I-131 Dose Conversion Factor, Again

I know this has been brought up before, but I think you need to reconsider the DCF you are using for I-131. The whole body dose is insignificant compared to the thyroid dose - we are talking about a an order or two of magnitude.

You are using a DCF of 6.8E+01mrem/uCi for water. The EPA MCL for I-131 is 3pCi/l. This is based on 730l/year consumed with a maximum exposure of 4mrem/year. If you calculate the DCF they must have used you get 1.8E+03mrem/uCi, a value over 26 times higher than the one you are using.

In the NCI publication "Estimated Exposures and Thyroid Doses Received by the American People from Iodine-131 in Fallout Following Nevada Atmospheric Nuclear Bomb Tests" Appendix 6, section A6.4.3, they give a conversion factor for the adult thyroid of 1.6rad/uCi, which is 1.6E+03mrem/uCi, a value over 23 times higher than the one you are using.

In an earlier post Owen Hoffman stated:

The most recent publication that addresses the uncertainty in the thyroid dose conversion factor as a function of age and gender, is Apostoaei AI, Miller LF., Health Phys. 2004 May;86(5):460-82. Uncertainties in dose coefficients from ingestion of 131I, 137Cs, and 90Sr.

For the most sensitive members of the population, infants, the absorbed dose per Bq ingested is 4 microGy, 4 E-6 Gy per Bq, or 4 Gy per MegaBq (this dose conversion factor is the arithmetic mean of a 95% uncertainty interval estimated for a 1-year old child).

Converting 4e-06Gy/Bq to mrem/uCi gives us 1.5E+04mrem/uCi, a factor of 220 greater than the DCF you are using.

Another way of looking at this is that the EPA says you should not have more than 2190pCi/year of I-131. The Boise, ID rain water on 3/27 had 390pC/l. You drink six liters of that and you are over your yearly dose, equivalent to the cross country airplane flight you give as a comparison.

Why does this matter? I live on the Big Island of Hawaii and lots of people have rain water catchments for their drinking water supply. What are the I-131 levels in them? Who knows? If you look at the air measurements here there is no reason to think that we have not had Boise level rains.

When I contacted the EPA they said that there was no evidence of a problem with rain water in Hawaii. Technically true, but functionally a lie. Absence of evidence is not evidence of absence. I have seen no published data on rain water here.

I also contacted the State Health Department had was told that no one should be drinking catchment water anyway so they aren't doing anything about it. What a joke, for two reasons. One, regardless of what people should do, they are doing it and one would think that the Health Department would take an interest. Two, the University of Hawaii gives out information about how to make your catchment water safe.

Anyway, until last month I would have put the quality of my water up against any municipal system. Until last month.

Sorry for ranting but this is all very frustrating and I would appreciate it if you would reconsider your DCFs.

DavidInHawaii