RESUMEN
A substantial part of the public's natural radiation dose is due to the inhalation of radon gas. Most of the exposure occurs indoors where the airborne concentration of radon and its decay products is usually higher than outdoors. Radon activity concentration has been measured twice in 50 classrooms in 5 levels [10 each] during summer and winter using solid-state nuclear track detectors [CR-39]. The indoor radon levels in the classrooms were slightly higher than exclusion limits recommended by the ICRP in the first and second floors and fill within the safe limits in the higher floors, where the average radon concentrations were found to be 57.6 +/- 3.33, 48.5 +/- 3.10, 34.5 +/- 1.71, 29.7 +/- 1.33 and 25.3 +/- 1.88 Bq/m3 for first, second, third, fourth and fifth floors with good ventilation, respectively; and 78 +/- 3.23, 66.9 +/- 2.84, 40.3 +/- 1.70, 34.4 +/- 1.42 and 28.8 +/- 1.75 Bq/m[3] for classes with poor ventilation respectively. The mean annual radiation doses obtained on inhalation exposure to Rn-222 and its degradation products were 0.85 +/- 0.37 and 0.67 +/- 0.23 mSvy[-1] for classes with closed and open windows, respectively. From the results obtained it can be concluded that values of radon and its daughter products and the resulting dose in the classrooms fill within the safe limit. Poor ventilation, construction materials, and radon exhalation from the ground are the main reasons for the relatively high radon concentrations in the lower levels