Answer to Question #11963 Submitted to "Ask the Experts"

We had a high radon reading of more than 2,200 becquerels per cubic meter (Bq m^-3) in our basement based on a recent test by a licensed inspector. I did not get a value for the first floor where we reside. Since the high reading, the radon has been mitigated, and we purchased a continuous working monitor, which gives average readings of 34 Bq m^-3.

I calculated my total hours in the home to be nine continuous years. For five of those nine years, the basement was finished, and I further estimated that I spent a year total down there. I consulted my doctor who looked at a recent chest x ray, and he did not seem overly concerned.

Is there a way to calculate my risk based on this information? I know the U.S. Environmental Protection Agency (EPA) calculates using a lifetime exposure but given how high the original reading was, I am a little panicked.

Although your concern is understandable given the relatively high levels of radon measured in your home's basement, you do not need to be overly worried about the health risk. From the perspective of the lifetime baseline (already existing) risk of cancer to the U.S. population and the relatively short time you were exposed in your basement (one year compared to an average lifetime of 70 years), the additional risk you would have received is quite small. Here is why.

Here’s what we know:

• The concentration of radon was measured at about 2,200 Bq m^-3. A becquerel is 1 atom decaying in a second.
• You estimate you were exposed for one year.
• The baseline lifetime risk of getting a disease like cancer for the US population is 20% (or 20 in 100). This means that for every 100 Americans, an average of 20 of us would be expected to get cancer in our lifetime.

For radiation exposure, we scientists conservatively assume the risk is directly related to how much radiation dose you get: the higher the dose, the more risk. (Although most scientists also believe that at low doses, there probably is no additional risk or it's too small to worry about. See, for example, the Health Physics Society's position statement Radiation Risk in Perspective.)

But for naturally occurring radon gas in our homes, the dose isn't really from the gas itself, but from the decay products (called "progeny") that the radon atoms turn into. These are particles of other types of atoms that the radon atoms become when they decay because of their physical (atomic) properties.

What does this mean? By estimating the radiation dose you would have received from the radon decay products and using US EPA factors that relate an amount of dose to an additional amount of cancer risk, we can then compare this to our baseline risk (20%) and see if it is significant or not.

In the US EPA's guidance for radon in homes developed some years ago (EPA 2013), they recognized that most of the radiation dose comes from the radon decay products (particles), not from the radon gas itself, and they assumed that in a typical home with average ventilation, the concentration of the decay product particles would be about 50% of the radon gas. Note that your current radon measurement of 34 Bq m^-3 is far below the EPA recommendation in this publication of 4 picocuries per liter or pCi L^-1 (150 Bq m^-3).¹

So assuming a concentration of about 2,200 Bq m^-3 radon for a year and using the currently accepted factor for converting radon decay product concentration to dose (ICRP 1994), we can calculate the annual dose to be about 20 millisieverts (mSv).

Is this bad? Not really. The average annual background radiation exposure from naturally occurring radioactive material in our soil, food, water, air, etc. (including from radon and its decay products) is about 3.5 mSv per year and about 7 mSv per year in Colorado (at high elevation and with a lot of minerals in the soil [NCRP 2009]). So for a 70-year lifetime, the difference between what the average American gets and the lifetime dose of the average citizen who lives in Colorado their entire life is about 70 years × (7 – 3.5 mSv) = 245 mSv. So the 20 mSv dose we estimate you received from the radon in your basement is less than 10% of the additional dose someone who chooses to live in Colorado gets over their lifetime compared to the US average.

Also, using the conversion factor from ICRP (1994) and changing to international units,¹ 20 mSv is the annual limit considered "safe" for occupational exposure to radon and decay products for miners set by the U.S. government (MSHA 1985, NRC 1991).

But what about cancer risk? Using the US EPA factor for converting radiation dose to cancer risk (EPA 2014), i.e., the number of cancers expected per unit of radiation dose, your 20 mSv in that one year would result in an additional risk of 0.17% (or 17 in 10,000). Compare that to the baseline existing cancer risk for Americans of 20% (or 2,000 in 10,000). So your additional risk is very small and you should not need to worry about it, particularly since you have gone ahead and mitigated your radon issue.

Steve Brown, CHP

References
Environmental Protection Agency. Home buyer's and seller's guide to radon. EPA 402/K-13/002; Washington, DC: US Government Printing Office; 2013.

Environmental Protection Agency. Radiation risk assessment at CERCLA sites: Q&A. OSWER 9285.6-20; Washington, DC: US Government Printing Office; 2014.

International Commission on Radiological Protection. Protection against radon-222 at home and at work. ICRP Publication 65; London, England: Sage Publications; 1994.

Mine Safety and Health Administration. Annual exposure limits. 30 CFR Part 57.5038; Washington, DC: US Government Printing Office; 1985.

National Council on Radiation Protection and Measurements. Ionizing radiation exposure of the population of the United States. NCRP Report 160; Bethesda, MD: NCRP; 2009.

Nuclear Regulatory Commission. Standards for protection against radiation. 10 CFR Part 20; Washington, DC: US Government Printing Office; 1991.