In 1993 and 1994 the householders of 96 UK houses were recruited for a long-term study of the year-to-year variation in radon levels. The houses were chosen because they had been found in earlier surveys (in 1991 and 1992) to have radon levels around 100 Bq m~(-3). This level was chosen because it was high enough to give reasonable statistical accuracy in estimating radon concentrations, but well below the UK radon Action Level of 200 Bq m~(-3) above which householders are advised to implement remedial measures to reduce the concentration. The initial measurements were carried out using passive radon detectors exposed for three months in both the main living area and an occupied bedroom in the house. Repeat measurements were carried out using the same type of detector in the same locations for the same three months in successive years until 1996. After that, the measurements for all houses were carried out simultaneously each year, to reduce the administrative burden of the survey. Each year a few houses dropped out of the study, reducing the number of participants to 70 by 2001. The annual average radon exposure was estimated for each house using two methods: correcting for the typical seasonal variation in radon levels found in earlier studies, or correcting for the change in levels caused by the average outdoor temperature during the measurement period [a]. The variations in corrected and unconnected results from year to year are being analysed using the BUGS and S-Plus statistical packages, and the results will be presented at the Symposium. Initial results using unconnected log transformed data give an estimated mean within dwellings year-to-year standard deviation of 0.40, with a 95% confidence interval of 0.37 to 0.43.
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