Air stops most cosmic radiation. Yet some particles do reach sea level. The cosmic radiation at sea level is estimated at around 300 microsieverts per year. Cosmic radiation also causes nuclear reactions in air that stops it. Many isotopes created by cosmic radiation are stable, or short lived and remain in air. Heaviest common nucleus in air is O-16. Of nuclei lighter than 16, only 4 are radioactive but have half-life over 21 minutes. These are T (12 years), Be-7 (53 days), Be-10 (1,5 million years) and C-14 (5700 years). What is the ratio of cosmogenic radiation to cosmic radiation on sea level?