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It’s hot out, way too hot to feel comfortable, but not so hot that it’s dangerous to run—if you slow down. But how much of a slow-down can you blame on the heat, and how much is just you being wimpy? Are there guidelines you can apply to adjust your workout and still feel good?

Here’s everything you need to know about running in the heat.

Slow Your Pace

Unfortunately, there isn’t much scientific literature on how to adjust your training to heat. What literature there is focuses on races, and much of that is about marathons. But that doesn’t mean there aren’t lessons in it for general training, too.

The leading study is a 2007 paper in Medicine and Science in Sports and Exercise by Matthew Ely and his team, then of the U.S. Army Research Institute of Environmental Medicine in Natick, Massachusetts.

Ely’s team examined decades of finishing times at seven North American marathons (Boston, New York, Twin Cities, Grandma’s, Richmond, Hartford, and Vancouver), looking to see how the weather affected not just the top finishers, but the 25^th-, 50^th-, 100^t-, and 300^th-place finishers as well. Using a measure of heat stress called the wet bulb globe temperature (, which differ from air temperature readings), they found a progressive slowing of top men’s performances at all temperatures above 40 degrees Fahrenheit.

The top women fared a bit worse, as did the 25^th-, 50^th-, 100^th-, and 300^th-place finishers, with slower runners being more strongly affected than faster ones. At WBGTs of 69°F to 77°F, for example, conditions in which the top male finisher might be slowed by 5–6 minutes, a 3:00 marathoner might be slowed by a whopping 20 minutes, partly because the day was continuing to heat up, and partly because they were simply out in the heat for a longer time.

Heart Rate and Perceived Exertion

Running in heat has a number of effects on the body.

One of the most important is that blood is diverted to the skin in order to facilitate heat loss. This causes the heart to beat faster because it’s now needing to provide blood not only to your muscles, but also the skin, according to Samuel Cheuvront, an exercise physiologist with entrinsic bioscience in Norwood, Massachusetts, who specializes in heat tolerance. And, the longer you run in heat, the higher your heart rate gets, especially if dehydration starts to shrink your blood volume, requiring your heart to work ever harder to keep it circulating. “It’s not just a function of how hard you are exercising, but how long,” Cheuvront says.

That’s important because science has found that perceived exertion is directly related to heart rate. “Exercise in heat feels harder than it does in a cooler environment,” Cheuvront says.

Added to that is the fact that diversion of blood to the skin means that the oxygen it’s carrying isn’t available to your muscles. It’s like running at altitude—enough so, in fact, that there have been studies examining the use of heat training as a substitute for altitude training.

Putting It into Practice

That’s the science. Applying it is more complex.

One possible solution is to use a heart-rate monitor and train at the same heart rate you would target in cooler conditions. That way, you let your heart rate set your pace, rather than your watch.

Another is to use a such as on the website of the Run S.M.A.R.T. coaching project, led by Jack Daniels, author of Daniels’ Running Formula. It predicts the effect of heat on race paces of any length (down to 1500 meters). In training, you could use it as a quick-and-dirty indicator of how to adjust your pace for workouts at any given race effort, such as 5K, 10K, or marathon.

That said, there are some caveats associated with it, the biggest of which is that it appears to give the same result for all distances. For examle, at 77 degrees Fahrenheit, it gives a 6:00/mile runner the same 10-second-per-mile slowdown for everything from marathon pace to a 1500-meters per-mile pace, and that just isn’t accurate. So consider it a rough ball-park, which is as close as we get when factoring in individual differences in heat tolerance and acclimatization anyway.

Another way to adjust training in heat is to do what my group does, which isn’t so much to slow down as to shorten the intervals and lengthen the recoveries. We may even leave the track and move to a shady street to do hill repeats ranging from 100 meters to 400 meters at full, cool-weather intensity, but with plenty of recovery time between them, to keep core body temperature from mounting.

Moving to shade plays a role in this, because air temperatures can be misleading. Which brings us back to wet bulb globe temperature.

Scientifically, WBGT is probably the best method for measuring the combined effects of air temperature, humidity, and intense sunlight. But it’s not something you yourself can measure unless you shell out a few hundred dollars for a specialized thermometer. Furthermore, the numbers it gives don’t translate directly into air temperature, which is why, if you compare the S.M.A.R.T. calculator to Ely’s results, they look vastly different. Humidity and solar intensity matter, as we all know from experience.

What this means, Cheuvront says, is that it’s better to think not so much in terms of air temperature but in terms of “heat stress,” whether that comes from temperature, humidity, or too much sun.

An in your location is available on the national weather service site.

And here’s the for race directors, which can guide you on how much you can expect the heat and humidity, or if you should workout at all.

Built-in Warning Systems

Also, important is to realize that your body has feedback mechanisms designed to protect you. In a fascinating 2010 study in Medicine and Science in Sports and Exercise, Cheuvront and Brett Ely (married to Matthew) had soldiers do 15-minute time trials on stationary bicycles, comparing their results at normal room temperature to those at a whopping 104 degrees Fahrenheit.

Interestingly, these cyclists still started fast, even in heat. But in the first three minutes of the hot time trial, they settled into a more sustainable pace, apparently without conscious plan. More interestingly yet, Cheuvront says, they made this change long before either their core temperatures or heart rates had had time to rise to levels that might affect fatigue. Somehow, he says, these people were picking up signals from their skin temperatures, saying “too fast, slow down.”

Other studies have shown that feeling hot will increase the perceived effort and slow your pace even when it isn’t affecting you physiologically.

Which, ironicaly, brings us to the oldest, simplest advice of all: listen to your body. Run your pace, not the pace of a running partner who might fare better in heat than you do. If you get too competitive, Cheuvront says, “you are running beyond yourself and [can] wind up collapsing.”

However you approach it, be aware of the signs that you might be taxing your body beyond the realm of wisdom. A sudden sense of feeling chilled on a hot day is a not-to-be-ignored danger sign of possible incipient heatstroke. So are nausea, dizziness, or a headache. Also, monitor yourself to make sure you’ve not stopped sweating.

If you experience any of these, figure it’s too hot for your current level training, and call it a day.