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Exhaled nitric oxide (FENO) in the non-smokers (never smokers are presented with open triangles and ex smokers with filled triangles) and smokers (non-symptomatic are presented with open squares and GOLD stage 0 COPD with filled squares). There was a significant difference between non-smokers and smokers while the difference between the subgroups was not significant. Mean values are shown with horizontal bars.
