How hot is too hot for the human body? Our laboratory has found that heat + humidity becomes dangerous faster than many people realize

How hot is too hot for the human body?  Our laboratory has found that heat + humidity becomes dangerous faster than many people realize

Heat waves are super-accelerated as the climate changes – lasts longer, becomes more frequent and simply gets warmer. One question many people ask is, “When will it get too hot for normal daily activities as we know them, even for young, healthy adults?”

The answer goes beyond the temperature you see on the thermometer. It is also about humidity. Our research shows that the combination of the two can become dangerous faster than scientists previously believed.

Scientists and other observers have been concerned about the increasing frequency of extreme heat coupled with high humidity, measured as “wet bulb temperature.” During the heat waves that swept South Asia in May and June 2022, Jacobabad, Pakistan, recorded a maximum wetball temperature of 33.6 C (92.5 F) and Delhi exceeded it – close to the theorized upper limit of human adaptability at humid heat.

People often refer to a study published in 2010 which estimated that a wet bulb temperature of 35 C – equivalent to 95 F at 100% humidity, or 115 F at 50% humidity – would be the upper limit of safety, whereby the human body can no longer cool itself by evaporating sweat from the surface of the body to maintain a stable body core temperature.

It was only recently that this limit was tested on humans in laboratory environments. The results of these tests show an even greater cause for concern.

The PSU HEAT project

To answer the question of “how hot is too hot?” we brought young, healthy men and women into the Noll Laboratory at Penn State University to experience heat stress in a controlled environment.

These experiments provide insight into what combinations of temperature and humidity are starting to become harmful to even the healthiest people.

A young man in shorts walks on a treadmill with a towel next to him in a glass-enclosed room while a scientist watches his body temperature and other conditions on computer screens on the other side of the glass.
S. Tony Wolf, a postdoctoral researcher in kinesiology at Penn State and co-author of this article, is conducting a heat test in the Noll Laboratory as part of the PSU Human Environmental Age Thresholds project.
Patrick Mansell / Penn State, CC BY-NC-ND

Each participant swallowed a small telemetry pill, which monitored their deep body or core temperature. They then sat in an environmental room and moved just enough to simulate the minimal activities of daily living, such as cooking and eating. Researchers slowly increased and monitored either the temperature in the room or the humidity when the subject’s core temperature began to rise.

That combination of temperature and humidity through which the person’s core temperature begins to rise is called the “critical environmental limit”. Below those limits, the body is able to maintain a relatively stable core temperature over time. Above those limits, core temperatures are constantly rising and the risk of heat-related diseases with prolonged exposure is increasing.

When the body overheats, the heart has to work harder to pump blood flow to the skin to dissipate the heat, and when you also sweat, it reduces body fluids. In the worst case, prolonged exposure can result in heat stroke, a life-threatening problem that requires immediate and rapid cooling and medical treatment.

Our studies on young healthy men and women show that this upper environmental limit is even lower than the theorized 35 C. It is more like a wet bulb temperature of 31 C (88 F). This will be equal to 31 C at 100% humidity or 38 C (100 F) at 60% humidity.

A graph allows users to see when the combination of heat and humidity becomes dangerous at each degree and percentage.
Similar to the National Weather Service’s heat index map, this graph translates combinations of air temperature and relative humidity into critical environmental boundaries, above which the core body temperature rises. The boundary between the yellow and red areas represents the average critical environmental boundary for young men and women at minimal activity.
W. Larry KenneyCC BY-ND

Dry vs. humid environments

Current heat waves across the globe are approaching, if not exceeding, these limits.

In hot, dry environments, the critical environmental boundaries are not defined by wet bulb temperatures, because almost all the sweat the body produces evaporates, cooling the body. However, the amount of people can sweat is limited, and we also get more heat from the higher air temperatures.

Keep in mind that these cut-off points are based solely on preventing your body temperature from rising excessively. Even lower temperatures and humidity can put stress on the heart and other body systems. And while obscuring these limits is not necessarily a worst-case scenario, prolonged exposure can be severe for vulnerable populations such as the elderly and those with chronic illnesses.

Our experimental focus has now turned to testing older men and women, as even healthy aging makes people less heat tolerant. By adding the increased incidence of heart disease, breathing problems and other health problems, as well as certain medications, it can cause an even greater risk of injury. People over the age of 65 make up about 80% -90% of heat waves casualties.

How to stay safe

Staying well hydrated and looking for areas to cool down – even for short periods – is important in high heat.

As more cities in the United States expand refrigeration centers to help people escape the heat, there will still be many people who will experience these dangerous conditions with no way to cool themselves.

The lead author of this article, W. Larry Kenney, discusses the impact of heat stress on human health with PBS NewsHour.

Even those with access to air conditioning may not turn it on because of the high cost of energy – a common occurrence in Phoenix, Arizona – or because of large-scale power outages during heat waves or wildfires, as is becoming more common in western areas. . USA

A recent study focusing on heat stress in Africa found that future climates would not be conducive to the use of even low-cost cooling systems such as “swamp coolers”, as the tropical and coastal parts of Africa become more humid. These devices, which require much less energy than air conditioners, use a fan to recirculate the air over a cool, wet pillow to lower the air temperature, but they become inefficient at high wet light bulb temperatures above 21 C (70 F).

All in all, the evidence is growing that climate change is not just a problem for the future. This is one that humanity is currently facing and has to tackle head on.