FIGURE 1.

Association^a,b between ambient total pollen concentrations during the preceding week and risk of delivery among preterm pregnancies, per IQR width (132.6 grains/m³), pooled across six cities in Ontario, Canada, April–October, 2004–2011. a: Hazard ratios adjusted for maternal age at delivery, parity, smoking during pregnancy, dissemination-area median family income, percentage of the dissemination-area adult female population 25–64 years with a university degree, proportion of dissemination-area population who are visible minority, month/year of follow-up, ambient temperature, relative humidity, NO₂, and O₃. b: The hazard ratios are shown as a smooth black line (red in the online version) and the pointwise 95% confidence intervals are shown in the shaded area.