Experts differ on risks from the low radiation doses resulting from Japanese nuclear reactors.

Dr. John Boice, a professor of medicine at Vanderbilt University, argues that there is little data on doses below about 10 rem (100 millisieverts), while Dr. David Brenner, director of the Center for Radiological Research at Columbia University, is among those who believe there is no threshold. Radiation damages DNA, he says, and just one damaged cell can become the seed of a cancer, though it takes decades to develop.

According to the Environmental Protection Agency, people receive 0.3 rem (3 millisieverts) per year from natural background radiation.

Many of today’s risk estimates are based on a study of 200,000 people who survived the atomic bombing of Hiroshima and Nagasaki in August 1945. More than 40 percent are still alive.