Answer to Question #11141 Submitted to "Ask the Experts"

I have a question about radon risk calculation based on an excess relative risk (ERR) of 0.008 per working-level month (WLM) for exposures of 100 WLM or less. I grew up in a house with approximately 1,500 Bq m^-3 in the basement and 800–1,000 Bq m^-3 in the upstairs living areas (that's what we measured about a year ago when my parents were thinking of selling their home; of course, I have no way of knowing if this was the case continuously or not). I recently saw the Health Physics Society's (HPS) position statement on radon risk and thought I would try to calculate my risk.

Assuming 20 years spent living there, I calculated that my personal exposure could have been around 100 WLM. From the literature I've seen, it appears that most sources state an average lifetime risk of lung cancer for a nonsmoker of 0.4%. Using the ERR of 0.008 and a lifetime lung cancer risk to a nonsmoker of 0.004 (0.4% risk, or 4 in 1,000 people getting lung cancer), I get 0.008 × 100 WLM = 0.8 ERR, and 0.8 × 0.004 = 0.0032 (or 0.4% × 80% = 0.32% excess risk). I then added 0.004 + 0.0032 to get 0.0072, or an overall risk of 0.72%.

So it seems like, on average, exposure to a nonsmoker to 100 WLM would raise lung cancer risk approximately three times. Does this seem like a logical calculation? It seems to follow rather well with the Environmental Protection Agency (EPA) calculations (a nonsmoker spending a lifetime at 740 Bq m^-3 has a risk of approximately 36 out of 1,000 or 3.6%).

I would feel much better about the whole situation if I knew this is indeed the appropriate way to calculate my risk in this case. Although the level in my house was quite elevated, it doesn't seem like my individual lung cancer risk is very high. My doctor says that I am at very low risk. Nonetheless, I take this rather seriously since I saw my uncle die of lung cancer. Would you say my risk is high or low?

Wow, you really have done your homework! If you have not already read this book, you will find Health Effects of Exposure to Radon (BEIR VI) of interest. The book can be downloaded for free from the National Research Council’s website.   

When you start getting into precise calculations of risk, the details get a little confusing. But the result is that indeed, your incremental risk from lung cancer is low, about 0.6% greater than "average." This is consistent with your doctor's opinion and may ease some of your concerns.

Before I show you how to calculate your risk, I want to make a few points:

1. The ERR of 0.008 per WLM is an age-dependent value. The ERR depends on (a) your age at initial exposure and (b) the duration of your exposure. I don't know your source for the ERR (the ERR is not stated in the HPS radon position statement), but the ERR that you are using seems to be a lifetime lung cancer risk rather than an excess relative risk. In addition, the 0.4% "average lifetime risk of lung cancer for a nonsmoker" you cited is actually the ERR for radon at 74 Bq m^-3 for a nonsmoker.
2. The radiation dose is not from the radon itself, but rather from the radioactive radon decay products that stick in your lungs when you breathe. Radon is an inert gas, so you breathe it in and breathe it out. When you state that the concentration is 800–1,000 Bq m^-3 in the living areas, I assume that is a measure of the radon, not the radioactive radon decay products. Proper accounting of the radon daughters will reduce the WLM you calculated by about 75%.
3. Radon concentrations are always changing. They cycle daily because of changes in the barometric pressure; they also change with season and weather. If you are living in the north where the ground freezes in the winter, it is likely that the measurement that you have (800–1,000 Bq m^-3) is not equivalent to the 20-year average concentration in your home.
4. In doing radon dose calculations, you have to account for breathing rate and time per year that you are in the house. You were not breathing radon for 24 hours a day and seven days a week at an average concentration of 900 Bq m^-3, because you went to school, to work, on vacation, etc. Not accounting for your changing breathing rate as you grew up, I calculate your annual exposure to be about 6.5 WLM (a high estimate because it assumes that the radon daughter concentration is the same as the radon concentration, which I know that it is not).
5. A working level month (WLM) is a unit used to quantify exposure in terms of a concentration and contact time. The unit is based on a miner working 170 hours (8 hours per day × 5 days per week × 4.24 weeks per month) in a month breathing air that has a radon concentration (in equilibrium with the short-lived progeny) that is 3,700 Bq m^-3. So based on this, your estimate of 100 WLM per year is a little high.
6. There are differences in risk based on gender, lifestyle (smokers vs. nonsmokers), etc. So there are a lot of nuances that go into calculating radon risk. After taking all of these factors into account you still have variability between individuals. The calculations that you are trying to make for yourself are intended to be used to assess risk for a population of individuals who have been exposed to the same radon concentration as you have been. That is the "average" individual. From a human health perspective you could be healthier than average or more sickly than average, so your risk could vary from the average individual's risk.
7. According to the American Cancer Society, the risk to a female from developing lung cancer is 0.0617, or 6.17% (it is not specified whether this is for a smoker or nonsmoker, so I am assuming that this risk value does not differentiate). This is the baseline risk. 

Given these complexities, a simple approach to calculating your risk might be to use the EPA website, which distills all the nuances down to a simple table. The values in the EPA table are linear, and if you want to do the calculation for 30 pCi L^-1 (about 1,100 Bq m^-3),* the excess relative risk would be 54 out of 1,000, or 5.4%. Note that this table assumes a lifetime of exposure, so if something has been done to your home to reduce the indoor air concentration, your risk will be lower. If nothing has been done to mitigate the radon in your home, I would recommend that you have a subslab depressurization system installed. Also note that the EPA table is for lung cancer deaths, not for contracting the disease. We are making some headway in that area, but we still have more to do.

A more precise calculation requires that we first identify the appropriate ERR. Lubin and Boice (1997) determined a relative risk (RR) of 1.14 at 150 Bq m^-3. Using the BEIR VI report, specifically Table ES-1 on page 12 (note that Dr. Lubin is on the committee for this report), 150 Bq m^-3 is equivalent to 0.58 WLM given 70% occupancy in the home and 40% equilibrium between the radon and the progeny (note that this value is less than the rough estimate discussed previously in point 4; assumptions are given in footnote b of Table ES-1). So the RR = 1.97 (1.14/0.58) per WLM, which is an ERR of 0.97 per WLM.   

From Table ES-1 (page 12, in BEIR VI), the female never-smoker estimated lifetime relative risk (LRR) is 5.317 to 7.440 (using a concentration of 800 Bq m^-3). The EPA recommends that the indoor radon concentration be less than 4 pCi L^-1 (150 Bq m^-3).* The LRR for a female never-smoker living in a home with a radon concentration of 150 Bq m^-3 is 1.819 to 2.229. That means that you are at a 2.92 (5.317/1.819) to 3.34 (7.440/2.229) times higher risk of contracting lung cancer than someone like you living in a home with a concentration of 150 Bq m^-3. This threefold increase in risk is for a lifetime of exposure, but you are no longer breathing 800 Bq m^-3, so we have to correct for that. As with smoking, if one stops breathing radon, the risk factor decreases.

A report from the National Council on Radiation Protection and Measurements (NCRP), Evaluation of Occupational and Environmental Exposures to Radon and Radon Daughters in the United States (NCRP 78, 1984), states that for 20 years of exposure beginning at the age of 1, your lifetime lung cancer risk is 0.002 (0.2%) per WLM (linear extrapolation of 0.00077 for 10 years of exposure and 0.0034 for 30 years of exposure). Based on this value from NRCP and your exposure of about 3.12 WLM per year (based on BEIR VI, Table ES-1), your lifetime cancer risk is 6.8%: baseline risk of 6.17% + (0.2% per WLM × 3.12 WLM). This is about 0.6% greater than the lifetime cancer risk to the "average" person (the baseline risk). Note the baseline risk of lung cancer is quantified accounting for the US indoor radon air concentration, which is not zero. 

Timothy A. DeVol, PhD, CHP 

* The radon concentration units are given here in pCi L^-1 (called traditional units) because that is the unit used by the Environmental Protection Agency. However, the Health Physics Society has adopted the SI (International System) of units and these are given in parentheses.