Answer to Question #10190 Submitted to "Ask the Experts"
Category: Instrumentation and Measurements — Surveys and Measurements (SM)
The following question was answered by an expert in the appropriate field:
I've received an inquiry from a local agency in San Diego, California, concerned that statements from California state agency officials inaccurately characterized the minimal risk of health effects from airborne radiation measured in California March–June 2011. My reanalysis of the California Department of Public Health filter samples (iodine-131, cesium-134, cesium-137) showed at most an increase of 3 µSv per year (annualized dose equivalent using 10 CFR Part 20). However, he insists that there is a health concern based on gross gamma and beta count rates archived at an Environmental Protection Agency (EPA) website. Unfortunately, that particular RadNet monitor was offline until several days after Fukushima. Do you have any suggestions on how to convince him that those cpm don't automatically convert to exposure nor dose nor health risk?
Unfortunately, when evidence exists that a particular nuclear event has resulted in detectable levels of radioactivity in the environment that are above background, many members of the public and some in the "scientific" community, including some who should know better, will assume and/or assert that such levels are hazardous to the public's health. The mere fact of detectability seems sufficient for some to make such statements, often with an unwillingness to consider the actual health significance of the increased levels. It appears that you are dealing with a similar situation. Even though we may employ sound logic, it is often difficult to convince some individuals that no harm has accrued and/or no hazard exists, but we must continue to try.
The airborne activity released from Fukushima took a minimum of about one week to arrive on our West Coast shores. I looked briefly at the available archived data at the site that you included. I looked at the San Diego air-filter data available from 18 January 2010 through 29 September 2011 and calculated the average airborne gross beta activity concentration for the 41 measurements to be 2.1 x 10-4 Bq m-3 with an associated standard deviation of 8.51 x 10-5 Bq m-3. Similarly, the average value and standard deviation calculated for the 25 measurements between May and December of 2011 was 2.69 x 10-4 + 1.40 x 10-4 Bq m-3. The reported concentrations for five measurements on 23 March, 25 March, 5 April, 13 April, and 17 April 2011 were, respectively, 3.48 x 10-3, 2.84 x 10-3, 7.40 x 10-4, 4.79 x 10-4, and 4.47 x 10-4. The first three, and most notably the first two, of these five measurements are well outside of the expected range based on the mean values and their standard deviations for times prior to or following these measurements. Such results would lead one to believe that airborne activity during this time in March was elevated compared to expectations for the typical radioactivity, and the association with releases from Japan is reasonable. Since no March readings prior to 23 March 2011 were reported, it may be true that some earlier readings might have been somewhat higher.
What is not necessarily reasonable is to associate these elevated readings with any significant health impact. The EPA RadNet gross beta activity concentration limit set as the screening level is 0.037 Bq m-3, a value more than 10 times greater than the highest reported measured value above. If an individual breathed air, contaminated at the screening level of 0.037 Bq m-3, at a normal rate of 20 liters per minute continuously for 30 days, his or her intake would be about 32 Bq. If all of this activity was represented by 137Cs, for which the committed effective dose per Bq of inhalation intake is given by EPA (Federal Guidance Report No. 11, 1988) as 8.63 x 10-9 Sv Bq-1, the committed effective dose to the individual would be 2.76 x 10-7 Sv. Such a dose number is of no health consequence, considering that the typical average natural background dose from external exposure plus intakes of naturally occurring radionuclides is about 9 x 10-6 Sv per day which would represent about 2.70x10-4 Sv in the 30-day assumed exposure interval. If the highest measured value of 3.48 x 10-3 Bq m-3prevailed, the 30-day effective dose would be even less, about 2.6 x 10-8Sv. We should note that if radioiodine (131I) were present at the same enhanced levels as we assumed above for 137Cs, the effective dose consequences would be only a few percent higher than the above estimates, based on an EPA effective dose conversion factor of 8.89 x 10-9Sv Bq-1.
I have not been able to access any specific gamma ray data for San Diego on the EPA site so I cannot make comparative assessments in the same way as for the gross beta data. The available EPA "Near-Real-Time Gross Gamma Ray Count Rate Data" for the March interval of likely interest appear to be deficient in the pertinent graphical displays. The plotted results that are shown are typical of similar EPA results over similar time periods.
There are some data available for specific radionuclide airborne concentrations measured in air samples collected in other southern California locations, and these are likely representative of the range of values that might have prevailed in the San Diego area. For example, there are some data for Anaheim and San Bernadino, with Anaheim showing somewhat higher reported values than San Bernadino.
The highest reported airborne 131I concentration reported for Anaheim (air cartridge collected on 21 March 2011) is 1.9 pCi m-3, which is equivalent to 0.070 Bq m-3. The highest reported 137Cs level for Anaheim (air filter collected 25 March 2011) is 0.031 pCi m-3, equivalent to 0.00115 Bq m-3. If these two concentrations prevailed for 30 days, which they clearly do not, they would translate to effective doses of 5.4 x 10-7 Sv and 8.6 x 10-9 Sv.
In summary and conclusion, we can state that the releases of airborne radioactivity from the damaged Fukushima reactors have resulted in measurable radioactivity reaching this country, but the dose impact of this activity has been inconsequential from a health standpoint. The typical annual effective dose to an individual from background radiation in California is on the order of 3 × 10-3 Sv from all natural sources of radiation, including radon. The variabilty in this number can be quite large, easily hundreds of µSv, depending on variations in natural phenomena and on the behavior of a given individual. The potential doses from the radioactivity from the Japanese disaster have all been much less than the statistical variations expected for the natural background doses. We have the significant advantage in the radiation dose-assessment field that the sensitivities of our measurement systems are very high, giving us the opportunity to detect very small amounts of radioactivity in environmental media. The fact that we are able to make such measurements sometimes leads to undue concern about public exposure in some situations.
George Chabot, PhD, CHP