Too much oxygen can harm the brain.

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Accordinding to a study researchers scanned the brains of 14 healthy children, ages 6 - 15, as they inhaled 100 percent oxygen through a mouthpice, and monitored their breathing and heart rates. After waiting eight minutes for the youngsters' breathing to return to normal, the team added 5 percent carbon dioxide to the gas mixture and repeated the scan. A comparison of the two scans revealed dramatic differences. "When the children inhaled pure oxygen, their breathing quickened, resulting in the rapid exhalation of carbon dioxide from their bodies. The drop in arbon dioxide narrowed their blood vessels, preventing oxygen from reaching tissue in the brain and hearth," said one of the researchers in neurobiology. That's when something surprising hapened on a MRI scan. Three brain structures suddenly lit up; the hippocampus, which helps contrl bllod pressure; the cingulate cortex, which regulates pain percention and blood pressure; and the insula, which monitords physical and emotional stress.