Re: Question RE <Working Level>

[Bernard L Cohen <blc+@pitt.edu>]

	The following is from a review article I recently prepared:

For dosimetry and health effect purposes, the. radon progeny are of much
more importance than radon itself, so it would not seem to be adequate for
these purposes to specify concentrations of radon gas in pCi/L. The most
accurate procedure would be to specify the concentrations of each of its
daughters, but this would be both difficult to measure and cumbersome to
use. As a compromise, the working level (WL) unit, developed for use  in
mines (originally, the maximum allowable level was 1.0 WL), is widely
used. The WL is defined as any combination of radon daughters in I 1iter
of air that will result in the ultimate emission of 1.3 x 10E5 MeV of
alpha
particle energy, which is numerically equal to the alpha particle energy
released by the progeny in equilibrium with 100 pCi of radon gas. 
	To show this, it may be noted that 100 pCi is equivalent to 3.7
decays per sec which is the initial decay rate for each member of the
series in equilibrium. The number of atoms of each daughter is then 3.7
times the half-life in seconds divided by In 2; this is 980 for Po-218,
8600  for Pb-214, 6300 for Bi-214, and <<I for Po-214. The latter three
are destined to decay with the emission of 7.68 MeV of alpha energy each
(the energy of the Po-214 alpha), so their energy releases are 6.6 x 10E4,
4.8 x 10E4, and 0 MeV respectively; while the Po-218 atoms  are destined
to
decay with emission of both their own alpha emission, which is 6.00 MeV
and the 7.68 MeV alpha to give a total alpha energy release of 1.3 x 10E4
MeV. When all of these energy releases are added, the sum is 1.3 x 10E5
MeV. If all of the daughter products were in equilibrium, a radon
concentration of 100 pCi/liter would thus give I WL. However, in nearly
all situations, the progeny are present in much less than the equilibrium
concentration, so I WL is typically equivalent to something like 200 pCi/L
of radon gas. The WL is often the quantity of interest, for if a particle
deposits in the lung, all of its eventual alpha particle decay energy will
be released at that point. Note that radon gas itself does not deposit in
the lung, so its concentration is not directly relevant. The ratio of WL
to radon gas concentration in units of 100 pCi/liter is called the
equilibrium factor, or the F-value.
	The unit of integrated exposure is the working level month (WLM)
which is the exposure at a level of I WL for 170 hr (a working month in a
mine).


Health effects depend on working level and unattached fraction, but the
latter quantity is quite difficult to measure. Since radon progeny
ordinarily attach to dust particles, the working level in a house can be
greatly reduced by removing dust from the air by inadvertent circumstances
(e.g. large areas of sticky surfaces) or by using electrostatic
precipitators, ion generators, or other such devices. But the paucity of
dust particles then results in fewer loci to which newly formed radon
progeny atoms can attach and hence a higher unattached fraction. This is
important because inhaled radon progeny attached to dust particles  have
only about 1-2% probability of sticking to the bronchial surfaces, whereas
an unattached progeny atom has about a 50% probability and hence has a
much greater health impact. Without measuring unattached fraction, working
level is therefore not a reliable indicator of health impacts in homes. 
The
concentration of radon gas, however, is not dependent on dust level.
Roughly, the effects of dust on working level and unattached fraction
cancel each other, leaving health effects dependent only on radon gas
concentration; it is therefore what is ordinarily measured.
	The situation is different in mines because dust levels are always
so high that unattached fractions are negligible, leaving health effects
dependent only on working level; it is therefore what is measured.



Bernard L. Cohen
Physics Dept.
University of Pittsburgh
Pittsburgh, PA 15260
Tel: (412)624-9245
Fax: (412)624-9163
e-mail: blc+@pitt.edu


************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html