Well, you might get sick. Certain people are more prone to serious heat illness, as listed in a 2011 Goldman's Cecil Medicine textbook chapter co-authored by Sawka. Particularly young and old people, as well as those who are dehydrated, have excess body weight or have a low level of physical fitness are predisposed. If you have certain health conditions, like a fever or viral infection, or are simply not acclimated to the heat (hello, first tip listed above), then you're more at risk, too.
Sawka lists a few issues you may encounter, in order of increasing severity: heat exhaustion, heat injury and heatstroke. The majority of incidences wind up being heat exhaustion, which people can recover from quickly, Sawka says. Often, this heat exhaustion is confused with heatstroke, and that's just fine, because, as he suggests, "Any time you have a heat illness, you should think of the worst, and be aggressive in treating that person."
So if you or a fellow exerciser is feeling the warning signs of heat illness, which Mayo Clinic identifies as nausea, headache and dizziness, among others, then get aggressive:
Cool off. Stop exercising, find shade, remove unnecessary equipment and clothes and, if possible, stand in front of a fan to improve evaporation of sweat. Boyd's overheated players have access to a tub of ice water – the "gold standard," he says – and both he and Sawka agree that immersing limbs or the whole body in cold water is a crucial step. This action helps reduce body temperature and cool the skin, which forces blood from the skin back to the heart to reduce cardio strain, Sawka says.
Rehydrate. Drink plenty of water as you're cooling off and throughout the following 24 hours to correct your dehydration. However, stop yourself before instinctively shot-gunning a bottle of Gatorade. The Goldman's Cecil Medicine chapter says when managing heat illness, "Rapid overcorrection of serum electrolytes (e.g. sodium) should be avoided."
Call a doctor. If you don't feel better within 30 minutes, call a doctor, Mayo Clinic suggests. Again, with heat illnesses, it's best to assume the worst. In this case, the worst is heatstroke, which can result in brain damage, organ failure and even death. Make the call so you're well enough to exercise during another steamy summer day.