How Hot Is Too Hot for the Human Body?

Extreme heat has been shattering records across Europe, Asia, and North America, leaving millions of people struggling with excessive heat and humidity that far exceeds 'normal' for long periods.

On July 16, 2023, Death Valley recorded a blistering 128 degrees Fahrenheit (53.3 degrees Celsius)—nearly the hottest day on record. Phoenix endured a historic 19-day streak of temperatures exceeding 110 F (43.3 C), with more hot days ahead and nights that never dropped below 90 F (32.2 C). In early July, Earth likely experienced its hottest week in modern history.

Heat waves are intensifying as the climate changes—lasting longer, occurring more frequently, and becoming increasingly unbearable.

A common question people are asking is: 'When will it be too hot for regular daily activities, even for young, healthy individuals?'

The answer isn't just about the temperature shown on the thermometer. It also involves humidity. Our research aims to determine the combination of the two, known as 'wet-bulb temperature.' Heat and humidity together create a far greater risk for humans, and this combination becomes hazardous at much lower levels than previously thought.

The Limits of Human Adaptability

Scientists and other experts are growing increasingly concerned about the rise in extreme heat events combined with high humidity.

A study conducted in 2010 suggests that a wet-bulb temperature of 95°F (35°C)—equivalent to 95°F with 100% humidity or 115°F with 50% humidity—could be the threshold beyond which human beings can no longer regulate their body temperature through sweat evaporation, leading to dangerous consequences.

It wasn't until recent years that this limit was directly tested on humans in controlled lab environments, and the outcomes from these tests raised even greater concerns.

The PSU H.E.A.T. Project

To explore the question of 'how hot is too hot?' we invited young, healthy men and women into the Noll Laboratory at Penn State University, where they were subjected to heat stress within a specially controlled chamber.

These experiments offer valuable insights into the specific combinations of temperature and humidity levels that start to pose serious risks, even for the healthiest individuals.

Each participant ingested a small telemetry pill that tracked their core body temperature in real-time. They then spent time in an environmental chamber, performing minimal daily activities like showering, cooking, and eating. Researchers gradually increased either the chamber's temperature or its humidity in various experiments to observe when the subject's core temperature began to rise.

The point at which a person's core temperature begins to rise continuously due to a specific combination of temperature and humidity is referred to as the 'critical environmental limit.'

When temperatures and humidity levels remain below these critical limits, the body is able to keep its core temperature stable over extended periods. However, if those limits are exceeded, core temperature starts to rise without stopping, which increases the risk of heat-related illnesses during prolonged exposure.

When the body overheats, the heart must work harder to pump blood to the skin to release heat. At the same time, sweating leads to fluid loss. In the most extreme cases, continuous exposure can cause heat stroke, a life-threatening emergency that demands immediate cooling and medical attention.

Our research on young, healthy individuals reveals that this upper environmental threshold is even lower than the previously estimated 35°C (95°F). It occurs at a wet-bulb temperature of around 87°F (31°C) when humidity exceeds 50%. For instance, this would correspond to 87°F at 100% humidity or 100°F (38°C) at 60% humidity.

Dry vs. Humid Environments

Global heatwaves are now surpassing these critical thresholds, approaching or even exceeding the theorized 95°F (35°C) wet-bulb limit.

On July 16, 2023, Asaluyeh, Iran, recorded a dangerously high wet-bulb temperature of 92.7°F (33.7°C). Both India and Pakistan have also experienced hazardous heat levels in recent years.

In arid, hot climates, critical environmental limits aren't determined by wet-bulb temperatures because most of the sweat produced by the body evaporates, cooling it down. However, the body's sweating capacity is not unlimited, and higher air temperatures also contribute to heat gain.

It's important to remember that these cutoffs are designed primarily to prevent the body’s temperature from rising too high. Even with lower temperatures and humidity, the heart and other bodily systems can still experience significant strain.

A recent study from our lab revealed that the heart rate begins to rise long before our core temperature does, as blood is pumped to the skin. While surpassing these limits may not always lead to the worst-case scenario, extended exposure can be harmful, especially for vulnerable groups like the elderly and those with chronic health conditions.

Our research focus has shifted to studying older men and women, as aging reduces heat tolerance. The rising incidence of heart disease, respiratory issues, and other health problems, along with certain medications, heightens their risk. People over 65 make up about 80% to 90% of heatwave-related deaths.

How to Stay Safe

Staying hydrated and finding cool places to rest, even briefly, is crucial during intense heat.

Although more cities in the United States are opening cooling centers to offer relief from the heat, many individuals will still face these hazardous conditions without any means to cool down.

Even with access to air conditioning, some people may not use it due to the high cost of energy, which is common in Phoenix, or because of widespread power outages during heatwaves or wildfires, a growing issue in the western U.S.

The mounting evidence makes it clear that climate change is not a distant issue, but a current challenge that humanity must confront directly.

This article is republished from The Conversation under a Creative Commons license. You can access the original article here.

W. Larry Kenney is an expert in the regulation of human skin blood flow, focusing on aging, the physiology and biophysics of heat exchange between humans and the environment, as well as thermoregulation during exercise and extreme conditions.

Daniel Vecellio is a postdoctoral researcher at Penn State's Center for Healthy Aging. He holds a master's in atmospheric science and a Ph.D. in geography. His work has included research on the effects of permafrost degradation on land-atmosphere interactions, and he is currently focused on the relationship between extreme heat and human health.

Rachel Cottle is a Ph.D. student in exercise physiology. Her research investigates the effects of heat on human bodies and athletic performance.

S. Tony Wolf is a postdoctoral researcher in the field of kinesiology. He contributes to the PSU H.E.A.T. (Human Environmental Age Thresholds) project, where he studies the effects of heat and humidity on the human body.