No doctor would recommend soaking up more than 300 mSv each year for three years, but it's also far from a death sentence. After all, we all receive radiation from naturally radioactive elements in our environment (such as the potassium in bananas) every day. Here in the U.S., we get about three millisieverts each year. Flight attendants live with five millisievert. People who work in nuclear power plants are allowed 20 mSv of annual exposure, and the residents of Ramsar, Iran, who build their homes from a local, radioactive limestone live their entire lives with a whopping 250 mSv annual dose. Fortunately, and inexplicably, cancer there occurs at similar rates to nearby areas, despite background radiation approaching that of the Martian surface.