United Nations.
"Suddenly, large portions of the global population would reach the threshold, even with moderate warming," Huber noted.
Climate change might also prolong periods of hazardous wet-bulb temperatures. A study published in the Journal of Scientific Advances in 2020 indicated that, by 2060, instead of lasting just an hour, dangerous wet-bulb temperatures could persist for six hours or more — posing lethal risks to those unable to seek shelter.
Overall, the frequency of extreme humid heat worldwide has doubled since 1979, as per the study's findings.
HAVE WE ALREADY ENCOUNTERED DEADLY WET-BULB TEMPERATURES?
Certain regions of the world have flirted with lethal wet-bulb temperatures, albeit briefly. However, these extreme conditions typically lasted for only one to two hours, avoiding catastrophic outcomes.
Jacobabad in Pakistan — dubbed the hottest city on Earth — has surpassed a wet-bulb temperature of 35C on at least four occasions.
According to the Journal of Scientific Advances study analyzing global weather station data, numerous other cities have briefly experienced peak wet-bulb temperatures exceeding 32C. These include La Paz, Mexico; Port Hedland, Australia; and Abu Dhabi, UAE.
DRY Vs HUMID ENVIRONMENTS
Current heatwaves across the globe are surpassing these critical environmental thresholds and nearing, if not exceeding, the theorized 95F (35C) wet-bulb limits.
In the Middle East, Asaluyeh, Iran, recorded an extremely hazardous peak wet-bulb temperature of 92.7F (33.7C) on July 16, 2023. India and Pakistan have also reached perilous levels in recent years.
In hot, arid environments, the critical environmental thresholds aren't defined by wet-bulb temperatures, as almost all sweat evaporates, aiding in cooling the body. Nonetheless, the body's sweating capacity is limited, and higher air temperatures contribute to increased heat gain.
It's worth noting that these thresholds are solely based on preventing excessive body temperature rise. Even lower temperatures and humidity levels can strain the heart and other bodily systems.
A recent study from our lab revealed that heart rate begins to rise well before core temperature, as blood is pumped to the skin. While exceeding these thresholds doesn't necessarily guarantee the worst-case scenario, prolonged exposure can pose dire risks, especially for vulnerable populations like the elderly and those with chronic illnesses.
HOW TO STAY SAFE
Maintaining proper hydration and seeking out cool environments — even briefly — are crucial during high temperatures. While more cities in the United States are expanding cooling facilities to help people escape the heat, many individuals may still find themselves in dangerous conditions with no access to cooling options.
Overall, the evidence continues to mount that climate change isn't just a future concern; it's a present challenge that humanity must confront head-on.
