Answer to Question #1241 Submitted to "Ask the Experts"

Q

I'm puzzled by some measurements I recently made. After a three-hour car trip during a fairly steady rainfall, several samples of "road grime" swiped off of my car onto a paper towel measured approximately 175 cpm. (Background measured about 32 cpm on the same meter). The next morning (about 15 hours later) these same samples measured at, or very near, background. A "fresh" sample from the car (it had been parked all night and it was still raining slightly) measured about 145 cpm. However, upon my return home (approximately 9-10 hours later) all samples previously taken, as well as "new" samples taken off the car, showed no activity above background. Could this have been "washout" of radon progeny that accumulated on the car? Or is it more likely that I encountered something on the road? In either event, should I be concerned about exposure? Some details that might help: Meter was a Medcom Inspector, 2" pancake GM, mica window; 3500 cpm mR^-1 hr^{-1 137}Cs (0.001-100 mR hr^-1). Roads were all paved interstate and local. The last 50 miles were rural (forested and/or clear cut) and far from any developed area (west coast, Washington State). In addition, I came across a reference to an event at Argonne National Labs (Health Physics, Nov. 1999) about rain/radon progeny triggering alarms. How can I locate the article?

A

Scavenging and deposition on surfaces by rain (rain out) of the short-lived radon daughters (²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, and ²¹⁴Po) is a phenomenon well known to the environmental surveillance community (for example, Klemic, 1996). Because the short-lived radon daughters also emit gamma radiation, background exposure rates can increase by as much as 2 to 3 microR per hour for a short period during and/or following rain. These excursions are readily measurable with instruments that are used to continuously monitor environmental exposure rates, such as pressurized ionization chambers. These events, while interesting phenomena, add little to annual average background and should not be of concern from a health perspective. This phenomenon is reviewed in a report of the National Research Council (1999) and a figure illustrating the variations in environmental exposure rate from several sources can be viewed on the National Academy Press website. Reference: Klemic G. 1996. Environmental radiation monitoring in the context of regulations on dose limits to the public. In: Proceedings of 1996 Congress of the International Radiation Protection Association, April 14-19, 1996. Vienna, Austria: Austrian Association for Radiation Protection. pp. 321-328. Thomas F. Gesell, PhD