Thousands of people living around the Japanese nuclear power plant most seriously damaged by the earthquake evacuated Saturday as authorities detected radiation leaks from the crippled facility, began screening evacuees for radiation exposure and announced plans to distribute pills to protect against thyroid cancer.
Government officials stressed that the amount of radiation that had been released by the Fukushima Daiichi reactor after an explosion at the facility appeared to be relatively low, and international authorities said the situation had not yet become a major public health threat.
To try to assess the extent of the exposures, workers at evacuation centers – wearing white masks and protective clothing – used handheld scanners to check everyone for radiation exposure as an estimated 170,000 people fled an evacuation zone that had been doubled to a 12-mile radius around the plant. The number of people possibly exposed to radiation could reach 160, the Japanese nuclear safety agency said.
"They must be planning for a worst-case scenario, which would be the core partially melting down or melting down," said John Boice, scientific director of the International Epidemiology Institute in Rockville, Md.
Three of those known to have been exposed had been chosen for random testing from among 90 patients and staff members at a hospital two miles from the plant who were awaiting evacuation by helicopter and needed to be decontaminated, officials said. None had yet shown physical symptoms of radiation poisoning, officials said.
As authorities in Japan know better than those in any other nation, the extent of the risk from a nuclear power plant ultimately depends on how much and what kind of radioactive material is released, where it travels and how many people are exposed for how long, experts said.
Radiation from nuclear power plants poses a host of health risks, ranging from severe toxic effects in workers exposed to high doses to long-term increased rates of many cancers.
Most of what is known about the risks of radiation comes from studying survivors of the atomic bombings of Hiroshima and Nagasaki during World War II, the 1986 Chernobyl nuclear disaster in Ukraine and radiation exposures for medical purposes.
In Chernobyl, about 30 firefighters who were exposed to very high doses of radiation while trying to douse the blaze at the plant died within a month.
"They had huge exposures," Boice said. "These were the kind of doses that just knocked out the blood system, the gastrointestinal system."
One worker at the Daiichi plant had died from injuries after becoming trapped in the exhaust stack of the plant, according to the World Nuclear Association. At least four other workers were reportedly injured in the explosion and had been hospitalized.
Beyond the deaths of firefighters in Chernobyl, the most well-documented health effect was an increase in thyroid cancer, primarily among children, due to exposure to iodine-131. An estimated 6,000 to 7,000 excess cases of thyroid cancer have occurred because of Chernobyl, mostly among people who were children at the time.
Japanese officials announced plans to distribute potassium iodide pills, which block radioactive iodine from accumulating in the thyroid glands, causing thyroid cancer, to people living around the Fukushima Daiichi facility and the damaged Daini plant about seven miles away.
Of the radioactive elements released in a nuclear plant leak, radioactive iodine has a relatively short "half-life" of eight days, which means that it essentially disappears within about 80 days, said Fred Mettler, a radiation expert at the University of New Mexico. In comparison, another radioactive substance released by nuclear power plants, cesium-137, has a half-life of about 30 years, meaning it poses a much greater risk because it gets into the food chain.