It's well-established that exposure to large amounts of radiation at once can cause acute sickness and even cancer. (A 1,000 mSv-dose can trigger acute radiation sickness, causing symptoms such as nausea and vomiting; 3,000 mSV can be lethal, according to Thrall.)
But there's no good data on the long-term risks of the low levels of radiation to which we're continually exposed.
According to the World Nuclear Association, annual exposure to 100 mSv or greater carries a measurable, though small, increase in cancer risk. Below that level, it's believed that your body's cells are able to heal themselves from radiation. "There are enzyme systems in the body that repair damage from these low levels of background radiation," says Thrall.
But even small levels of radiation exposure may impact cancer risks later in life.
This has been of particular concern in the medical community, where some experts worry that increasing use of diagnostic CT scans (which has skyrocketed from 3 million annual scans nationwide in 1980 to 70 million in 2007, according to MedPage Today) will impact future cancer rates. For example, in one 2009 study, National Cancer Institute researchers estimated that one in 270 women and one in 595 men who had a heart CT at age 40 would eventually develop cancer related to the test.
While the health benefits of necessary diagnostic imaging usually outweigh the small risks of secondary cancers, it's always a good idea to talk to your doctor before any procedure involving radiation to understand exactly what you're getting, why you need it, and what the potential health risks may be.
Bottom line: Americans are exposed to far more radiation in their daily lives — and especially from certain medical tests — than from dispersed particles traveling across the Pacific. "With what we know now about the situation in Japan, there are no personal or public health risks apparent for people in the United States," Thrall says.
