Re: 200 to 230

[FRAMEP@ORAU.GOV]

On Fri, 27 Feb 1998 JMUCKERHEIDE@delphi.com wrote:

> Hi Paul, others,
> 
> Interesting. Thanks. Do you, anyone, know what the basis is for this
NCRP
> reported uncertainty of converting the WLM to dose to the bronchial
> epithelium? 

Dr. Cohen is right, Naomi Harley would be the one to ask. 

I would look at NCRP 78, "Evaluation of Occupational and Environmental
Exposures to Radon and Radon Daughters." It might not be real current
but it does reflect the NCRP thinking wrt the dosimetry.   It discusses the
uncertainty associated with each of the assumptions used in the
dosimetry calculations.   

Let me throw out a few random unconnected thoughts regarding the
uncertainty of the dose estimates. This is just inexpert guesswork,
probably highly inaccurate, and I'd appreciate someone elses thoughts .

Just calculating the exposure in WLM has to be tricky ( the general
assumption is 0.2 WLM per year). I don't think the WL concentrations in
homes are nearly as well known as the Rn concentrations. Also, there
has to be a lot of uncertainty wrt the time spent at home versus the time
at work or outdoors.

Going from WLM to a dose has lots of problems too e.g. what is the
appropriate target tissue. The normally assumed target is the basal cells
of the bronchial epithelium, but I've heard that other cells might be at risk
that are at shallower depths than the basal cells. The dose to the latter
would be greater than the dose to the basal cells. On the other hand, the
depth to the basal cells is assumed to be 22 um but this is the shallowest
of the basal cells and I believe that average depth is nearly twice that. 
The distribution of the radon daughters in the lungs depends on the
particle size distribution and this might not be well known. With alpha
particles there has to be self absorption in the mucous of the lungs but
calculating the self absorption requires knowledge of the distribution in
the mucous of the radon daughters. 

Although the concentration of the unattached fraction of the radon
daughters is small, I believe that the dosimetry equations imply that the
unattached daughters  contribute significantly to the dose. The idea is
that the unattached daughters are all assumed to be deposited in the
sensitive regions of the respiratory tract. Nevertheless, I've heard that its
possible that for routine breathing almost all of the unattached fraction
ends up in the nasopharynx and not the lung.

Then, when we've got a dose in rads to the bronchial epithelium, we
have to apply a quality factor for the alphas -  a quality factor that must
have uncertainty in it. And an even larger uncertainty has to be
associated with converting the dose equivalent to the bronchial
epithelium into an effective dose equivalent. I believe that the assumed
weighting factor (of 0.06 or 0.08) has to be based more on guesswork
than on hard data.

PS I believe the calculations work like this

0.2 WLM per year is the average exposure. This is multiplied by 600
mrad/WLM  to give 120 mrad per year to the bronchial epithelium. This
converts into 2400 mrem per year to the bronchial epithelium (quality
factor of 20). This is then multiplied by an assumed weighting factor of
0.08 to give an effective dose equivalent of 192 mrem which is rounded
up to 200.  

If we want to increase the assumed annual radon exposure to the public
based on new numbers for radon concentrations in homes, multiplying
the ratio of the new to old radon concentrations (1.25/1.0) by the
rounded off number of 200 would seem incorrect. The ratio would be
better applied to the 192 number (assuming what I've done is correct).

Best wishes

Paul Frame
Professional Training Programs
ORISE
framep@orau.gov