In a previous blog post, I talked about how all of us are exposed to natural radiation on a daily basis. It mostly comes from space (called cosmic rays) as well as natural radioactive materials found in the soil, water and air. Some people were wondering what cosmic rays are.
Cosmic rays are actually energetically charged particles that originate in outer space (some from the Sun and some from elsewhere in the universe). About 90% of incoming cosmic rays are simple protons and 10% are helium nuclei. We’re protected from most of the cosmic rays by the earth’s magnetic field and also our atmosphere. Since our atmosphere provides some of our screening from these particles, the higher you go in the atmosphere the less protection you have. For that reason, there is slightly more exposure to cosmic radiation in Flagstaff when compared to Yuma (although the difference is of no health consequence).
So when I said earlier this month that if the situation in Japan would continue as-is for several weeks, the total exposure in N. America would be comparable to the amount of radiation that you would get flying from AZ to New York (about 0.04 milli-Sieverts- or about a 1% increase from average annual background), I was talking about the added radiation exposure you get from increased exposure to cosmic rays by spending time at 35,000 feet for a few hours.