How hot is too hot?

Extreme heat has been scorching a vast part of Bangladesh, with millions of people sweltering in heat and humidity well above "normal" for days on end.

A dangerous combination of heat and humidity has been sweeping Bangladesh for the last couple of weeks, hitting cities from Jashore to Nilphamari.

Chuadanga, for example, on Friday recorded 42.7 degrees Celsius temperature. In fact, the whole Khulna division as well as some parts of the Rajshahi and Barishal divisions have been baking in severe heat.

However, climate experts say air temperature alone can be misleading.

Meteorologists are particularly worried about the "wet-bulb" temperature — a more holistic measurement that accounts not just for air temperature but also how much moisture it holds.

Today at 6:00pm, Dhaka's temperature was 38 degrees Celsius and the humidity level was 70 percent.

WHAT IS WET-BULB TEMPERATURE?

One question a lot of people are asking is: "When will it get too hot for normal daily activity as we know it, even for young, healthy adults?"

The answer goes beyond the temperature you see on the thermometer. It's also about humidity. Together, heat and humidity put people at greatly increased risk, and the combination gets dangerous at lower levels than scientists previously believed. At the upper end, wet-bulb temperatures can cause serious health effects if people can't find a way to quickly cool down.

HOW IS WET-BULB TEMPERATURE MEASURED?

A wet-bulb measurement is taken by covering a thermometer with a water-soaked cloth. The process of the water evaporating from the cloth, thus lowering the temperature, mirrors how the human body cools down with sweat.

At 100% relative humidity, wet-bulb temperature will be the same as the dry air temperature, but with less humidity it is lower.

HOW DO HIGH WET-BULB TEMPERATURES AFFECT HUMAN BODY?

High wet-bulb temperatures are dangerous because humans lose around 80% of heat through sweating, so when both humidity and air temperature are high it becomes harder to shed excess heat.

Sweat evaporates very slowly, if at all, in very humid conditions.

Internal body temperature is around 37C (98F). But humans also generate more heat through exercise.

"You have to lose that - if you don't lose the heat, you just slowly heat up and that can't be good," said Matthew Huber, a global expert on heat stress at Purdue University in the US.

If the body cannot cool down it will eventually overheat, triggering respiratory and cardiovascular issues, and even death.

WHAT'S THE THRESHOLD?

This is an area of ongoing research among scientists. A landmark study, opens new tab co-authored by Huber in 2010 found that a wet-bulb temperature of 35C (95F) persisting beyond six hours could induce hyperthermia in people and cause serious health consequences or death.

While Huber's study assumed more of an ideal scenario —perhaps a person in the shade drinking water — real people might be out exercising in direct sunlight.

Research published in the Journal of Applied Physiology in 2022 suggested that limit could be lower.

Scientists placed young, healthy adults in chambers that mimicked high wet-bulb temperatures and had them perform tasks that mirrored those of daily life. They found the limit could be much lower — between 30C (86F) and 31C (88F).

"That's kind of a game changer if that's right," said Huber.

WHAT AREAS ARE AT HIGHEST RISK?

Tropical regions with a lot of humidity, especially those along the monsoon belt, are generally at greatest risk of experiencing lethal wet-bulb temperatures.

China, India, Bangladesh, Pakistan and Africa's Sahel region are all considered key risk zones.

HOW DOES CLIMATE CHANGE AFFECT WET-BULB TEMPERATURES?

Climate change is predicted to affect not just how high wet-bulb temperatures get but also how long they persist. The upper range of wet-bulb temperatures scales directly with the climbing global average temperature.

"If you warm the world by about 1C, then the maximum wet-bulb temperature that might be experienced over much of the world increases by about one degree," said Huber.

The world is on track for 2.7C (4.9F) of global warming by 2100 under current government policies, the United Nations says.

"All of a sudden you have large parts of the world in terms of population that reach the threshold, even with moderate warming," said Huber.

Climate change may also cause dangerous wet-bulb temperatures to last longer. A 2020 study in the Journal of Scientific Advances found that, rather than lasting for only an hour, dangerous wet-bulb temperatures could persist for six or more hours by 2060 — killing anyone who can't take cover.

Overall, extreme humid heat worldwide has doubled in frequency since 1979, the study found.

HAVE WE ALREADY SEEN DEADLY WET-BULB TEMPERATURES?

Small slivers of the world have flirted with lethal wet bulb temperatures. But these punishing conditions only lasted one to two hours at a time, avoiding deadly consequences.

Jacobabad in Pakistan — dubbed the hottest city on Earth — has surpassed a wet-bulb temperature of 35C on at least four occasions.

But many other cities, opens new tab have briefly seen maximum wet-bulb temperatures exceeding 32C, said the Scientific Advances study, which assessed global weather station data. These include La Paz, Mexico; Port Hedland, Australia; and Abu Dhabi, UAE.

DRY Vs HUMID ENVIRONMENTS

Current heat waves around the globe are exceeding those critical environmental limits, and approaching, if not exceeding, even the theorised 95 F (35 C) wet-bulb limits.

In the Middle East, Asaluyeh, Iran, recorded an extremely dangerous maximum wet-bulb temperature of 92.7 F (33.7 C) on July 16, 2023. India and Pakistan have both reached hazardous levels in recent years, as well.

In hot, dry environments, the critical environmental limits aren't defined by wet-bulb temperatures, because almost all the sweat the body produces evaporates, which cools the body. However, the amount humans can sweat is limited, and we also gain more heat from the higher air temperatures.

Keep in mind that these cutoffs are based solely on keeping your body temperature from rising excessively. Even lower temperatures and humidity can place stress on the heart and other body systems.

A recent paper from our laboratory showed that heart rate begins to increase well before our core temperature does, as we pump blood to the skin. And while eclipsing these limits does not necessarily present a worst-case scenario, prolonged exposure may become dire for vulnerable populations such as the elderly and those with chronic diseases.

HOW TO STAY SAFE

Staying well hydrated and seeking areas in which to cool down – even for short periods – are important in high heat. While more cities in the United States are expanding cooling centres to help people escape the heat, there will still be many people who will experience these dangerous conditions with no way to cool themselves.

All told, the evidence continues to mount that climate change is not just a problem for the future. It is one that humanity is currently facing and must tackle head-on.