The concentration-response relationship between daily ambient inhalable particle (particulate matter [less than or equal to] 10 [micro]m; P[M.sub.10]) concentrations and daily mortality typically shows no evidence of a threshold concentration below which no relationship is observed. However, the power to assess a relationship at very low concentrations of P[M.sub.10] has been limited in studies to date. The concentrations of P[M.sub.10] and other air pollutants in Vancouver, British Columbia, Canada, from January 1994 through December 1996 were very low: the 50th and 90th percentiles of daily average P[M.sub.10] concentrations were 13 and 23 [micro]g/[m.sup.3], respectively, and 27 and 39 ppb, respectively, for 1-hr maximum ozone. Analyses of 3 years of daily pollution (P[M.sub.10], ozone, sulfur dioxide, nitrogen dioxide, and carbon monoxide) concentrations and mortality counts showed that the dominant associations were between ozone and total mortality and respiratory and cardiovascular mortality in the summer, and between nitrogen dioxide and total mortality in the winter, although some association with P[M.sub.10] may also have been present. We conclude that increases in low concentrations of air pollution are associated with increased daily mortality. These findings may support the notion that no threshold pollutant concentrations are present, but they also raise concern that these effects may not be effects of the measured pollutants themselves, but rather of some other factor(s) present in the air pollution-meteorology mix. Key words: air pollution, mortality, nitrogen dioxide, ozone, particulate matter. Environ Health Perspect 111:45-51 (2003). [Online 14 November 2002] doi:10.1289/ehp.5276 available via http://dx.doi.org/
An association between short-term changes in ambient pollutant concentrations, especially particle concentrations, and daily mortality has been observed in many settings (1,2). These settings have included a range of mean particle concentrations and of particle emission sources and different types and concentrations of other ambient pollutants. When concentration-response relationships between particle concentration and daily mortality have been evaluated, most studies have observed that the relationship is reasonably linear, even down to the lowest concentrations observed in any given study (3-5). One implication of such observations is that no clear threshold concentration can be identified below which no effects of particle pollution are observed.
Conclusions about the linearity of the concentration-response relationship extending down to the lowest observed concentrations should be tempered by the relative scarcity of observations at the lowest, as well as the highest, particle concentrations in most studies. This is reflected in the larger confidence intervals around the fitted relationship typically observed at both low and high concentration extremes (6). Further, error in measuring individual particle exposures due to the use of only a few pollution monitors to reflect individual exposures would be expected to blur any threshold concentration, if one exists. This occurs because some individuals will have true exposures above their "thresholds" when measured exposures are below these thresholds, and some will have true exposures below their "thresholds" when measured exposures are above these thresholds (7). The observed linearity of the concentration-response relationships may therefore partly be a result of measurement error.
Vancouver, British Columbia, Canada, is a large urban area that has low levels of air pollution relative to other large urban areas (2,8). For example, the mean daily P[M.sub.10] (particulate matter [less than or equal to] 10 [micro]m in diameter) concentration in Vancouver during 1994-1996 was lower than that reported for any of the 90 cities studied in the National Morbidity and Mortality Air Pollution Study …