Results
Mean (SD) PM2.5 concentrations ranged from 21.1 (4.7) μg/m3 in 1979–1983 to 14 (3) μg/m3 in 1999–2000 with an average of 17.6 (3.7) μg/m3 observed for the two time periods (Table 1). The three PM2.5 measures were strongly correlated (r = 0.72–0.96), whereas weak inverse correlations were observed between PM2.5 and radon (Table 2).
The significance of an interaction term between PM2.5 and follow-up time was used to assess the plausibility of the proportional hazards assumption.
All analyses were conducted using SAS version 9.2 (SAS Institute Inc., Cary, NC) (21). Ethics approval was obtained from the Ottawa Hospital Research Ethics Board.