According to the United States Nuclear Regulatory Commission, the lethal dose of radiation is in the range of 400 to 450 rem. This level of radiation causes death to around 50 percent of a population that has been exposed over a period of 30 days.
According to MIT News, radiation exposure on average in the United States from sources of natural radiation is generally 300 millirems in a year. The exposure to radiation is higher in areas of higher elevations. Areas such as Denver have recorded radiation exposure of around 400 millirems in a year. Natural radiation exposure includes radon and cosmic radiation.