Answer to Question #1476 Submitted to "Ask the Experts"

Q

I live in Pennsylvania where radon levels are quite high and many homes have basement radon ventilation systems. What is my dose sitting on my deck approximately 30 to 40 feet away from my neighbor's exhaust pipe? I know the radon level in my basement is 9 pCi L^-1 and assume the neighboring house would be similar.

A

It is impossible to provide an accurate estimate of the concentration of radon by your deck on the basis of the radon level in your basement. To even guess at the concentration, we would need to know such things as the release rate of radon from your neighbor's vent (for example, picocuries per second), the wind speed, the dimensions of the house, the atmospheric stability, and the ambient background concentration of radon in air. And that would only give us an estimate for when the wind blows towards your deck. Having said that, we will now proceed to do the impossible. What we are about to do is pure guesswork, as much for fun as anything. As such, we will refer to the neighbor as an imaginary one. If you are truly concerned about your radon exposure on your deck, make a measurement. That is the only way to know what the concentration is. Based on the radon concentration in your basement, we will estimate that the emanation rate of radon from the soil in your area is about five times the national average, that is, 2 picocuries per square meter per second (pCi m^-2 per second). The national average is about 0.43 pCi m^-2 per second. Assume that an imaginary neighbor's house covers 140 square meters (1,500 square feet) of ground. Given our assumption that the emanation rate is about 2 pCi m^-2 per second, there would be a natural emission from the soil under the home of about 280 pCi per second (140 m²×2 pCi m^-2 per second). Let's assume that this is the release rate from your imaginary neighbor's vent. The turbulence on the downwind side of this neighbor's home spreads out the radon released from the vent in what is called the building wake effect. Assuming a 50 m² cross-sectional area of the home and a wind speed of 1 meter per second, we might imagine that the radon released from the vent is distributed into approximately 50 cubic meters of air each second (50 m²×1 m per second). This equals 50,000 liters per second. The concentration of radon in the air that reaches your deck when the wind is blowing towards it would therefore be 0.0056 pCi L^-1 (the release rate of 280 pCi per second divided by 50,000 liters per second) due to the release from the vent. This would be so low as to be immeasurable given the fact that the background concentration of radon in outdoor air is about 0.2 pCi L^-1 and highly variable. It is also worth noting that it is far lower than typical indoor radon concentrations. A final point is that dose to lung is delivered by the radioactive decay products of radon (isotopes of Pb, Po, Bi), not the radon itself. As the radon is pumped out of the basement and transported through a vent pipe, most of the decay products will likely be plated out on the fan blades and vent pipe surfaces. New decay products take several tens of minutes "grow in" as the radon decays. With the hypothetical neighbor only 30 to 40 feet away from you, the radon discharged from the vent will reach you in a fraction of a minute, insufficient time for any significant production of radon decay products. Of all the hazards associated with your deck, and there are a number of them, your radon exposure is the last one to worry about. Enjoy your deck with confidence! Paul Frame, CHP, PhD Thomas F. Gesell, PhD