TNF and chronic obstructive pulmonary disease: The objective of this study was to evaluate whether there is a dose response relationship of IL-1β and TNF-α levels with COPD airflow limitation staging according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) criteria and whether there are differential responses in these biomarkers between biomass smoke COPD (BMS-COPD) and tobacco smoke COPD (TS-COPD).