Vigorous exercise such as running, bicycling or even hiking uphill in intense heat can lead to heat stroke and possibly a heart attack with few warning signs, researchers say.

The danger becomes even greater when the air contains high levels of carbon monoxide and ozone, both of which put additional stress on the heart and lungs, one of the researchers said in an interview Tuesday."I don't think it's generally recognized that even in these short races, when you're exercising very intensely and you're not trained or acclimated, you can very quickly produce and store enough heat to kill you," said Edward Coyle, director of the Human Performance Laboratory at the University of Texas in Austin.

Carbon monoxide can trigger a heart attack in exercisers who unknowingly have latent coronary artery disease, said Peter Raven, an exercise physiologist at the Texas College of Osteopathic Medicine in Ft. Worth.

Ozone irritates the lungs, causing the tiny airways in the lungs to shrink and decreasing the amount of breath taken in with each inhalation, Raven said.