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Measurement of indoor Concentrations were performed in Zulfi city of Saudi Arabia, using CR-39 track etch detectors .This investigation focused on the influence of different parameters, namely different locations, school categories, school building types, and room type as well as on the existence of differences in radon concentration at floor levels. We divided the Zulfi city into five regions, keeping in mind their geographical locations between Tuwaiq Mountains and Al-Thuwayrat sands. The measured average radon concentrations for regions 1-5 respectively are: 87.0 ± 14.2 Bq/m 3 , 83.4 ± 6.0 Bq/m 3 , 61.6 ± 6.4 Bq/m 3 , 63.7 ± 5.4 Bq/m 3 and 87.5 ± 6. Bq/m 3 and the minimum concentrations are 28.0 Bq/m 3 , 5.5 Bq/m 3 , 1.1 Bq/m 3 , 1.0 Bq/m 3 and 24 Bq/m 3 respectively. These results are still within normal limits and below the action level of 148 Bqm -3 set by the U.S. Environmental Protection Agency (EPA). A test of significance using Minitab program was applied to investigate if radon levels in regions are significantly different from each other. We tried all combinations, and found the following results. The "within regions" (different location) test yielded, region 2 is not significant versus region" 1" (p=0.783) and versus region "5" (P=0.646) , whereas it is significant versus region "3" ( P=0.0160) and also versus region "4" (p=0.018). We investigated government and rented school's building also and none was found significantly different (p=0.052). Floors of the same building were tested in order to examine the radon concentration as a function of storey level. No significant difference was observed at floor levels (p=0.009). When girl's schools versus Boys and kindergartens schools were tested they were found significantly different. It is believed that this significant difference is due to geographical nature of the area, since most of the girl's schools were selected from regions 2 and 3, these regions are relatively close to the Tuwaiq mountains whereas other regions are near to the Al-Thuwayrat sands.

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