Fig. 3

The plots of the joint/interaction effect of \(\text {PM}_{10}, \text {SO}_2, \text {NO}_2, \text {O}_3\), and CO on COPD. The x-axis represents the standardized value for the first exposure factor, and the y-axis represents the exposure-response function estimate for the first exposure factor according to the quantile of the second exposure factor when the values of the remaining exposure factors are fixed.