Modeling Ozone Levels in and around Southern California Homes

To investigate residential ozone (O3) concentrations and their relationship to regional monitoring data, we studied 126 southern California homes between February and December 1994. Using a controlled flow sampler, 481 samples were collected over 24 h sampling periods, both inside (n = 241) and immediately outside (n = 240) residences. Indoor O3 levels (13 ± 12 ppb, arithmetic mean) were almost always below observed outdoor measurements (37 ± 19 ppb). Low outdoor concentrations resulted in uniformly low indoor concentrations, but high outdoor levels resulted in a range of indoor levels. Indoor/outdoor ratios (0.37 ± 0.25) were greater during the summer pollution period. Using information collected from interviews performed before and after sampling, we explored relationships between measured indoor O3, home operating characteristics, and ambient O3 reported at the closest regional monitoring station. Indoor O3 levels were largely determined by outdoor O3 levels and the duration of time that windows were kept open. Ozone measured adjacent to study homes predicted indoor levels no better than station ambient values. These data suggest that ambient O3 measured at regional stations, coupled with information about how homes are operated, predict in-home O3 levels moderately well and are potentially useful for future exposure assessment purposes.