Tree resulting from C&RT analysis illustrating the joint effects of CO, NO2, O3, and PM2.5, treated as ordinal variables by quartile, for pediatric asthma ED visits in Atlanta from 1/1/1999 – 12/31/2009. The tree was grown using an alpha of 0.15 and a minimum node size of 60 observations. Nodes are numbered such that each node, n, produces two child nodes numbered 2n and 2n + 1. Nodes with a bold border are terminal nodes for two-sided α = 0.15, labeled T1 – T13, as indicated by the circle in the upper right-hand corner and are colored according to the strength of association; redder colors indicate a more harmful association. The dotted lines indicate how the tree would appear under different levels of α. For each split of the tree the branch with the more harmful association is bolded.