The Electret Ion Chamber Anomaly
A radon professional uses electret ion chambers (EICs) for a 7-day test in a school. Two EICs are placed in the same room: one on a shelf 5 feet above the floor and one on a desk 2.5 feet above the floor. The room has a ceiling fan running continuously. After exposure, the electrets are read with a voltage reader. The initial voltages were 500 V and 510 V. The final voltages are 350 V and 370 V, respectively. The lab's calibration factor is 0.05 pCi/L per volt. The professional calculates radon concentrations of 3.0 pCi/L and 2.8 pCi/L. The school's action level is 4 pCi/L. The professional must decide if the results are valid and what to report.
1. Evaluate the validity of the EIC measurements considering placement, fan operation, and voltage readings. Are the calculated concentrations consistent and reliable?
Read the worked answer
The EIC measurements appear valid but with considerations. Placement at different heights can yield different radon concentrations due to stratification, but in a room with a ceiling fan running continuously, air is well mixed, so the difference should be minimal. The voltage drops are 150 V and 140 V, which are within typical ranges for a 7-day exposure at these concentrations. The calculated concentrations using the calibration factor are 3.0 pCi/L and 2.8 pCi/L, a 0.2 pCi/L difference (about 7%), which is within the combined uncertainty of the method (typically ±10-20%). The fan operation is good for mixing but could cause some plateout of radon progeny, potentially reducing the measured concentration slightly; however, EICs measure radon gas directly, so plateout is not a significant issue. The reliability is acceptable. The professional should average the two results (2.9 pCi/L) and compare to the action level. Since both are below 4 pCi/L, the room is currently below the action level, but because it's a school, confirmatory testing is recommended, especially if this was a short-term test. The professional should also verify that the electret reader was calibrated and that the EICs were not exposed to high humidity or temperature extremes.
2. If the professional later discovers that the ceiling fan was turned off during the last two days of the test, how would that affect the interpretation? What follow-up actions are needed?
Read the worked answer
If the fan was off for the last two days, the air mixing was reduced, leading to potential stratification. The EIC on the shelf (higher) might have measured lower radon if radon-rich air is denser and settles, but radon is a gas and mixes relatively well; however, without mixing, local variations could occur. The difference between the two EICs might increase, but the measured concentrations are still likely representative of the average over the exposure period, though less certain. The professional should consider the test as having a potential bias: the average might be slightly different. The follow-up actions: 1) Document the fan operation issue. 2) Consider the test as valid but with increased uncertainty; if the result is near the action level (2.9 is not near 4), it may still be acceptable. 3) If the result had been close to 4, a follow-up test would be warranted. 4) Recommend a follow-up test with controlled conditions (fan on continuously) and possibly use a continuous monitor to assess temporal variations. 5) Report the results with a note about the fan operation and its potential impact.