The sun is blazing. The temperature is rising. You’re soaked with sweat.

Ugh.

But is it more than a matter of discomfort? Researchers at Pennsylvania State University are finding out that we humans can’t withstand as much heat and humidity as once thought.

This has implications for the elderly and infants, who are more susceptible to the effects of heat, as well as healthy young adults and children.

The Philadelphia area is bracing for a heat wave this week, with temperatures peaking close to 100 degrees on Wednesday and Thursday.

The death in June of 38-year-old Dallas Cowboys running back Marion Barber III was caused by heat stroke, according to an autopsy report, and is a reminder that heat-related illnesses can affect anyone. People who are extremely active — adult and youth athletes, for instance — are at risk of heat stroke if they over-exert themselves in high heat.

» READ MORE: A fever, racing heart rate, and altered mental state: Why was this teen athlete so sick?

Pinpointing how hot is too hot is increasingly important, as heat waves become more frequent and intense due to climate change.

We recently spoke with W. Larry Kenney, professor of physiology and kinesiology at Penn State, and principal investigator on the project.

What is the HEAT Project?

We’ve been studying aging and heat stress — the effects of extreme environments and dehydration on older men and women — for the last 30-plus years.

Most of those studies involved recruiting a group of men and women over the age of 65 and bringing them into our lab, where we have two environmental chambers — large rooms in which we can regulate heat and humidity. We would measure what happens to them — sweating, blood flow to the skin, core body temperature — and compare it to a group of college-age students. That taught us a lot about what happens as we age with respect to our ability to regulate our body temperature.

The PSU HEAT Project — for Human Environmental Age Thresholds — is different in that we are now addressing the question of “how hot is too hot?” What combinations of temperature and humidity are unsustainable for even young men and women and which ones put an extra burden on the elderly?

As the first phase of that, we tested a large group of young men and women to establish a baseline and published a series of papers looking at the upper limits of heat and humidity on the ability of younger, healthy people to control their body temperatures. Now, we’ve turned our attention to people over age 65.

For years, the climatology and meteorology community has theorized that the upper limit for human adaptability — the point at which the body can no longer dissipate enough heat and the core temperature begins to rise — was a “wet bulb temperature” of 95 degrees Fahrenheit. Wet bulb temperature is the air temperature at 100% humidity, which is more of a theoretical parameter. We have not seen a wet bulb temperature of 95 degrees anywhere on Earth, at least for a sustained period of time.

But for the first time, we have tested that “critical upper limit” of temperature and humidity on people of varying ages in our environmental chambers. In some studies, they just sit quietly. In other studies, we have them lightly exercise at intensities mimicking daily activities like cooking or light housework. We either keep the temperature in the chamber constant and start increasing the humidity, or we keep the humidity constant and start increasing the temperature.

Before the subjects go in, they swallow a telemetry pill that is Bluetooth-connected and that allows us to monitor their core temperature. We look at the environment where their temperature doesn’t remain stable anymore, where it starts to increase.

How do our bodies react to extreme heat?

As body temperature rises, we eliminate heat primarily by sweating — as long as that sweat can evaporate — and by pumping more blood to the skin. In humid heat, our ability to regulate our body temperature is limited by the amount of sweat that evaporates. In extreme heat, body core temperature goes up, and that can lead to heat illnesses, like heat exhaustion and heat stroke.

It’s a three-pronged impact on the body. First would be the increased body temperature. Second would be the strain that extreme heat places on the cardiovascular system, in particular the heart, which now has to pump harder and increase its heart rate to maintain the blood flow to the skin. Third would be the impact of heat on all the co-morbidities that people have, such as respiratory disease, kidney problems, diabetes and obesity. All of those factor into our ability to tolerate conditions of high heat and humidity.

Can we get sick and die from extreme heat?

Yes. The most severe heat illness is heat stroke, which is a life-threatening emergency. It is defined by two things. One is an elevated body core temperature, usually over 104 degrees. The second is some sort of cognitive impairment or dysfunction. The brain’s delicate nerve tissue is the most vulnerable to higher temperatures. People suffering from heat stroke may not know where they are or how they got there. They might lose consciousness. If the body is not cooled promptly, it can lead to coma and death.

» READ MORE: How to avoid heatstroke and heat exhaustion

There are two different types of heat stroke. Classic heat stroke happens during heat waves and typically affects people over the age of 65.

The second type is exertional heat stroke. A large part of the problem is the lack of heat acclimation. It takes humans who are exercising in the heat somewhere between nine and 14 days to fully acclimate.

The less severe form of heat illness is heat exhaustion. That is usually the result of dehydration associated with sweating during high-intensity exercise. It can be treated by getting the person in a shady area and, if conscious, giving them fluid to drink. If not, they should be given fluid by IV.

Generally, the elderly and infants are at higher risk for the adverse effects of heat. We are just now starting to collect data on the elderly. For infants, there is no data whatsoever. But they are at higher risk because they have an underdeveloped ability to regulate their body temperature. They are beholden to parents and caregivers to make sure they are hydrated and kept out of those heat-stress conditions.

What have you found about our heat limits?

One of the most interesting findings so far is that humans can withstand less heat and humidity than we thought. The theoretical limit has been a wet bulb temperature of 95 degrees Fahrenheit. But our laboratory studies show that even for young, healthy people, the limit is more on the order of a wet bulb temperature of 88 degrees, which equates to 95 degrees in 75% humidity, or 104 degrees in 55% humidity.

It may seem like we get that a lot, but we don’t. Those conditions — a wet bulb temperature of 88 degrees — are just starting to appear on Earth. They are going to become more common with climate change and more and more heat waves start to develop and get more severe.

We are testing those upper limits on human volunteers so we can get the data to help determine when health alerts should be issued, or what policies should be enacted.

What about climate change?

Climate change is interesting because humans are tropical animals. We can tolerate conditions of high heat and humidity for short periods of time. When we hear about climate change, we hear about the earth’s average temperature increasing. That’s not what creates health risks for humans. It’s the extremes. With climate change, there are more frequent heat waves, they last longer, and they’re more severe. So, we always say, “don’t fear the means, fear the extremes.”

The types of temperatures and humidity — these critical environmental limits — that we’ve simulated in the lab have not been exceeded yet on Earth. But we’re moving in that direction.