Radioactive particles in fallout can be inhaled, 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 a 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 type of particles are in the fallout, whether you're standing under a roof where they've accumulated or shielded you from them, etc., he said.

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. Long term, radiation can damage DNA and raise the risk of many types of cancer years down the road.

Most people get around three-tenths of a rem (a 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 time period are not a health concern.

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; within two weeks at 1,000 rems, the EPA says.

A chest X-ray delivers about one-tenth of a rem of radiation, a CT scan of the abdomen and pelvis about 1.4 rems. A person's dose accumulates over time. That's why medical experts say we should avoid unnecessary tests that involve radiation.

Potassium iodide pills can block uptake of radioactive iodine and protect the thyroid gland, but they must be used quickly. "Ideally, you'd give it before they're exposed or at the time" of exposure, Mettler said. "After 12 hours, it's hardly useful" unless exposure is continuing.

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

No. That Russian plant had no containment vessel around its reactor. When an explosion occurred, large chunks of radioactive fuel from the core spewed out. That fuel contained cesium, a longer-lasting and more hazardous radioactive material than the shorter-lived radioactive iodine that has mostly been released in Japan. Still, there have been reports of some cesium release in Japan, prompting worries that a meltdown may be occurring.