According to William Roberts, M.D., medical director of the Twin Cities Marathon in Minneapolis-St. Paul, ambient temperature is only a small factor in predicting heat stress. "At Twin Cities, we've had cases of heatstroke at 50 degrees," he says. "And there have been marathons in Rio de Janeiro without any problems. So much depends on your acclimatization levels."
Dr. Roberts has researched cases of fatal heatstroke among athletes and found that the runner's general health and use of medications were contributing factors, just as important as dehydration and weather conditions. He offers this checklist to determine if you're ready to run in the heat. If you answer "no" to any of questions 1 through 6 or "yes" to question 7, Dr. Roberts advises either exercising indoors or keeping your run very short and very easy.
1. Are you acclimatized—have you been in similar temperatures during the last two weeks?
2. Are you well rested (having gotten at least seven hours of sleep last night), and have you been in cooled/air-conditioned environments for some part of the last 24 hours?
3. Are you hydrated? (If you are hydrated, your urine will be pale yellow in color. If it is dark like apple juice, you're dehydrated.)
4. Are you healthy—no recent illnesses?
5. Are you well nourished?
6. Have you avoided alcohol in the last 24 hours?
7. Are you taking medications with ephedrine or other prescription medicines that might interfere with your thermal regulation (information you can find in the literature that comes with the medication or get from a pharmacist)?