RE: Question RE <Working Level>

["william a. goldsmith" <wagoldsmith@mindspring.com>]

The dosimetry of radon-222 and its progeny is one of the most complex
problems in radiation protection.  In spite of much research, it has never
been unequivocally solved.  Radon is simply a radioisotope generator.  The
short lived progeny of radon deliver the dose.  How much dose and where it
is delivered depend on many factors such as the concentrations of daughters
inhaled, aerosol particle size distribution, free ion fraction of the
progeny, how the progeny are inhaled (nose vs mouth breathing), etc.
Correlation of dose with radon concentration is further complicated by the
deposition (plate out) of progeny on surfaces - particularly
electrostatically charged surfaces.
The literature is full of references on this subject, but some of the more
useful ones may be NCRP 78, FRC Report 8, and a very useful article by R. D.
Evans with a title something like "An Engineer's Guide to the Behavior of
Radon Daughters" in Health Physics journal in the mid 1970s.
The Evans article gives an excellent derivation of the working level (WL).
The key to the derivation is that the WL is defined as "ANY concentration of
short lived radon daughters in one liter of air that can produce an emission
of 130,000 MeV of POTENTIAL alpha energy."  A quick "do it yourself" guide
to deriving the 130,000 is as follows:  (1) Calculate the number of atoms of
Po-218, Pb-214, Bi-214, and Po-214 when each is present at 100 pCi of
activity.  (2) Calculate the total POTENTIAL alpha energy contributed by
each of these four: (a) Multiply Po-218 atoms by the sum of Po-218 and
Po-214 alpha energies per disintegration and (b) multiply Pb-214, Bi-214,
and Po-214 atoms by the Po-214 alpha energy.  (3) Sum the total products
obtained in (2) and round to two significant figures.
Short-term, real-time measurements of airborne progeny have been made by
many techniques for counting air filters including multiple alpha counts,
alpha spectroscopy, alpha and beta counting, and various combinations of
these.
This message is much longer than I intended, but this is a complex subject
that has defied simple answers.  Hope this helps.
Bill Goldsmith
Wagoldsmith@mindspring.com

-----Original Message-----
From:	radsafe@romulus.ehs.uiuc.edu [mailto:radsafe@romulus.ehs.uiuc.edu] On
Behalf Of Ivor Surveyor
Sent:	Tuesday, January 12, 1999 2:47 AM
To:	Multiple recipients of list
Subject:	Question RE <Working Level>

Dear Radsafe,

I wonder if anybody be kind enough to help me with some questions on the
definition of <working level>.

Epidemiological studies on Radon and its progeny use a unit called <working
level>.

I understand that is a historical unit, but is still very much favored by
some epidemiologists.

I am interested to know the origin of the unit.
How did the definition 1.3 x 10E5 Mev/L of alpha energy arise?
How did the original workers measure this quantity? Or intend it to be
measured?
Is it necessary to assume a form of secular equilibrium between  radon and
its progeny in order to use the unit?

I read that 1 WL corresponds to a Rn-222 concentration of approximately
100pCi/l or 3.7Bq/l, and/or 7.5 pCi/ or 277mBq/l of Rn-220.  Are these neat
conversions just a coincidence?

I have also read 1 WLM corresponds to an  equivalent dose  of 10 mSv, but
the Olympic Dam mine in South Australia uses the conversion 1 WLM = 5 mSv.
ICRP 90 (B126) page 138. Points out that the complex relationship between
dose to target tissue and concentration in WLM. Matters such as the
aerodynamics of particles, breathing patterns and biological characteristic
of lungs, the lung model used etc.
I note that ICRP gives  a conversion to the tracheobronchial region of
between 4 to 13 mGy per WLM.

This raises two further questions.

In any given industrial site is the conversion of WLM to equivalent dose a
compromise based on management, worker negotiation?

Is variation in conversion from WLM to mSv a problem in comparing findings
from different epidemiological sources?

I hope these question are not considered to be too simplistic.







Ivor Surveyor		[isurveyor@vianet.net.au]
Emeritus Consultant Physician,
Department of Nuclear Medicine
Royal Perth Hospital.

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