  In 1956, H.L. Kusnetz proposed a quick method for radon progeny concentration
measurement in mine atmosphere using a single gross-alpha count of a
membrane-filtered air. The method is still widely used today and is based on a
number of impractical assumptions. An instantaneous sampling time (less than
ten seconds), is one of these assumptions that ignores the build-up and decay
of the progeny on the filter paper during the sampling period. Of special
concern is the 214Bi decay during the sampling period, since 214Po alphas are
lost during the sampling time and cannot be accounted for during the counting
time. In addition, the method assumes that 214Bi activity during the counting
period is constant. This inaccurate assumption can result in a smaller count
rate, especially when counting times are long. Hence underestimated working
levels are expected when using Kusnetz factors without correcting for the
sampling and counting times. In this technical report, exact sampling and
counting time corrections to the method are provided along with the updated
Kusnetz factors that correspond to common equilibrium conditions to correctly
estimate the Working Level in air. Additionally, time corrections to the
commonly used self-absorption correction formula and the lower level of
detectability (LLD) equation used for any sample measurement are given.
