Percentage
The mapped percentages
are estimates based on the table of 48,000 measurement results,
interpreted in a study 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 in a 1987 Wisconsin-US EPA random survey of 1100 houses.
The study showed the year main floor radon test results ranged from 10% to
100% of the corresponding basement short-term results, with most clustered
near the average, 40%. So basements over 10 pCi/L correspond,
statistically in the table (though not for any particular house) to
main-floor year averages over the EPA 4 pCi/L guideline. The more
measurements there are in the table for a given zip code, the more
statistically representative is the estimating procedure.
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