Figure 8: Kinetics of pulmonary fibrotic changes in mouse lungs after repeated cycles of O2, IR or O2+IR exposure Separate cohorts of mice (n=5-15 / group) were exposed to 100% O2 for 8 hours only (O2), 0.25 Gy total body ionizing gamma irradiation (IR) only, or a double-hit combination of both challenges (O2+IR) followed by intermittent normoxia (21% pO2) for repeated cycles 3 times a week for 4 weeks. Mice were sacrificed at weeks 1, 2, and 4 weeks (acute) and 8, 12, 16 and 20 weeks (chronic). Lung fibrosis was determined by measuring lung hydroxyproline content. Data is represented mean ± SEM. Letter a indicates p< 0.05 each challenge cohort as it compares with the untreated control at the respective time point.