MMP9 and silicosis: Compared with healthy controls, silicosis patients had remarkably higher plasma concentrations of matrix metalloproteinase-2 (MMP-2), matrix metalloproteinase-9 (MMP-9), collagen alpha-1(I) protein (COL1A1), and collagen alpha-1(III) protein (COL3A1) (all p < 0.05).