How hot is too hot? Specifically, at what temperature do humans stop functioning well? While there is no specific air temperature, there is a wet-bulb numeric value that impacts our well-being. Are there places on Earth that exceed this value? How will the world’s hottest cities bear the heat?

As a refresher, here is how we measure temperature. Engineering Toolbox explains, “The dry-bulb temperature, usually referred to as ‘air temperature,’ is the air property that is most commonly used.” This is generally the number reported by the weather forecast (http://bit.ly/3W3hLza). 

“The wet-bulb temperature is the adiabatic saturation temperature,” the article continues. “Wet-bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer bulb and the cooling effect is indicated by a wet-bulb temperature lower than the dry-bulb temperature in the air.” 

In simplistic, nonengineering terms, wet-bulb readings consider the influence of humidity. 

Humidity and sweat

Humidity is a key accelerant to overheating. Humans produce sweat to disperse heat, which is automatic, latent cooling. The evaporative cooling effect of sweat beads dissipating into the surrounding air transports heat away from the body. 

The ability to sweat effectively to cool down depends on the humidity of your surroundings. A little bit of sweat is enough to cool you down or even make you cold if you are hiking at high elevations in the Rocky Mountains in the summer because the humidity is so low. Even if you sweat buckets in a place like New Orleans, you may still feel toasty. 

A Science Advances article explains our ability to self-regulate heat in more detail (http://bit.ly/3uRM5AT): “Humans’ bipedal locomotion, naked skin and sweat glands are constituents of a sophisticated cooling system. Despite these thermoregulatory adaptations, extreme heat remains one of the most dangerous natural hazards, with tens of thousands of fatalities in the deadliest events so far this century. The additive impacts of heat and humidity extend beyond direct health outcomes to include reduced individual performance across a range of activities, as well as large-scale economic impacts.” 

What temperature marks “the upper limit of survivability under sustained exposure”? A wet-bulb temperature of 95 degrees is a high limit for humans. 

The article continues: “Humans’ ability to efficiently shed heat has enabled us to range over every continent, but a wet-bulb temperature of 95 degrees marks our upper physiological limit, and much lower values have serious health and productivity impacts.” 

We can outrun our body’s ability to act as an evaporative cooler. Put another way, we might be unable to sweat our way back to the comfort of a warming world. 

A grist.org video explains wet-bulb temperature differences in a few simple examples (www.youtube.com/watch?v=35Orkd9vzHQ). A common dry-bulb temperature in Death Valley, Calif., is 120 degrees. However, with a humidity of 17 percent, the wet-bulb temperature would be 77 degrees. In Florida, an 86-degree dry-bulb temperature with 67 percent humidity could lead to the same wet-bulb temperature of 77 degrees. 

The risk of heat exhaustion and heat stroke increases as the wet-bulb temperature climbs. Beyond a wet-bulb 95-degree temperature, the evaporative cooler of the human body exceeds capacity. 

The human cost of heatwaves

Is a wet-bulb temperature of 95 degrees a distant reality? No. In the last 40 years, this temperature has been measured 14 times in Pakistan and at a few coastal locations on the Arabian Peninsula, the Science Advances article explains. When humid air from a shallow sea blows over extremely warm, dry land, the wet-bulb temperature spikes. Jacobabad, Pakistan, is a perfect storm for these high wet-bulb temperatures, recording six of the 14 95-degree instances. 

As of November 2022, the World Health Organization (WHO) calculated at least 15,000 deaths from reporting European countries (http://bit.ly/3YrGUVV). The majority of deaths occurred during the summer heatwave. Essentially, climate change has outpaced the air conditioning basis of design for homes in Europe. 

“Temperatures in Europe have warmed significantly over the 1961-2021 period, at an average rate of about 0.5 C per decade,” a WHO statement notes. “This is the fastest-warming region, according to a report launched this week by the World Meteorological Organization. Extreme temperatures accounted for more than 148,000 lives lost in the European Region in the previous 50 years. In just one year since, we lost at least another 15,000 lives.”

Cities are more likely to feel the wrath of heatwaves than rural areas. The urban heat island effect is a combination of man-made factors that lead to more extreme, longer-lasting heat events in cities. 

The EPA defines the scenario (http://bit.ly/3Wha3BX): “Urban heat islands occur when cities replace natural land cover with dense concentrations of pavement, buildings and other surfaces that absorb and retain heat. This effect increases energy costs (e.g., for air conditioning), air pollution levels and heat-related illness and mortality.” 

Cities get hot and stay hot overnight since they retain heat in the hardscape. 

How will the world’s hottest cities bear the heat? In areas with the resources to plant more shade trees, improve minimum insulation values for new construction and install/maintain the latest cooling equipment, buildings can become more resilient to heat. Put another way, rich communities will be uncomfortable but OK. 

In communities with limited resources, more people will be at risk. Unlike extreme cold temperatures, we cannot huddle together to fight heat waves. Cities with harsh conditions may see increased emigration, which will, at a minimum, leave a smaller tax base to undo the urban planning decisions made in a cooler past. 

Productivity could decrease in careers where people work outside all day. People may spend more of their waking hours focused on the bottom section of Maslow’s hierarchy of needs pyramid: physical safety and survival. 

Another practical note is to design based on the 2021 ASHRAE climatic design information dataset. Also, anticipate that these numbers will become outdated more quickly than other dataset periods in the past.