Evaluation of indoor ozone concentration and predictors of indoor-outdoor ratio in Mexico City

As part of a study on the potential adverse health effects of ozone exposure on the respiratory health of young children residing in Mexico City, we used passive ozone monitoring devices to determine microenvironmental ozone concentrations. Indoor and outdoor ozone concentrations were measured at 145 homes and at the schools of participating children. In addition, outdoor concentrations were also measured with continuous monitors at the schools and at stationary outdoor monitoring sites. At the children's homes, indoor ozone levels were 10-30% of the outdoor ozone concentrations. The mean indoor-to-outdoor (I/O) ratio was 0.20 (SD = 0.18). The highest I/O ratios were observed in homes where windows were usuary open during the day, and where there was carpeting or air filters. At school during class hours, the I/O ratio was higher (0.3 to 0.4) than at the children's homes, due to periodic opening of the doors and windows. Given the large disparity in ozone concentrations between different microenvironments, we concluded that, in epidemiological studies conducted in Mexico City, microenvironmental ozone concentrations and time spent in these microenvironments needed to be considered to adequately estimate personal ozone exposure.