And the current heat wave in Europe has brought it all swirling back. "I started getting tunnel vision, and I could feel my body weakening, and I collapsed, like just I completely blacked out," he told AFP, recounting how he was evacuated just short of the finish line in 95F (35C) heat that day in California while vomiting nonstop.Krizman says he can feel himself "getting on edge" when he sees news alerts about extreme heat across Europe, where scores of deaths attributed to the heat have been reported."I can feel my heart (rate) elevating," he said.Krizman says that ever since his brush with heat stroke, his tolerance for extreme heat has significantly diminished.While children under the age of five and the elderly are most at risk in heat waves, teens and healthy adults can also suffer serious symptoms that can ultimately be fatal when exposed to high temperatures, especially when engaging in physical activity.At the time, Krizman was a competitive runner, logging about 70 miles (more than 110 kilometers) a week. "I wasn't an Olympic-level athlete, but I was probably more physically fit than 90 plus percent of the population," he said. "I had been hydrating all day long."Human 'threshold' for heat