As we’ve had the hottest year ever after the hottest year ever, there’s been a growing popular understanding of just how dangerous high temperatures can be: it’s the deadliest form of extreme weather, and it disproportionately affects Black Americans. There have been headline-grabbing instances of Black people dying at places like work and in prison during heatwaves.
And as we’ve been adjusting how we think about heat, scientists have been looking at how even hotter heat waves could come with even greater increases in global temperatures, which would continue to have an outsized effect on Black people in the U.S. What the research suggests is that humans have a lower threshold for incredibly hot, humid temperatures than previously believed.
That’s where so-called wet bulb temperatures come into play — and the danger they represent could have a disproportionate impact on Black America.
What Is a Wet Bulb Temperature?
Wet bulb temperatures are a measure of both heat and humidity — conditions that, at extreme levels, can make it so no one can continue to regulate body temperature, even a healthy person who has access to shade and water. The term comes from scientists who wrapped the bulb of a thermometer with wet cloth, and as the water evaporated, it cooled the thermometer down — unless the humidity was too high. That limit has been understood to be a wet-bulb temperature of 35 degrees Celsius — or 95 degrees Fahrenheit. Literally deadly heat.
Past that limit, “no amount of sweating or other adaptive behavior is enough to lower your body to a safe operating temperature,” Colin Raymond, an expert on extreme heat and humidity at NASA’s Jet Propulsion Laboratory told NASA News in 2022.
“If you’re sitting in the shade with unlimited drinking water in California’s Death Valley, conditions may not be pleasant, but they’re survivable,” Raymond explained. “But in humid regions, once you approach wet-bulb temperatures of 34-to-36 degrees Celsius (93-to-97 degrees Fahrenheit), it doesn’t matter what you’re doing. You can’t survive for extended periods of time.”
Because wet-bulb temperatures account for both heat and humidity, that threshold can be hit in a number of ways, including, for example, 95 degrees Fahrenheit at 100% humidity, as well as 115 degrees Fahrenheit at 50% humidity. But because heat waves that extreme are, thankfully, still many years off in our warming trajectory, the 35 degrees Celsius wet-bulb temperature is a largely theoretical threshold.
A Lower Tolerance
But new research is showing that, in reality, the absolute limit of heat and humidity for humans is likely much lower.
Last week, the New York Times reported on wet-bulb research being conducted at Penn State by exposing volunteers to increasingly high levels of heat and humidity in a kind of scientific sauna and seeing at which point their bodies are no longer able to regulate their temperature.
While the head researcher, Larry Kenney, only tested a small cohort of 24 people, what the research found is that the test subjects could not keep their internal body temperature steady after being exposed to a wet-bulb temp of 31 degrees Celsius — approximately 87.8 degrees Fahrenheit — which is significantly lower than the theoretical threshold of 35 degrees Celsius.
In 2023, parts of the southeast U.S., where more than half of Black Americans live, did experience wet-bulb temperatures that hit 90 degrees. However, the real risk of heat waves that could be deadly to anyone and everyone is probably still far off in the climate crisis timeline.
Mapped against climate models, Kenney found that wet-bulb temperatures of 31 degrees Celsius would begin to affect the United States if we tip past 4 degrees Celsius of warming. And those deadly temperatures would hit major American cities with large Black populations first — places like Chicago, Houston, and Washington, D.C.
