Examination of underground miner data for radon progeny size reduction as cause of high radon 'inverse' dose rate effect.

摘要

Radon progeny measurements of particle size distributions show that at high radon levels the distributions decrease in arithmetic mean diameters (AMD)--apparently due to increased neutralization rates affected by high radon concentrations. The radon concentration threshold for this diameter decrease effect (enhanced deposition effect, EDE), decreases with increased humidity. From graded screen array measurements, in a test chamber the AMD's are known vs. radon concentration. Knowing all the necessary parameters and using the ICRP 66 human respiratory tract model, the reduction in human lung dose rate per unit radon concentration was computed to be a factor of 2.5 in the test chamber. Supported by other work, this implies that the underground miners must experience a similar lung dose reduction. A prior estimate of a factor of 4 was obtained for the miners, but with only an estimate of the crucially important mine humidity distributions. Here a re-evaluation, using more realistic values for a representative mine work area surface to volume ratio and using in-mine measured humidity data at 108 mine work locations in 15 different mines, the re-computed estimate for the EDE miner lung dose rate reduction was 4.3. Analyzing separately in-mine data in unventilated locations finds a greater transition rate of the EDE lung dose reduction. These results are affirmed by measured mine progeny to radon concentration ratios, mine deposition rates and miners' individual work shift exposures at the North Alice, Utah mine. In comparing our data with other radon "inverse" dose rate effect (IDRE) data, a significant portion of the IDRE must be from reduction in lung dose from this EDE and not from decreased lung cancer incidence per progeny lung dose. We offer the only currently plausible answer for IDRE. This does not negate any bystander effects at the very low radon concentrations where other data are least accurate.