HealthTraining & Performance
Sweat Science

The Misunderstood Science of Exercising on Sunny Days

To figure out how your body will respond in hot conditions, consider your “physiological equivalent temperature”

Full sunlight heats up your body about as much as speeding up by 30 seconds per mile. (Photo: Marco VDM/iStock)
Everything is better on a open road

We all know that air temperature isn’t the whole story. In the winter, heading out without considering the wind is a recipe for pain; in the summer, it’s the humidity that gets you. For a long time, I didn’t appreciate how much of a difference direct sunlight also makes. A few years ago, researchers in Japan showed that the equivalent of a clear sunny day cuts time to exhaustion in half compared to an overcast day, even with temperature and humidity held constant. In fact, full sunlight heats up your body about as much as speeding up by 30 seconds per mile.

That’s one reason why race directors and exercise physiologists don’t just look at a thermometer (or even the Heat Index, which factors in humidity but not solar radiation or wind) to guess how runners are going to fare on hot days. Instead, the scale of choice is the “wet-bulb globe temperature,” or WBGT, which combines measurements from three types of thermometer: an ordinary dry bulb to measure air temperature, a wet bulb that incorporates the effects of humidity and wind, and a globe thermometer that measures solar radiation. WBGT is simply a weighted average of the three measurements, based 70 percent on the wet-bulb reading, 20 percent on the globe, and 10 percent on the dry-bulb.

Numerous studies over the years have looked for links between WBGT at races and the number of runners who collapse or need medical attention. The result is a set of thresholds, based on guidelines from the American College of Sports Medicine, that guide runners and race organizers. A WBGT above 82 degrees Fahrenheit (28 Celsius) suggests that the race should be canceled; above 73 degrees (23 Celsius) calls for extreme caution; below 50 degrees (10 Celsius) presents a risk of hypothermia; and so on. Those are the thresholds for safety, but there are a separate set of guidelines that tell you when your pace will be impaired—above 59 degrees (15 Celsius) for a marathon, for example.

This is all pretty well-established and uncontroversial: the first WBGT guidelines for races were developed back in 1983. But a new study in the British Journal of Sports Medicine argues that we can do better with a different scale called the Physiological Equivalent Temperature, or PET, which predicts risk more effectively, especially on sunny days.

The PET is defined as the temperature you’d need indoors to replicate the feeling you’re experiencing outdoors. If you’re outside in the shade on a summer day when the temperature is 86 degrees, the wind is 1 meter per second, and the vapor pressure is 21 mbar (in this case, that means the relative humidity is 50 percent), your body will experience the same heat flow as if you’re indoors in a room at 84 degrees. That means the PET in the shade on that summer day is 84 degrees. But if you step out of the shade into the sunlight, the PET increases to 109 degrees. Solar radiation really matters!

While WBGT simply involves measuring temperature with a few different thermometers, calculating PET is much more complicated because it tries to model how the body actually responds to the temperature, humidity, sunlight, wind, and so on. That involves solving a heat balance equation that includes your metabolic rate, heat generated by movement (the PET assumes “light activity” generating 80 watts of heat), heat lost during breathing, sweat evaporation, heat entering and leaving the body via radiation and convection, and on and on. You use a computer program to solve the equations and spit out the PET for a given set of parameters.

In the new study, a research team from the University of Gothenburg, in Sweden, analyzed data from the 2010 to 2017 Gothenburg Half-Marathons, which draws more than 60,000 runners a year. It’s held in mid-May, which means the temperatures can range from the 50s to the 80s. Using WBGT and PET (along with a similar scale called the Universal Thermal Climate Index), they try to predict the number of runner collapses and ambulance rides. PET comes out on top, predicting 71 percent of the variation in the number of collapses from year to year and 72 percent of the variation in the number of ambulance assistances. That means if you know the PET before a race, you already have a pretty good sense of how many people will need medical help that year. In comparison, WBGT only captures 56 of the variation in each case.

The results are heavily influenced by a couple of hot and sunny years in 2010 and 2013. In both cases, the WBGT was around 80 degrees, just below the threshold for canceling. The PET, in contrast, sounded a more urgent warning with values around 99 degrees, capturing the risks of solar radiation more effectively. Of course, with just eight years of data points, we should be cautious about concluding that PET works better in all cases. As the researchers themselves point out, it will need to be tested for many other races and data sets before any conclusion can be drawn.

I’d never heard of the PET before, so I asked a couple of experts of thermal physiology and endurance what they thought. Turns out they were pretty much in the same boat. “I too had to look it up,” admitted William Roberts, a professor at the University of Minnesota and longtime medical director of the Twin Cities Marathon. Roberts is one of the pioneers of using WBGT in endurance sports, and in fact has been lobbying to have it included in standard weather broadcasts and forecasting. (“We were intending to present our case to the National Weather Service, but the government shutdown closed down the conference a couple of years ago and the project stalled out,” he says.) He saw the Gothenburg results presented at a conference a few years ago, and thought they were interesting, but notes that calculating PET seems complicated.

Stephen Cheung, an environmental physiologist at Brock University in Canada and the co-author of the comprehensive reference book Cycling Science, had a similar reaction. PET has mostly been used in building design and urban planning, he pointed out. It has some nice features, like the fact that it can be applied in both cold and warm conditions, rather than using separate wind chill and humidity scales. And he’s open to the possibility that PET might be better at predicting heat illness than WBGT—but its usefulness depends on it being easily available or simple to calculate, which it isn’t currently.

My own takeaway? I doubt PET is going to take over the world. It’s simply too unwieldy, as far as I can tell. But I do think we need some sort of scale that better captures the effects of strong sunlight. That contrast between a PET of 84 degrees in the shade and 109 degrees in the sun is the first time I’ve seen anything that validates my subjective sense of how much the sun saps me. I always thought it was just that I was a weak, sun-phobic Canadian—but now I know the physiology backs me up.

For more Sweat Science, join me on Twitter and Facebook, sign up for the email newsletter, and check out my book Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance.

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Lead Photo: Marco VDM/iStock

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