Houses with levels over 10 pCi/L
In a 1987 Wisconsin-US
EPA random study of 1100 houses, 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.
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%.
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.