The steam plume drifting from the Fukushima, Japan, nuclear plant that exploded after Friday's 8.9-magnitude earthquake and the looming possibility of a meltdown there have U.S. scientists warning of possible serious health risks.
Although the steel container protecting the plant's No.1 reactor was not damaged in the explosion, radiation levels near the plant rose to roughly twice that which constitute an emergency situation, according to Japanese officials, prompting a doubling of the evacuation radius to 20 kilometers.
Japan's nuclear safety agency has since reported a malfunctioning cooling system at a second reactor in the same plant.
The partial meltdown at the Three Mile Island Nuclear Generating Station, which ended nuclear power construction in the United States, has never been definitively linked to deaths or cancer. But radiation exposure following the Chernobyl meltdown has been directly implicated in more than 50 deaths and suspected in many more.
The true extent of the disaster's effects, such as the cancer risk posed by widespread low-level radiation, remains unclear.
"No one has yet conclusively assessed this question 25 years after the Chernobyl Accident," said John Williams, professor of nuclear and energy engineering at the University of Arizona.
The acceptable level of radiation exposure, up to 10 millisieverts per year, is based on a "no threshold" model, which assumes a direct relationship between radiation dose and cancer risk and implies anything beyond natural levels is harmful. But some experts argue that low-level radiation may be less harmful on a per unit basis.
Jacky Williams, director and core leader of the Center for Biophysical Assessment and Risk Management Following Irradiation at the University of Rochester Medical Center, called the 20-kilometer evacuation radius an "extremely conservative safety zone to protect against fallout."
The radiation dose following the Hiroshima and Nagasaki bombings was cut in half every 200 meters from ground zero, Williams said.
But the fallout from a Fukushima could vary greatly depending on the nature of any leak -- whether it's through an explosion of the core, as in Chernobyl, or a slow, controlled release of pressure-building gases -- and the wind.
"Members of the public are not in imminent danger at a distance of 20 kilometers, so long as they are not downwind," Williams said.
But while the breadth of the evacuation zone may limit the risk of acute radiation sickness, the potential for chronic conditions, such as cancer and cardiovascular disease remains.
Edwin Lyman, a senior scientist with the Union of Concerned Scientists, said wind over the Fukushima prefecture could boost radiation to cancer-causing levels up to 100 miles away. Tokyo, home to nearly 13 million people in 2009, is roughly 200 miles away.
The plume from the Chernobyl explosion drifted over large parts of Europe and the Soviet Union, prompting the evacuation of more than 300,000 people.
Workers clad in masks and protective clothing are reportedly screening Fukushima evacuees with handheld scanners at evacuation centers bordering the more than 1,200-square kilometer zone.
Such protections, as well as ensuring the food and water supply stays uncontaminated, will be crucial in the days and weeks to come, Williams said.
The distribution of potassium iodine is another important precaution, as a meltdown could release radioactive iodine into the air.
Iodine is taken up by the thyroid -- a butterfly-shaped gland in the neck that produces hormones that regulate metabolism. Radiactive iodine can cause thyroid cancer.
Since Chernobyl, distributing potassium iodide to children has been a standard response when risk of radiation release is high.