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Health risks of radiation depend on dose, duration, exposure; Amid concern over radiation leaks from Japan's nuclear reactors, here are answers to questions about possible effects.

Concern is mounting about the potential health risks of radiation from the crippled nuclear reactors in Japan. How much radiation you get depends on the dose, duration and method of exposure. Some types of radioactive particles are more dangerous or longer lasting than others.

Some basics:

Q. How are people exposed to radiation?

A. Radioactive particles in fallout can be inhaled into the lungs, fall on the skin or be ingested through contaminated food or water. The level can vary greatly even between short distances, said Dr. Fred Mettler, a University of New Mexico radiologist who led an international study of health effects after the 1986 Chernobyl disaster.

"You can come around a corner, and the dose rate can be very high, and you get back behind a column and the dose rate is much lower," depending on what types of particles are in the fallout, whether you're standing under a roof where they have accumulated or have been shielded from them, he said.

Q. How does radiation harm?

A. In the short term, radiation damages rapidly dividing cells -- hair, the stomach lining, bone marrow. That can cause nausea, vomiting, fatigue, loss of infection-fighting blood cells and clotting problems. Children are most at risk because they have so many rapidly dividing cells.

One type of radiation, radioactive iodine, is taken up by the thyroid gland and can lead to thyroid cancer if pills are not taken right away to prevent this uptake. In the long term, radiation can damage DNA and raise the risk of many types of cancer years down the road.

Q. How much radiation is unsafe?

A. Most people get around three-tenths of a rem (a measurement unit of dose) each year from radiation in the environment, mostly from radon gas in the soil. The U.S. Nuclear Regulatory Commission says doses of less than 10 rems over a long period are not a health concern.

Q. When does radiation threaten health?

A. Symptoms of radiation sickness -- nausea, vomiting and hair loss -- can occur at exposures of 50 to 100 rems, according to the U.S. Environmental Protection Agency. Death within two months becomes a possible risk at around 400 rems and within two weeks at 1,000 rems, the EPA says.

Q. What about medical radiation?

A. A chest X-ray delivers about one-tenth of a rem of radiation; a CT scan of the abdomen and pelvis is 1.4 rems. A person's dose accumulates over time, which is why medical experts recommend avoiding unnecessary tests that involve radiation.

Q. If fallout is occurring, should people flee or stay?

A. Each situation is different and can change rapidly. Japanese officials urged tens of thousands of people to evacuate from a 12-mile zone around the reactors but now have told many more in a broader region, about 20 miles from the troubled plant, to seal themselves indoors. U.S. officials want its citizens 50 miles removed from the reactors.

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