Just 0.3%, says David Brenner, Director, Center for Radiological Research at Columbia University Medical Center in New York.

FORBES: Putting Numbers On Japan's Radioactivity Risk

Now, cancer-related risk estimates are "no more than a rough guesstimate, " David Brenner, an expert on radiation-induced cancer at Columbia University, said in an email.

NPR: Astronauts Face Radiation Threat On Long Mars Trip