One clear trend stands out: Mental distress tends to linger longer after man-made disasters, like an oil spill or radiation leak, than after purely natural ones, like a hurricane.

The average American receives 360 millirems of radiation each year, 300 from natural sources and 60 from man-made activities. (a rem is a unit of radiation exposure.)

Radiation also is released from man-made sources such as X-ray machines, television sets, and microwave ovens.