Radon gas is a naturally occurring, odorless and colorless gas that forms from the radio-active decay of uranium and thorium, two elements that naturally are present in rocks and hence in the soils that form such rocks, and in ground water that has some residence time in such rocks and soils. According to Churchill (1997), long-term exposure to radon concentrations of 4 picocuries per liter (pCi/L) of air or more should be avoided because of the risk of lung cancer which is actually caused by the inhalation of two “daughter” elements formed from the decay of radon, polonium-214 and -218. Preliminary work suggests that average concentrations of radon in indoor and outdoor air is 1.3 and 0.4 pCi/L, respectively, in the United States.

Churchill's initial study indicated that locally, the highest concentrations of radon are emitted from sections of the Monterey Shale and granitic rocks of the Santa Monica batholith. However, other formations, including alluviums also may constitute significant sources. Radon can enter houses through various pathways including pores and fractures in concrete slabs and walls, construction joints and other openings, in water from wells, and from natural rock materials that may be incorporated in the interior house design. Moreover, radon gas, being heavier than air tends to concentrate in crawl spaces, lower floors and basements, and particularly in sheltered areas such as closets or wine cellars where there is less ventilation.

Subsequent work by Churchill (2005) identifies part of California having relative risks of radon contamination based on a statistical study of randomly selected homes. He most recently has reported (Churchill, 2009) that "non-geologic survey approaches" are useful in areas of simple geology, although not suited to determining "hot spots." Maps at a scale of 1:100,000 have been prepared showing parts of California where there is a potential radon risk. From the abstract of his 2009 report, Churchill explains the basis of his mapping as follows:

Four radon potential zones were developed for the map: High, High-Qa (areas of recent alluvium with increased radon potential), Moderate, and Low. Comparisons of radon data for each zone show that the zones are statistically different from each other. The estimated percentages of buildings in each zone with indoor radon levels of 4.0 (pCi/l) or higher are: 28.3 percent in the High zone; 20.6 percent in the High-Qa zone; 9.7 percent in the Moderate zone, and 2.4 percent in the Low zone. The High, High-Qa, and Moderate Zones comprise about 15 percent of the southern Los Angeles County.

His map of southern Los Angeles County shows the area of Malibu he regards as having a "moderate" potential risk, i.e., where he estimates that 9.7 percent of the buildings have radon concentrations of 4.0 pCi/l or higher. Presumably, it is based on the distribution of the Monterey Formation in Malibu as mapped by Dibblee (1993) and Dibblee and Ehrenspeck (1993), although they are not cited specifically in his references. The area in Malibu thus defined, according to Churchill's map, includes about the eastern half of the Malibu Park area immediately north of Zuma Beach, all of Point Dume, and most of the rest of Malibu to its eastern boundary. As a matter of safety, it is prudent to obtain the services of a laboratory to test for radon. The California Geological Survey or the Environmental Protection Agency may be able to provide information about reaching certified radon contractors.

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