Lung cancer and time-related exposure to residential radon in the air.

It is now generally recognized in most countries that Radon-222 is the principal source of potentially hazard1,2 ous radiation source in the general population. Simple devices used to measure Radon levels in the air record the number of alpha particle tracks, evidence of the radioactive disintegration of Radon. The unit of radioactivity of nuclides (isotopes of elements that disintegrate spontaneously) is the Curie, defined as the amount of radioactive nuclide that produces 3.70 x 10l° disintegrations per second. For very low radiation levels the picoCurie (pCi per liter) is used. A pCi is one trillionth of a Curie, or Curie x 10"~2, or 0.037 disintegrations per second. This is equivalent to about one disintegration every 27 seconds. Radon has a very short half-life, but, unfortunatel~ most of its radioactive products are non-gaseous solids that remain in the lungs. One of these, the 210 isotope of lead, has a halflife of 19.4 years and emits penetrating gamma rays (alpha and beta particles have a very short range of penetration in body tissues). The danger of radiation damage, including lung cancer, comes more from this lead isotope and other "radon daughters" than it does from the Radon-222 gas itself. It is due to the effect of steady accumulation of very small doses of radon daughters that cannot be discharged from the body in expired air. In the U~S. a threshold dose of 4 pCi/liter has been set for average room air level in any house or working-place.2 Background radiation averages about 02 pCi/l outdoors, and I pCi/1 indoors. Sweden is a country in which much of the pioneer research has been carried out on the lung cancer risk of exposure to low doses of radon. This recent case-control study1 presents data on 1281 lung cancer cases for which Radon meas-