To cause death within hours of exposure to radiation, the dose needs to be very high, 10Gy or higher, while 4-5Gy will kill within 60 days, and less than 1.5-2Gy will not be lethal in the short term. However all doses, no matter how small, carry a finite risk of cancer and other diseases.

How much radiation is too much?

Radiation exposure is commonly measured in millisieverts (mSv). The average person in the U.S. can expect to receive no more than 3 mSv of exposure per year from naturally occurring background radiation. An exposure of greater than 20 mSv is considered high, while greater than 3 mSv to 20 mSv is considered moderate.

What is normal radiation level?

Naturally-occurring background radiation is the main source of exposure for most people. Levels typically range from about 1.5 to 3.5 millisievert per year but can be more than 50 mSv/yr.