Studies of HBECs of COPD patients reported that repeated exposure to PM with an aerodynamic diameter of 4μM (PM4) led to increased release of IL-1, IL-8, tumor necrosis factor (TNF)-α and GM-CSF, and that these cells had a decreased capacity of metabolizing organic chemicals from particles comparing to those cells of healthy subjects (52, 53). Here, TNF is linked to chronic obstructive pulmonary disease.