As workers hurry to cool the exposed fuel rods at the Daiichi nuclear plant in Japan's quake-battered Fukushima prefecture, health officials are screening evacuees from the 12-mile danger zone surrounding the plant for radiation.
Nineteen people have shown signs of radiation exposure following the two hydrogen blasts at the plant's No.1 and No. 3 reactor buildings. And 141 more are feared to have been exposed while waiting for evacuation, including a group of 60 people removed by helicopter from a high school, according to Japan's Nuclear and Industrial Safety Agency.
Although the health impact of radiation at low doses is controversial, the National Research Council maintains that no level of above what occurs naturally is safe. Prior to the latest emergency at the Daiichi plant, radiation levels at the plant reached 3,130 microsieverts per hour -- roughly half the average annual dose in the U.S.
Jacky Williams, director of the Center for Biophysical Assessment and Risk Management Following Irradiation at the University of Rochester Medical Center, called the 12-mile evacuation radius an "extremely conservative safety zone to protect against fallout."
But even if a meltdown is avoided, the possibility of low-level radiation circulating in the air and contaminating the soil following the two steam-releasing explosions is very real, according to Dr. Janette Sherman, author and specialist in internal medicine and toxicology from Alexandria, Va.
"To assume that steam containing radioisotopes found in nuclear reactors is not going to have health effects, I think, is wishful thinking," Sherman said.
Those radioisotopes, such as iodine-131, strontium-90 and cesium-137, get taken up by the body. As they decay, they give off energy in the form of gamma rays, beta rays that penetrate deep through tissues, and alpha rays that damage DNA. Sherman likens them to harmful chemicals that settle in various tissues of the body.
"We know that radioactive iodine, which goes to the thyroid, can cause cancer and stunt children's growth," said Sherman, adding that exposure during pregnancy can damage the fetal brain. "We know strontium 90 goes to bones and teeth and is linked to leukemia and immune dysfunction. And we know cesium goes to soft tissues, like muscle and breast tissue."
For pregnant women, the risk of birth defects and miscarriages may also rise, Sherman said.
The Japanese government has evacuated 184,670 residents from 10 towns in the 20-kilometer exclusion zone surrounding the plant -- a distance that Sherman said might not be sufficient.
"We know nuclear radiation [from Chernobyl] drifted as far as North America," Sherman said.
Drifting fallout could also contaminate food and water beyond the evacuation zone.
"They shouldn't eat or drink anything contaminated by cesium," Sherman said. "All food and drink have to come from outside the area."
The Japanese government has distributed 230,000 units of potassium iodide to evacuation centers bordering the danger zone as a precaution, in case radiation levels surge. Potassium iodide can block radioactive iodine from entering the thyroid -- a butterfly-shaped gland in the neck that produces hormones that regulate metabolism.