Stricken victims' overall appearance testified that they felt unwell. They had confused countenances or lethargic demeanors.
But one August day, Shanahan experienced heat-related illness firsthand while participating in a mountain bike race in southern Missouri.
"Right away, I had the cramps in my legs," said Shanahan, 29, who at the time was in his mid-20s.
"It was hot out there, no doubt about it ... I was barely sweating, and I was drinking a lot of fluids ... my body was not functioning properly."
After Shanahan cooled down by drinking water, splashing water on his body and seeking shade and a fan, he was fine, he said.
"It didn't require going to the hospital seeking advanced medical help," Shanahan said. "It was just me knowing that I needed to cool down for awhile."
With warmer weather comes increased risk of heat-related illnesses -- heat cramps, heat exhaustion and heat stroke -- which can occur when the body doesn't cool properly or enough.
Although a mild form of heat-related illness, heat cramps are painful muscle spasms that usually occur in the legs and abdomen, according to the American Red Cross.
Heat exhaustion is a moderate illness in which the patient's skin is cool, moist, pale, flushed or ashen; headache, nausea and dizziness plus weakness and exhaustion also occur.
Treatment for both heat cramps and heat exhaustion includes moving a person to a cooler place and providing cool water to drink.
Heat stroke, however, is life-threatening and causes a change in consciousness. Victims also suffer a high body temperature; red, hot skin that's either dry or moist; rapid or weak pulse and rapid or shallow breathing. If someone has symptoms of heat stroke, call 911.
Becoming educated about heat-related illnesses is an important step in preventing them, said Shanahan, health and safety coordinator with the American Red Cross.
"We're really just trying to give people a basic idea of what would happen, heat cramps obviously being the first step," Shanahan said.
"If those aren't addressed, it will progress into heat exhaustion. If that's not addressed, it goes into heat stroke. Obviously, if that's not addressed, you're looking at some pretty serious damage being done."
Approximately 400 deaths ascribed to heat occur each year in the United States, said George Luber, epidemiologist with the national Centers for Disease Control and Prevention's National Center for Environmental Health in Atlanta.
"Heat is often a missed underlying cause of death, or it could contribute to existing conditions such as heart disease," Luber said.
"What that implies is that the actual number of deaths that are attributable to heat might be actually higher because the attribution of death from heat doesn't always happen."
Heat stroke can affect the brain, kidneys and muscles, said Dr. Regina Kovach, associate professor of general internal medicine at Southern Illinois University School of Medicine.
"It causes the cells to die, so there's actually a death to muscle cells and release of enzymes from muscles. It causes the cells in the brain not to function correctly," said Kovach, who also is director of internal medicine clerkship at SIU School of Medicine.
"People can become very confused, comatose," she said. "They can have seizures. It can cause the kidneys to fail, and part of that is actually related to muscle damage. It can affect the kidneys primarily, also."
Heat-related illnesses can affect anyone, but they are more likely to affect children (who sweat less than adults, making it harder for them to cool off) as well as the elderly and athletes.
"Anyone during a heat wave who ... isn't thinking clearly, isn't drinking properly or is feeling ill should be evaluated," said Kovach, who added that evaluation would rule out other illnesses such as meningitis, encephalitis, stroke, infection, dehydration, urinary tract infection, heart attack and blood clotting.
People who play sports or are physically active in hot weather also are at risk, Kovach said.
Heat, humidity, an athlete's level of conditioning and hydration are the biggest factors to consider, said Dr. Diane Hillard-Sembell, an orthopedic surgeon with AthletiCare, a program of the Bone & Joint Institute at St. John's Hospital.
Hillard-Sembell remembers when several athletes participating in past Ironhorse triathlons in Springfield suffered from heat cramps and heat exhaustion.
"We did have to send one athlete, in the past, to the hospital who probably had just severe heat exhaustion," said Hillard- Sembell, who also is a physician of arthroscopy, sports medicine and orthopedic surgery at Springfield Clinic.
"You put somebody in a situation like that -- high heat, high humidity, possibly inadequately trained and inadequately acclimatized if they're coming from other areas -- you've got ... risk factors there."
Hillard-Sembell and Ryan Townsend, a certified athletic trainer with AthletiCare, agreed that certain prevention methods are important in avoiding heat-related illness.
Athletes should practice in cooler, early morning and evening hours. During the first couple of days of practice, they should wear lightweight and light-colored clothing. It's also important for athletes to stay adequately hydrated, they said.
Athletes should drink eight glasses of water a day or more, Hillard-Sembell said. A couple of hours before exercise, they should drink 16 ounces of water or a sports drink, and then about 8 ounces of fluid about 15 minutes before exercise. Every 15-20 minutes they should continue to consume 8 ounces of water or sports drink.
"It's important to actually schedule drink times because a lot of people, they'll only drink when they get thirsty, but by the time you're thirsty ... you're dehydrated already," Townsend said.
As little as 1 percent dehydration can have serious effects on performance and safety, Hillard-Sembell said.
"They can weigh themselves before an event and then weigh themselves after," said Hillard-Sembell, adding that for every pound lost, athletes need to drink 16 ounces of fluid to replace any weight loss from exercise.