times, respectively, larger than the equilibrium equivalent DACs for radon and thoron. Thus, the DACs in
the DOE system become 3,333 pCi/L for pure 222Rn and 3,730 pCi/L for pure 220Rn (with 216Po).
The 1993 UNSCEAR Report (Annex A, Table 24) has "effective dose" coefficients for radon and
thoron gas (pure), both indoors and outdoors, in nSv per BqAhAm!3. These are given in Table V. The
stochastic derived air concentration corresponds to 2.5 mrem per hour (i.e., 25 :SvAh!1 or 25,000 nSvAh!1),
so a "5-rem per year" DAC for pure radon or thoron gas can be calculated by dividing 25,000 nSvAh!1 by
the effective dose coefficient. Note that these values, about 3,975 pCi/L and 6,143 pCi/L for radon and
thoron, are comparable to the values derived above from ICRP Publication 32, even though the
approaches are dramatically different and even the dose quantities are different (effective dose equivalent
and effective dose).
Table V. Effective Dose Coefficients for Radon and Thoron Gas (Pure), Both Indoors and
Effective Dose Coefficient
nSv per Bq.h.m-3
7.5.7 Choice of and Use of Assigned Protection Factors for Respirators in Radon and
Thoron Dose Calculations
Equilibrium factors inside respirators have not been measured. Clearly, for HEPA-filtered air-
purifying respirators, the equilibrium factor would be close to zero, since virtually no particles pass
through a respirator. However, radon and thoron are noble gases and will pass unimpeded through a
particulate air filter in an air-purifying respirator. The use of activated carbon filters may impede the
passage of 56-s thoron considerably, perhaps permitting some of it to decay. The use of activated carbon
filters for radon is unlikely to be effective for prolonged exposures, since it will merely retard the passage
of the radon. Using the rule-of-thumb observation that "one gram of carbon acts like 4 liters of air," a 50-
g charcoal canister will act as if it were 200 liters of air, or about 10 minutes' worth of intake by a worker.
Adsorbed radon will begin to desorb after a while and eventually radon will desorb as fast as it absorbs.
Until there are measurements, it is not acceptable to use an assigned protection factor (APF) for radon gas
or thoron gas greater than 1.
Radon and thoron gas concentrations may limit the APF for an air-purifying respirator.
Three options are available for determining APFs for radon, thoron, radon progeny, and thoron
progeny, as summarized in Table VI. The first, best, and simplest option, is to accept the ANSI Z88.2-
1992 APFs for radon progeny and thoron progeny, and to accept APFs of 1 for radon gas and thoron gas.
In the second option, regardless of the actual filtering ability of a respirator, an APF for radon and
thoron progeny in combination with radon and thoron gas is the lesser of either the ANSI Z88.2-1992
(ANSI 1992) value or