Years ago as a high school student, I attended a summer running camp in Pennsylvania. During a routine six-mile run, I stumbled, and a coach stopped me and asked me if I was okay. “I’m fine,” I assured him, and kept running.

But I wasn’t fine. By the end of the run I had collapsed with heatstroke and had fallen unconscious. The athletic trainers quickly submerged me in a tub of ice to lower my body temperature, and I spent the next week in a hospital, lucky to be alive. Later my coach visited me, angry at himself for letting me continue my run. But I had no memory of our earlier exchange. “You were so convincing,” he told me.

Athletic researchers say one of the most frustrating aspects of heat illness is that the sufferer is often completely unaware of his or her own symptoms.

“Your cognitive ability and your capacity to make a rational decision is indeed compromised,” said Michael F. Bergeron, director of the environmental physiology laboratory at the Medical College of Georgia. “You hear all the time that the athletes themselves didn’t know enough to stop and didn’t want to stop and have no memory of what happened.”

While preparing for a study last year, Dr. Bergeron put himself on a treadmill in a heated room. But knowing the tendency for the heat to distort judgment, he gave his assistant specific instructions before getting started. “I told her that when I got to a certain point, no matter what I say, you stop me,” he said. “I told her that however adamant I was about wanting to keep going, don’t let it happen.”

The study was stopped without incident when Dr. Bergeron’s body temperature reached 103.5 degrees. “I hadn’t lost cognitive control,” he said. “But I remember not being totally aware of exactly what I was doing.”

The lesson is that if a problem with heat illness is suspected, asking someone if they are O.K. is not enough. In addition to high body temperature, the obvious signs of heat illness are red, hot and dry skin with little or no sweating, as well as a rapid strong pulse. A person with heat illness may start stumbling or appear less coordinated than usual. Ask the person if she or he has a headache, nausea or dizziness. Talk to them about a variety of topics to see if they exhibit symptoms of confusion.

If you suspect a person is having a problem with the heat, err on the side of caution and insist they get into shade and cool down. Have them drink water and spray their body with cold water or rub them down with ice or a cold cloth. If they don’t cool down quickly, seek medical advice.

Dr. Bergeron notes that after incidents of heatstroke among student athletes, it often becomes clear that other students had noticed the player “didn’t look quite right.” Kids should be instructed that if their friends start acting funny, confused or mumbling, they should alert an adult.

As a result, athletic researchers recommend that kids and adult exercisers alike should adopt a buddy system when playing or exercising in the heat.

“The athlete is the worst one to make the decision,” Dr. Bergeron said. “We strongly recommend that you have people and kids in like positions sort of assigned to each other so you have a buddy system. It’s your buddy or friend who is likely to notice the behavioral change first.”

The Centers for Disease Control and Prevention answers frequently asked questions about heat illness here.