The chief sources of radiation exposure in the United States, as tabulated by the NCRP, are indicated in Table 15-2. The largest single source of exposure is from radon, which is produced in the decay of radium-226 in the soil and enters a house through openings at the base. The "radon" dose arises mostly from the inhalation of the progeny of radon-222, and varies widely from house-to-house depending upon the radium content of the underlying soil, its porosity, and the house construction. The average effective dose of 2.0 mSv/yr (200 mrem/yr) corresponds to the average radon concentration, but there are more than one million homes with radon levels that are more than five times as great. Appendix D has more information on the average annual radiation exposure and its sources that are received by the U.S. population.