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The relationship between exhaled nitric oxide (expressed in particles per billion, ppb) and Tiffeneau–Pinelli index (expressed as FEV1/FVC %) (upper panel a) or active MMP-9, assessed in exhaled breath condensate (EBC) and expressed in ng/ml (lower panel b). The p value below 0.05 was considered as statistically significant (by Spearman correlation test). NS non-significant
