Wet Bulb Temperature as a Measure of Heat: How Hot Is Too Hot?

Extreme heat has been breaking records worldwide, with millions suffering from heat and humidity well above "normal" for days.

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Measuring Heat With Wet Bulb Temperature

As heat waves become supercharged as the climate changes, most people ask about the temperature that is considered too hot for normal daily activities. The answer goes beyond the temperature normally seen on the thermometer but is also about humidity.

Experts have developed a combination of dry air temperature and humidity, measured as "wet-bulb temperature" (WBT). In essence, it is a measure of heat-stress conditions in humans. Together, heat and humidity put a lot of people at increased risk, and this combination gets dangerous at lower levels than previously thought.

The term "wet-bulb temperature" comes from the way it is measured. If a wet cloth is slid over the bulb of a thermometer, the evaporating water from the cloth will cool the temperature. This lower temperature is the wet-bulb temperature and cannot go above the dry temperature. Suppose the humidity in the surroundings is high. In that case, the air is already more saturated with water, less evaporation will occur, and the wet-bulb temperature will be closer to the dry temperature.

According to climate scientist Kristina Dahl from the Union of Concerned Scientists, the wet-bulb temperature reading will change depending on how humid it is. That is the real purpose of this method: to measure how well humans can cool themselves by sweating.

Humidity and temperature are not the only things that affect a person's body temperature since solar radiation and wind speed are also factors to be considered. However, wet-bulb temperature is critical to measuring indoor environments since deaths often occur in heat waves.

When Does WBT Get Dangerous?

Concerns are raised regarding the "threshold" or critical wet-bulb temperature for humans also referred to as the point at which a healthy person can survive for only six hours. The threshold is usually considered 35 degrees Celsius, almost equivalent to an air temperature of 40 degrees Celsius with a relative humidity of 75%.

In a normal setting, humans regulate their internal body temperature by sweating. Above the wet-bulb temperature, we can no longer cool down this way, steadily increasing our body temperature. This marks a limit to human adaptability to extreme heat. If we fail to escape this condition, the core of our body can rise beyond the survivable range, and our organs will start failing.

The often-cited 35 degrees Celsius value comes from a theoretical study in 2010. In 2022, a new study was conducted by Daniel J. Vecellio and his colleagues where they discovered that the real threshold that our body can tolerate could be far lower. The result of the study also suggests that a wet-bulb temperature threshold cannot be applied to human adaptability across all climatic conditions.