Explained! How harmful is rising incidence of fatal wet-bulb temperature for humans?

Since March 2022, a spate of heatwaves has engulfed South Asia, continuing an alarming trend of surpassing previous temperature records

The discussions on persistent excessive heatwaves has given rise to a new phrase in our climate change terminology: wet bulb temperatures. Concerns about wet-bulb temperatures — a measure of combined heat and humidity — and their implications on human health have arisen as heatwave conditions spread over the subcontinent.

Since March 2022, a string of heatwaves has engulfed South Asia, continuing an alarming trend of shattering previous temperature records.

Thankfully, these high temperatures were not accompanied by the high fatality rate seen in prior heatwaves like the one in 2015. The mystery of why those heatwaves were so lethal has yet to be answered.

Humidity a critical factor for measuring heat exposure

Sweat evaporates on the skin, allowing heat created within the body to escape. This evaporation’s cooling impact is critical for keeping a constant body temperature.

The same sweat does not evaporate when humidity rises — just as clothes take a long time to dry in humid climates — making it difficult to regulate body temperature. This is why humans are more uncomfortable in humid environments.

Wet-bulb temperature factors in humidity levels

The temperature of a wet-bulb is normally lower than that of a dry bulb, and the difference between the two grows substantially as the air becomes dryer.

The temperature that would be measured by a thermometer covered in a moist towel is known as the wet-bulb temperature. The evaporation of water from the bulb’s surroundings produces a cooling effect.

The lowest temperature to which air may be cooled by the evaporation of water into the air is known as the wet-bulb temperature. It is calculated by taking into account the temperature and humidity levels.

Theoretically, if the wet-bulb temperature hits 35° Celsius – its greatest point – humans will no longer be able to sweat off their internal body heat and cool themselves. Heat strokes might result as a consequence of this.

Sustained exposures to wet-bulb temperature fatal

Humidity is particularly significant for evaluating the physiological stress that excessive heat has on the human body, according to the recent Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC).

The alternative metric of ‘wet bulb temperature’ has been used to assess exposure to severe heat instead of the ‘dry bulb’ temperature that is generally measured with a conventional thermometer.

Persistent exposure to wet bulb temperatures above 35°C is lethal, according to the findings, while sustained exposure to wet bulb temperatures above 32°C is harmful to vigorous physical activity. However, concerns about the 35°C threshold and whether regions of South Asia would become “unsurvivable” in the next years are mounting.

Average global temperatures on the rise

Global average temperatures are presently 1.1 to 1.3 degrees Celsius higher than pre-industrial levels.

Even if countries keep their pledges to reduce emissions and remove carbon from the atmosphere, global temperatures are on course to climb by the same amount by the end of the century.

Wet-bulb temperature debate obscures concerning issues

The media frenzy about survival thresholds in reaction to wet-bulb temperatures obscures fundamental physiological and political concerns.

The body’s failure to maintain a stable core temperature might be caused by a variety of factors. Increased heart strain during periods of high temperature, for example, can be catastrophic for people with pre-existing cardiac problems and is the biggest cause of mortality during heatwaves.

The theoretical limit of what people can tolerate is a wet-bulb temperature of 35°C. It would, however, attain a temperature of 45°C if relative humidity was 50%, or around 39°C if the humidity was 75%. Sweat can no longer cool the body below this temperature, leading people to overheat and, in effect, cook. The swelling of cells, the deformation of proteins, and the failure of organ systems, all lead to death.

Even young healthy persons wearing light clothes, regardless of whether they are sitting in front of a fan, in the shade, or have unlimited water to drink, are expected to die in around six hours at wet-bulb temperatures above 35°C. Wet-bulb temperatures that are even significantly lower can be harmful. Anything that elevates body heat, such as walking or working outside, is dangerous at a wet-bulb temperature of 32°C.

Death can also be caused by pre-existing respiratory issues or diabetes. As a result, the body’s capacity to efficiently transmit heat to the environment is harmed.

Dehydration is a less evident problem. Due to the scarcity of toilets in workplaces, many workers, particularly women, purposefully dehydrate themselves. Dehydration can lead to less sweating and, as a result, an increased risk of heatstroke during heat waves.

Such public health issues can significantly affect survival thresholds and underestimate the population’s real risk.

Building a more climate-resilient nation imperative

In conclusion, the recent attention on rising temperatures and humidity is a positive step toward raising awareness of climate change’s effects.

However, it is critical to recognise that our susceptibility to heatwaves is a long-standing systemic issue that is not only dependent on rising greenhouse gas emissions.

We can construct a more climate-resilient nation by refocusing our objectives to expose glaring gaps in our urban and health infrastructure that routinely fail to protect the most vulnerable among us.