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

Q

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.

A

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).

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).

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.

1 The radon concentration units are given here in pCi L-1 (called traditional units) because that is the unit used by the EPA. However, the Health Physics Society has adopted the International System (SI) units, and these are given in parentheses. In addition, we have prepared a conversion table to help in converting SI to traditional 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.
Answer posted on 26 April 2017. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.