PM_2.5: particles with a 50% cut-off aerodynamic diameter of 2.5 µm; PM₁₀: particles with a 50% cut-off aerodynamic diameter of 10 µm. ^#: Pearson correlation coefficients (r) between any two pollutants. The data in bold represents correlations during the warm season (March 16, 2005 to June 30, 2005), whereas the un-bolded data shows correlations in the cold season (October 1, 2004 to March 15, 2005). ^¶: correlations are estimated using 24-h averages over a 30-day lag before each subject's exhaled nitric oxide fraction (F_eNO) measurement except ozone, which used averages of 10:00–18:00 h ozone concentrations over a 30-day lag prior to F_eNO testing.