[ RadSafe ] Questions about radon

[Otto G. Raabe]

At 05:34 PM 4/22/2009, Steven Dapra wrote:
>Is anything true in this quote?
>
>         "The "action level" recommended by the Environmental 
> Protection Agency for radon in the air is 4 picocuries/liter of 
> air. It is difficult to convert air concentrations to actual 
> exposures in rems or sieverts, but estimates are in the range of 4 
> to 14 rem per year at that concentration. That makes it greater 
> that all the other routine environmental exposures combined."
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April 23, 2009

Airborne radon-222 gas with its decay products, which are 
constituents of the uranium-238 decay series, is the most significant 
source of natural background radiation exposure yielding an annual 
dose by inhalation of about 2.4 rem to the target bronchial 
epithelium of the average American (BEIR V, 1990). Since the tissue 
weighting factor for the bronchial epithelium is 0.08 (the whole lung 
is 0.12) this translates to an effective dose of about 200 mrem per 
year. This is about 2/3 of the annual dose from naturally occurring 
ionizing radiation sources. "That makes it greater that all the other 
routine environmental exposures combined."

Following inhalation of radon and its decay products most of the 
absorbed ionizing radiation dose is deposited in the tracheobronchial 
region of the lungs by radon's short-lived alpha-radiation emitting 
daughters, that are solid metals rather than gases. Less than about 
5% of the absorbed dose to the target tissue is from radon gas. The 
dose delivered depends on the so-called working level (WL) of 
daughter rather than on the radon concentration and the exposure is 
commonly described in working level months (WLM).The daughters 
deposit in the lung airways during inhalation attached to naturally 
present airborne particles or as small molecular clusters (so-called 
unattached daughters). The amount of dose delivered by the decay 
products depend on the so-called unattached fraction and the size 
distribution of the airborne dust particles. Hence, the dose 
delivered for a given concentration of radon gas varies widely 
depending on the local airborne particle and daughter equilibrium 
situations. They ar different in homes from that in uranium mines.

The WL unit is defined as any combination of the short-lived radon 
progeny in one liter of air that will result in the emission of 
130,000 MeV of alpha particle energy. Air having a radon-222 
concentration of 3.7 kBq/m3 (100 pCi/liter) with the progeny in 
secular equilibrium would represent 1 WL. The exposure associated 
with a typical work month in a uranium mine 170 h at 1 WL is called 
an exposure of 1 working level month, WLM. Dosimetric models indicate 
that the nominal dose to the bronchial epithelium associated with 
inhalation of radon decay product aerosols by a uranium miner is 
about 6 mGy/WLM. Assuming an alpha radiation quality factor of 20, 
this yields an absorbed dose of about 120 mSv/WLM or 12 rem/WLM. With 
a cancer weighting factor of 0.08 for the bronchial epithelium, the 
effective dose for cancer induction is about 10 mSv/WLM or 1 rem/WLM.

At 4 pCi/liter radon per liter in home room air, 12 hours per day 
residence in the home for one year and assuming a uranium mine 
atmosphere (which room air is not), the calculated annual exposure 
would be 12 hours time 365 days per year = 4380 hours or 4380/170 = 
26 working months at 4/100 WL =  1 WLM. This suggests an epithelial 
tissue absorbed of 12 rem. The statement:  ["The action level 
recommended by the Environmental Protection Agency for radon in the 
air is 4 picocuries/liter of air. It is difficult to convert air 
concentrations to actual exposures in rems or sieverts, but estimates 
are in the range of 4 to 14 rem per year at that concentration."] is 
reasonable given the occupancy, aerosol, daughter equilibrium 
uncertainties. Of course, the effective dose for lung cancer is 8% of 
these values, or in the range from about 0.3 to 1 rem.

In contrast, he annual average absorbed dose to the lungs' bronchial 
epithelium from Po-210 for a typical cigarette smoker is estimated to 
be about 16 rem, about 7 times the U.S. annual average dose of 2.4 
rem from natural radon-220 and its decay products (NCRP-95, 1987d).



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Prof. Otto G. Raabe, Ph.D., CHP
Center for Health & the Environment
University of California
One Shields Avenue
Davis, CA 95616
E-Mail: ograabe at ucdavis.edu
Phone: (530) 752-7754   FAX: (530) 758-6140
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