Heat and Humidity - The Unbearable Limit

Over the 30-year average, excessive heat caused more fatalities per year in the United States than any other type of hazardous weather. Heat and humidity take their toll on the elderly most acutely, but even for young and otherwise physically fit individuals there is an ultimate limit to how much heat and humidity the human body can take before perspiration can no longer cool the body down, even without doing physical activity. That limit is a wet-bulb temperature of 35°C which is equivalent to a heat index or “feels like” temperature of 160°F (71°C). While considerably rare on Earth, these conditions are not unheard of especially in areas around the Persian Gulf and during the monsoon in southern Asia. A 2020 study from researchers at Columbia University, as well as Loughborough University, projects that by the third quarter of this century, instances of extreme humid heat will become more common in these parts of the world.

The research team analyzed global surface weather station data from 1979-2017 to find instances of extreme humid heat and found many instances of wet-bulb temperatures exceeding 31°C in portions of India, Bangladesh, Pakistan, northwestern Australia, along the coast of the Red Sea and near the Gulf of California. However, locations that experienced conditions near the survivability limit were confined to areas near the Persian Gulf and the Indus River Valley in Pakistan where copious amounts of moisture evaporate off of some of the warmest sea and ocean water on the planet, and are transported over already very hot coastal areas.

At present, these instances of extreme humid heat, approaching the survivability limit, are short-lived events, reaching the 35°C wet-bulb mark for only 1 to 2 hours on the hottest day of the year. However, projecting the current rate of warming seen worldwide, which has seen a doubling of these extreme events since 1979, by the third quarter of this century these conditions are expected to be longer duration and more common events. Although modern conveniences like air conditioning make such outdoor weather survivable, those who live in less-developed countries, where electricity is a luxury, may find regions that regularly reach such extremes of humid heat uninhabitable.

The human survivability limit for humid heat, even when in the shade and with adequate hydration, is a wet-bulb temperature of 35°C which equates to a heat index of 160°F (71°C).

High humidity decreases the body's ability to cool itself through the evaporation of sweat

As global temperatures rise, 35°C wet-bulb readings, while rare and short-lived today, are expected to become more common and of longer duration by the third quarter of this century.