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.

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. Long term, radiation can damage DNA and raise the risk 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-term period are not a health concern.

Q. When does it threaten health?

A. 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.

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 say we should avoid unnecessary tests that involve radiation.

Q. What’s the antidote once there is radioactive fallout?

A. Potassium iodide pills can 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.

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

A. 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.

Q. Is this like Chernobyl?

A. No. That Russian plant had no containment vessel around its reactor, so 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.