Continuous measurements of concentrations of radon and radon decay products, and equilibrium factor were carried out in the indoor air of 12 schools at the beginning of 2000. Average values of the parameters measured, obtained during working time only and during the whole period, were compared and applied to calculate exposure of a person breathing in this air. Results of short-term continuous measurements are proposed to be used to correct the exposure, generally based on the radon concentration obtained by etched track detectors, and the justification of applying this procedure is discussed.