Everyone is exposed to some radiation. It's the level of exposure that determines whether there's any harmful effect.
But how much radiation is a lot? Here are a few numbers for comparison.
(A microsievert is a unit that measures the biological effects of radiation.)
- Limit on whole-body exposure for a radiation worker for one year: 50,000 microsieverts
- One year's worth of exposure to natural radiation from soil, cosmic rays and other sources: 3,000 microsieverts
- One chest X-ray: 100 microsieverts
- One dental X-ray: 40-150 microsieverts
- One mammogram: 700 microsieverts
- CT scan (abdomen): 8,000 microsieverts
- Full-body airport X-ray scanner: 0.0148 microsieverts
- Airplane flight from New York to Los Angeles: 30-40 microsieverts
- Smoking a pack a day for one year: 80,000 microsieverts
- Average dose to people living within 10 miles of 1979 Three Mile Island accident: 80 microsieverts
- Average radiation dose to evacuees from areas highly contaminated by the Chernobyl disaster: 33,000 microsieverts (Of 600,000 of the most-affected people, cancer risk went up by a few percentage points -- perhaps eventually representing an extra 4,000 fatal cancers on top of the 100,000 fatal cancers otherwise expected.)