Answer to Question #13127 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation — Radon

The following question was answered by an expert in the appropriate field:

Q

We have a radon bubbler on the well and a charcoal filter capturing the remainder in the house. Just bought a Geiger counter and it's off the charts, 4.96 µSv h-1 for the charcoal filter.
Question: the charcoal filter is in the basement, but my son plays with LEGOs right over it on the main floor. Is the radiation reaching him? Should we just scrap the charcoal filter? Our radon in the water is about 1,000,000 Bq m-3 before the bubbler.

A

Your question is very timely. Potential radiation dose is a common concern with charcoal filters on residential water systems due to the buildup of radon decay products. However, it is unlikely that the radiation dose to your son on the main floor of the house is much greater than the natural background radiation dose because the dose rate decreases significantly with distance from the filter. Given the relatively high concentration of radon in the well water, I would not recommend scrapping the charcoal canister system.

While you don't specify where the exposure rate measurement was taken, I assume it was at the surface of the filter cartridge or the housing. In addition to the mitigating factor of distance from the filter, the solid material between your charcoal system and the main floor will provide shielding, further reducing the exposure rate to occupants of the home. However, to set your mind at ease, I suggest that you take an exposure rate measurement in the location where your son normally plays. Depending on where you live, the natural background dose rate from cosmic radiation (radiation from outer space) and radiation from the ground surface (terrestrial radiation) ranges from an average of 0.32 mSv y-1 to 0.87 mSv y-1 (or an average of about 0.04 μSv h-1 to 0.1 μSv h-1). The annual average dose from indoor radon is approximately 2 mSv y-1 (or an average of about 0.3 μSv h-1) for a total of about 3 mSv y-1 (or about 0.4 μSv h-1).

Without knowing the fraction of the radon removed by the bubbler at the well head, it is not possible to estimate the concentration in the water if the charcoal filter were to be removed. The concern with radon in domestic water is that it is transferred to the air in the residence. The National Academy of Sciences National Research Council report on the risk from radon in drinking water concluded that the radon concentration in residential air resulting from 1,000,000 Bq m-3 in water would be about 100 Bq m-3 (2.7 pCi L-1)* added to the indoor radon levels in air from soil and ambient outside air. While that level would not exceed the Environmental Protection Agency guidance of 4 pCi L-1 (150 Bq m-3) or other international guidance values for indoor radon, it still represents a potential risk. The bottom line is that while it is not possible to accurately predict the radiation dose rate at any specific location in your home without more information, it would be relatively easy to take measurements with your Geiger counter. It would also be prudent to restrict direct access to your charcoal filter because the potential radiation dose depends on the amount of time spent in close contact with the system. Again, I would not recommend scrapping the filter. Depending on whether you have a radon mitigation system in the home and the actual radon concentration in the water at the entry point to the charcoal system, the potential radiation dose from the increased radon emanating from the water without the charcoal filter would likely be greater than the dose from the charcoal filter under normal occupancy conditions.

Jan Johnson, CHP
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* Note that the radon concentration units are given here in pCI L-1 (called traditional units) because that is the unit used by the US Environmental Protection Agency. However, the Health Physics Society has adopted the SI (International System) of units.

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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