• Click on Northeast, Northwest or Southern portions of the map to see enlarged portions for those areas

The conversion to the main-floor average mapped is based on a random survey of 1100 houses, in which one-year follow-up measurements were done on the first floors in 200 houses where basement 2-day screening levels were over 4 pCi/L (picocuries per liter).  Year-average main-floor radon levels for individual houses were typically 40% of the corresponding basement short-term levels.  They ranged from 10% to 100% of the basement short-term results, with the distribution peaking at 40%, the average.  For the map, basements with short-term test results over 10 pCi/L were taken to correspond statistically (though not for any particular house) to main-floor year averages higher than the EPA 4 pCi/L guideline.  The more measurement results there are for a given zip code, the better this estimating procedure is.

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