PM characteristics and acute changes in respiratory function models, whereas they disappeared in the two- pollutant models. In our primary analyses, we used the OP of aggregated PM fractions. A secondary investigation of OP of the small- est available fraction (PM0.18) did not show consistent associations with the measured respiratory parameters, nor did it reduce the associations observed for PNC and NOx. Our characterization of OP focused on PM and did not include oxidative properties of other co-pollutants, such as O3 and NO2. However, in the two-pollutant models with those co-pollutants, we did not observe con- sistent associations with OP. Our measure- ment of AA or GSH depletion is one of the existing methods of PM OP determination, and it is possible that other methods would show associations that we did not observe. 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