The investigators recent study has found that ambient melamine exposure can cause the increase of biomarkers of renal tubular injury in melamine manufacturing workers. Thus, in this year, the investigators will examine whether the intervention by installing ventilation in the workplace and providing personal protective equipment and education can decrease ambient melamine exposure and further decrease urinary biomarkers of renal tubular injury such as N-acetyl β-D-glucosaminidase (NAG) and β2-microglobulin (β2-MG).