Effects of exposure to CS and filtered air (FA), duration of exposure (10, 16 and 22 weeks), mouse strain (C3H and BL/6), and mouse gender on the development of emphysema. Representative micrographs of the left lungs of (A) male and (B) female mice exposed to CS or FA were processed, sectioned, and stained with hematoxylin and eosin (H&E). Scale bar, 200 µm. (C) The mean linear intercept (Lm, µm) showing interactions among strain, exposure, and duration (P < .0001) and (D) strain, exposure, and gender (P < .05, 4-way ANOVA). Significant differences between individual groups (n = 6–8 mice per group, Tukey-Kramer pair-wise tests, P < .05) are indicated by symbols above the bars as follows: for (C), *duration-, and strain-matched differences from FA controls; #strain-matched differences from 10-week CS exposure; $duration-matched differences between CS-exposed C3H and BL/6 strains, and for (D), *strain- and gender-matched differences from FA groups; #gender-matched differences between CS-exposed C3H and C57Bl/6 strains.