Answer to Question #12932 Submitted to "Ask the Experts"

I cannot find the answer to my question despite exhaustive online searching. At what "rate" does the radon level increase over the years in an unmitigated home? Even with daily/weekly and seasonal spikes, a trend of increasing levels is obvious on a six-month graph. When I bought my home 14 years ago I vaguely remember being told that the radon level was normal, though I never knew what the numerical value was, nor was I present when the testing was allegedly done by the "inspector."

Recently it was tested and the average over two days was 17.7 pCi L^-1 (655 Bq m^-3). (I'm having mitigation done.) Assuming the average was close to 4 pCi L^-1 (148 Bq m^-3) 14 years ago, would this quadrupling of the average have been a STEADY rise (though peppered with spikes)? Or is it possible that for the first, say, five years of living here, the average was still close to 4 pCi L^-1 (148 Bq m^-3) but then only in the last nine years it began rising? I asked the detection tech if he had any idea what the average would have been five years ago and he couldn't answer. And is it possible that it's been 17.7 pCi L^-1 (655 Bq m^-3) for all 14 years?

My townhome is a garden level (lower level halfway underground) plus second level; the recent test was done on lower level.

In summary, does an increase in average radon concentration necessarily follow a course, such that, for instance, if after 20 years the average is 20 pCi L^-1 (740 Bq m^-3) this would mean that 10 years ago the average was 10 pCi L^-1 (370 Bq m^-3), and five years ago the average was 5 pCi L^-1 (Bq m^-3)? Or could a high increase occur suddenly over only a short period, like two or three years?

I need to know just how long I've been breathing in very high concentrations!

First of all, you are taking the right steps in mitigating the radon in your home. A radon concentration of 17.7 pCi L^-1 (655 Bq m^-3) is quite high and would indicate the need for mitigation. However, it is not possible to assess the reason for the discrepancy between the earlier "normal" measured concentration and the recent measured concentration or estimate the potential radon concentrations during the interim without more information. Radon does not build up in a residence over time due to its short half-life (3.8 days).

Many conditions could affect the measured radon concentration over time in a residence. Major factors include season of the year, area of the home where the measurement is taken, changes in the residence itself such as increased or decreased ventilation rates, and atmospheric conditions during the measurement period. Indoor radon concentrations tend to be higher during seasons when the home is closed up, i.e., winter in colder climates. Concentrations are generally higher in the basement or, in your case, garden level, than in the upper levels in the residence. The radon exposure would also depend on whether the garden level or the second level is the area primarily occupied by the residents.  Changes in atmospheric pressure may also affect the influx of radon into the home. The measured concentration could vary over short periods of time even if the average concentration over the longer period remains the same. It depends on the conditions under which the measurements are taken.

Radon has a relatively short half-life; that is, half of the radon present will decay in a bit less than four days. Therefore, radon in the home will not build up over time as long as the source and other characteristics of the residence remain the same. The increase you observed in the measured radon concentration over the 14-year period is most likely due to other factors such as those described above.  

The type of measurement, short term (e.g., two days) or long term (three months) could also be a factor in the inconsistency between the measurements. A long-term measurement would take into account the variability in radon concentration over time (months). While you note that the recent measurement was done on the lower level, you apparently do not know where the earlier measurement was taken. A measurement taken in your second level would most likely have shown a lower radon concentration than a measurement in the garden level. Also, changes in ventilation rates during the 14-year interval between measurements, potentially related to efforts to conserve energy, could affect the indoor radon concentrations. Your technician was correct in that it would not be possible to reconstruct the radon concentrations over the 14-year period based just on the two measurements.

With regard to the risk associated with the elevated radon levels in your home, a relationship between indoor radon concentration and lung cancer is well established. However, the risk is much lower if you are not, or have never been, a smoker. While you can't do anything about the radon levels you might have been exposed to in the past, you are doing the right thing by mitigating the current and future radon levels. You should take a long-term (three month) radon measurement in your home in the area where you and your family spend the most time to get a reasonable estimate of the concentrations to which you are actually exposed. The Health Physics Society has published a position statement on indoor radon titled Update on Perspectives and Recommendations on Indoor Radon.

I'm sorry that we can't give you a definitive answer with regard to the radon concentrations to which you have been exposed during the interim between measurements. 

Follow-Up Question: Thank you for the in-depth response. In the meantime, I have a new simple question: Experts recommend radon tests every two years. This short interval implies that levels can really jump in a short period. But your message says otherwise (save for normal fluctuations from seasons, weather, ventilation). So, I'm wondering why take a measurement every two years if, at least from my new understanding, radon levels tend to be stable? I can understand that baseline test, but I'd think it'd be good for much longer than two years.

Follow-Up Answer: In general, unless conditions change, the average radon concentration indoors should not vary appreciably over time. Short-term variations are to be expected as atmospheric conditions may have an effect. The EPA Citizen's Guide to Radon has general advice with regard to retesting your home. The agency suggests the following:

“If your living patterns change and you begin occupying a lower level of your home (such as a basement) you should retest your home on that level. Even if your test result is below 4 pCi/L, you may want to test again sometime in the future.”

The Health Physics Society Position Statement on indoor radon, Update on Perspectives and Recommendations on Indoor Radon, does not include a specific recommendation on retesting your home but does contain other information that may be helpful.

There are certain factors that could result in a change in the average radon concentration in your home, including renovations such as adding insulation to reduce energy costs, changes in lifestyle, or perhaps even changes in a domestic water source. (Under some circumstances, the water supply adds to the radon concentration in air.) Retesting periodically in the future would give you assurance that your radon mitigation system is working as it should. Radon tests are relatively easy and inexpensive and might provide you with peace of mind.

Janet A. Johnson, PhD, CHP