Characterizing Indoor and Outdoor 15 Minute Average PM[sub2.5] Concentrations in Urban Neighborhoods.

While a number of studies have looked at the relationship between outdoor and indoor particulate levels based on daily (24 h) average concentrations, little is known about the within-day variability of indoor and outdoor PM levels. It has been hypothesized that brief airborne particle excursions on a time scale of a few minutes to several hours might be of health significance. This article reports variability in measurements of daily (24 h) average PM[sub 2.5] concentrations and short-term (15 min average) PM[sub 2.5] concentrations in outdoor and indoor microenvironments. Daily average PM[sub 2.5] concentrations were measured using gravimetry, while measurements of 15 min average PM[sub 2.5] mass concentrations were made using a light scattering photometer whose readings were normalized using the gravimetric measurements. The measurements were made in 3 urban residential neighborhoods in the Minneapolis-St. Paul metropolitan area over 3 seasons: spring, summer, and fall of 1999. Outdoor measurements were made at a central monitoring site in each of the 3 communities, and indoor measurements were made in 9-10 residences (with nonsmoking occupants) in each community. Residential participants completed a baseline questionnaire to determine smoking status, sociodemographics, and housing characteristics. Outdoor PM[sub 2.5] concentrations across the MinneapolisSt. Paul metropolitan area appear to be spatially homogeneous on a 24 h time scale as well as on a 15 min time scale. Shortterm average outdoor PM[sub 2.5] concentrations can vary by as much as an order of magnitude within a day. The frequency distribution of outdoor 15 min averages can be described by a trimodal lognormal distribution, with the 3 modes having geometric means of 1.1 µg/m³ (GSD = 2.1), 6.7 µg/m³(GSD-1.6), and 20.8 µg/m³ (GSD = 1.3). There is much greater variability in the within-day 15 min indoor concentrations than outdoor concentrations (as much as &sim... [ABSTRACT FROM AUTHOR]