How hot is too hot? The temperature threshold that the human body can handle

In a revelation that highlights the grave consequences of climate change, research indicates that the temperature threshold at which the human body cannot survive may be significantly lower than previously thought. 

This alarming discovery sheds light on the stark reality that thousands face across the globe as extreme heat events become more frequent, intensifying under the unrelenting progression of human-induced global warming.

Wet bulb temperature 

A healthy young individual might endure six hours at 35 degrees Celsius (95 Fahrenheit) coupled with 100 percent humidity before succumbing to heat-related fatalities. 

Beyond this critical point, referred to as the “wet bulb temperature,” the body’s primary cooling mechanism – sweat – fails to evaporate, causing heatstroke, organ failure, and eventual death.

The occurrence of this wet bulb temperature, at 35 degrees Celsius, has been extremely rare in human history. 

“It has only been breached around a dozen times, mostly in South Asia and the Persian Gulf,” Colin Raymond of NASA’s Jet Propulsion Laboratory told AFP. During these incidents, none lasted more than two hours, sparing populations from “mass mortality events” linked to this survival limit, according to Raymond.

Temperature thresholds vary 

But it’s not just this extreme level that poses a risk to human life. As experts warn, individual thresholds vary widely depending on factors such as age, health, and socioeconomic status.

The terrifying reality of heat-related mortality became evident last summer when more than 61,000 people died in Europe due to heat, where humidity rarely reaches the dangerous levels of wet bulb temperatures. As global temperatures surge, hitting record highs, scientists predict that perilous wet bulb events will multiply.

Serious hazard 

Raymond explained that the frequency of these events has at least doubled over the past 40 years, labeling this increase a “serious hazard of human-caused climate change.” 

His study projects that if the world continues on its current trajectory, warming 2.5C degrees above preindustrial levels, wet bulb temperatures exceeding 35C will become a regular occurrence in several regions in the upcoming decades.

Human survival limit 

The term “wet bulb temperature” might sound abstract to some, but its measurement is grounded in the very fundamentals of human physiology. 

Initially gauged by covering a thermometer with a wet cloth, it simulates how quickly water – or sweat – evaporates off the skin. This temperature threshold of 35C wet bulb temperature corresponds to 35C of dry heat with 100 percent humidity, or 46C at 50 percent humidity.

Recent experiments at Pennsylvania State University in the United States have shed light on this human survival limit. 

Researchers measured the core temperatures of healthy subjects inside a heat chamber, finding that the “environmental limit” was reached at 30.6C wet bulb temperature, alarmingly below the previously theorized 35C. 

The experts estimated that “really, really dangerous core temperatures” would manifest between five to seven hours under such conditions, stated Daniel Vecellio, who contributed to the research.

Study implications 

The impact of these findings resonates deeply across diverse populations. Joy Monteiro, a researcher in India, noted that most deadly heat waves in South Asia occurred well below the 35C wet bulb temperature threshold. Vulnerabilities are “wildly different for different people,” he told AFP. 

Small children, with their reduced ability to regulate body temperature, and older individuals, who have fewer sweat glands, face the greatest risks. A staggering 90 percent of the heat-related deaths in Europe last summer were among those aged over 65.

Those who work outside in sweltering temperatures, or lack access to cooling spaces, are also at heightened risk. Monteiro pointed to other factors like limited access to toilets, leading to dehydration. 

Climate impacts

“Like a lot of impacts of climate change, it is the people who are least able to insulate themselves from these extremes who will be suffering the most,” said Raymond. 

His research also highlighted the link between wet bulb temperatures and other climate events, such as El Niño events and ocean surface temperatures, both of which have seen concerning trends recently. 

In fact, the world’s oceans recorded an all-time high temperature last month, surpassing the previous 2016 record, according to the European Union’s climate observatory.

This study’s revelations are more than a warning; they are a chilling testament to the immediate and dire need for global action to mitigate the advancing peril of climate change. 

More about heat and the human body

High temperatures can have a significant impact on the human body, affecting its physiological processes and overall functioning as we surpass our built-in temperature threshold. Here’s a brief overview:

Impaired heat regulation

The human body constantly tries to maintain an internal temperature of around 37°C (98.6°F). In high temperatures, the body uses processes like sweating and increased blood flow to the skin (vasodilation) to help cool down. However, in extreme heat, these processes might not be sufficient, leading to overheating.

Dehydration

Sweating is the body’s primary means of cooling down, but it also results in fluid loss. Without adequate fluid intake, this can lead to dehydration, which further impairs the body’s ability to regulate temperature and affects bodily functions.

Heat-related illnesses – surpassing the temperature threshold

Heat cramps

Muscle cramps due to loss of salt and water from sweating.

Heat exhaustion

Symptoms include heavy sweating, weakness, cold or clammy skin, a fast but weak pulse, nausea, and fainting.

Heat stroke

A life-threatening condition where the body’s temperature regulation fails. Symptoms include hot and dry skin, rapid heartbeat, confusion, and even unconsciousness.

Cardiovascular strain

The heart works harder in high temperatures. As the body sends more blood to the skin to cool down, it can strain the cardiovascular system.

Respiratory challenges

Hot temperatures, particularly when combined with humidity, can make breathing more difficult and may exacerbate conditions like asthma or COPD.

Cognitive impairments

Extreme heat can affect the brain’s functioning, leading to symptoms such as confusion, fatigue, and reduced ability to concentrate or perform tasks.

Kidney stress

The kidneys play a key role in regulating body fluids. In hot conditions, they work harder to maintain balance, and without adequate hydration, they might be stressed, potentially leading to kidney problems.

Electrolyte imbalance

Sweating not only leads to fluid loss but also to the loss of essential salts and minerals. This can lead to an electrolyte imbalance which affects muscle function and other bodily processes.

Skin irritations

Excessive sweating can lead to skin problems, including heat rashes, which are characterized by small blisters or red lumps.

Vulnerability in certain populations

Some groups, such as infants, older people, and people with certain medical conditions or on medications that affect temperature regulation, may be more susceptible to the effects of high temperatures and have lower temperature thresholds.

Given these potential impacts, it’s crucial to take precautions in extreme heat, including staying hydrated, wearing appropriate clothing, seeking shade or air-conditioned environments, and avoiding strenuous activities during the hottest parts of the day.

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