Six years ago this month, I did my first really long triathlon as a 30th birthday celebration. It was a lovely idea, but in practice it didn't really translate to a fabulous present to myself, since I trained in the New York City spring (cool, dry) and raced in Panama City Beach, Fla., (hotter than hell, humid) in mid-May. The 1.2-mile ocean swim was great, the scorching 56-mile bike ride considerably less fun, and by the time I got to the 13.1-mile "run" on black asphalt, I was wishing I were 29 again. I walked most of it, finished an hour slower than my goal time, and capped the whole party off with a half-hour on a chaise longue in the medical tent, covered by an ice-cube-studded towel.
Thinking back on that disastrous race compels me to share what I've learned since: that the key to continuing an outdoor exercise routine in the warmer months is to listen to your body, whose physiological mechanisms have evolved over hundreds of thousands of years to protect you from dangerously overheating. You have to take action, too, says Jonathan Dugas, an exercise physiologist who, with Ross Tucker, writes the Science of Sport blog. When you first feel hot you will probably take off some clothing layers, then you'll slow your pace a bit, and finally you'll start to walk or stop entirely and get yourself somewhere cooler, he says. "Your brain and body have pretty good ways of getting you to do what they want you to do, and eventually, it becomes so uncomfortable you'll say, 'I'm out of here,' " he says. (Dugas and Ross have been writing on their blog about heat-related changes in performance.)
Because you generally slow down before serious problems arise, it's actually really hard to get exertional heat stroke, a potentially deadly illness that occurs when your body temperature rises above about 104 degrees. Most cases arise during a competition or under such extreme circumstances as military boot camp or an August football practice in full gear, when either your own ego or someone else is yelling at you to continue, says Paul Thompson, a cardiologist at Hartford Hospital in Connecticut. Thompson completed last year's stultifyingly hot Chicago Marathon, coming in second in the men's age 60-to-64 division, but he consciously slowed to about 10-minute miles during the last 4 miles of the race, he says. So if you do listen to your body and take it easy when the conditions aren't ideal, you're not likely to get into trouble.
That goes for drinking, too. Exercisers often want elaborate formulas for how much fluid they should be taking in when it's hot. But those aren't particularly useful, says Dugas, because there are so many variables: gender, body size, training, and just individual biology. Follow your urge to drink, and you'll optimize your fluid balance. "Thirst is 100 percent accurate," he says. And overdrinking—whether water or exercise drinks—has its own dangers. It's not so much a problem for recreational athletes going out for an hourlong run or bike ride, but people who exercise for a long time and drink excessively risk getting hyponatremia, a rare condition that has killed a handful of runners. (I wrote about this condition in 2002, following the death of a runner in the Boston Marathon.) Dehydration, by contrast, may make you uncomfortable and affect performance, but it isn't likely to seriously hurt you. (I wrote last summer about when you should consider sports drinks instead of water.)
The good news is that as the summer wears on, you'll get somewhat used to the heat and humidity, and the same level of exertion will be easier. That acclimatization—involving changes like increased sweat production and sodium conservation through less salty sweat—is key to avoiding problems, says Brendon McDermott, an athletic trainer with the University of Connecticut who worked on the National Athletic Trainers' Association's heat illness prevention tips. "The problems come when you're in Minnesota, and all of a sudden you get a heat wave and it's 90 degrees and humid," he says. "Then you're most at risk, because your body isn't used to dissipating the heat." It can take between 10 and 21 days of training in similar heat and humidity to really acclimate, says Dugas.