Table 3.

Synergy index and relative risk for exposures to PM_2.5 and O₃ on total, cardiovascular, and respiratory mortality

Endpoint by category	Relative risk* (95% CI)
Total mortality
Low-low	Reference
Low-high	1.001 (0.998 to 1.003)
High-low	1.007 (1.005 to 1.009)
High-high	1.015 (1.013 to 1.018)
Synergy index	1.92 (1.46 to 3.33)
Cardiovascular mortality
Low-low	Reference
Low-high	1.002 (0.998 to 1.006)
High-low	1.006 (1.002 to 1.010)
High-high	1.011 (1.006 to 1.016)
Synergy index	1.37 (0.92 to 5.56)
Respiratory mortality
Low-low	Reference
Low-high	1.001 (0.994 to 1.008)
High-low	1.012 (1.005 to 1.019)
High-high	1.018 (1.012 to 1.025)
Synergy index†	1.35 (0.90 to 9.21)

CI=confidence interval; O₃=ozone; PM_2.5=fine particulate matter with aerodynamic diameter ≤2.5 μm.

^*Risk for certain strata of low/high levels of air pollutants to days with low levels for both pollutants. Low and high designations were based on whether levels were above or below the median values of air pollutants across cities. First exposure relates to PM_2.5 and second to O₃; low-high=relative risk (RR)₀₁; high-low=RR₁₀; high-high=RR₁₁.

^†Calculated as [RR₁₁−1]/[(RR₁₀-1)+(RR₀₁−1)].