An improved ion chromatographic (IC) method has been developed for the separation of nitrate in filter extracts in the presence of high concentrations of nitrite. This analytical method was successfully used for an indirect measurement of ozone (O3) in outdoor and indoor air, following its collection using a nitrite-impregnated passive sampler. The limit of detection and the limit of quantification, using the modified IC method, were 6 microg l(-1) (3sigma) and 20 microg l(-1) (10sigma), respectively. Improved detection limits and low baseline noise were obtained with the use of eluent generator and high-capacity ion exchange column. The optimized method was used for assessing O3 concentration in both indoor and outdoor environments of 28 child care centers (CCCs) located in different parts of Singapore. The O3 concentrations ranged from 0.1 to 11.95 parts per billion (ppb) in indoor and from 3.2 to 21.7 ppb in outdoor environments during the study period. It was found that, among the CCCs investigated in this study, air-conditioned CCCs and those located in close proximity to traffic emissions had significantly lower O3 concentrations indoors.