New Delhi, Aug. 8 -- She had spent three hours at the beach under a hazy sky, the kind of August afternoon where the clouds seemed to offer cover. By evening, she could not keep water down. Her skin was blistering. Her temperature was 102 degrees. Her family drove her to the emergency room thinking she had food poisoning. The attending physician told them she had the other kind.

Sun poisoning, the informal name for a severe systemic reaction to ultraviolet radiation, is one of the most commonly misunderstood warm-weather medical conditions, partly because its name implies something exotic and partly because its early symptoms mimic a sunburn so closely that many people spend the first hours treating the wrong thing. As heat intensifies a...