Temperatures expected to rise into the 80s apparently forced 4,300 runners to sit out the 116 th Boston Marathon, which kicked off this morning in Hopkinton.
The no-shows, some 16 percent of the 27,000 registered runners, likely took marathon organizers up on an offer to defer into next year's race, an unprecedented gesture prompted by the weather forecast.
"We're asking runners who haven't run previously to think about tomorrow and maybe coming back next year," Boston Mayor Tom Menino told runner at a pre-race pasta dinner Sunday, the AP reported. "We don't want to have any accidents out there, or anybody overtaken by the heat."
Runners keep cool by sweating. But heat and high humidity impede the body's cooling process, according to Dr. Corey Slovis, chair of emergency medicine at Vanderbilt University Medical Center in Nashville, Tenn.
"In temperatures above 70 degrees, the body begins to lose its ability to cool itself. And once the temperature hits 80 degrees, people begin to suffer heat illnesses, such as heat exhaustion and heat stroke," Slovis said.
Runners with heat exhaustion might feel faint or dizzy, and have a headache or muscle cramps. If they don't take it easy, the condition can worsen to heat stroke, a serious heat illness marked by a 104-degree body temperature causing confusion and even the loss of consciousness.
Defending champion Geoffrey Mutai of Kenya dropped out of the race at the 30-kilometer (18-mile) mark because of cramping, the Associated Press reported.
Staying hydrating can help stave off heat illness, but drinking too much water can cause a dangerous imbalance in sodium and other electrolytes.
"It's probably best to drink a combination of water and dilute sports drinks," Slovis said. "Mixing and matching is a great way to go."
Slovis said marathon runners would normally acclimatize themselves to heat in advance of a hot race to avoid heat illness.
"But it shouldn't be this warm so early in the year," he said, explaining that many runners haven't had the chance to train in 80-degree weather. "I very much hope that people do well and we don't see a lot of it."
The Boston Athletic Association said it's prepared for medical emergencies along the 26-mile course. But runners should listen to their bodies and bow out before they need emergency care, Slovis said.
"Trying to run through a cramp or knee pain is one thing. But if people start to have difficulty concentrating, difficulty with vision, or start getting a chill in the heat, it's time to pull over to rest, drink and consider whether you want to continue walking or stop altogether," he said.
Slovis said runners who collapse from heat stroke need plenty of water and air.
"We say, 'wet and windy' is the best way to dissipate heat," he said. "Get them wet and get plenty air circulating around them."
The stress of running in the heat also raises the risk of a heart attack, Slovis said.
"We hope we don't see this, because it's almost always fatal," he said. "But doing compression-only CPR [while waiting for emergency medical services to arrive] can certainly save a life. Pump hard and pump deep 100 times a minute."
The humidity, which stands at 69 percent, is another obstacle in today's race, according to Dr. Robert Truax, a sports medicine expert at UH Case Medical Center in Cleveland.
"As the humidity goes up, you need fewer clothing layers," Truax said. "But that increases the risk of sunburn."
Truax said runners should wear sweat-resistant sunscreen or lightweight running clothing to limit their sun exposure.