The investigators through intervention trial design to examine whether the use of ventilation in the workplace can decrease ambient melamine exposure, oxidative stress markers (such as 8-hydoxy-2-deoxyguanosine, malondialdehyde)and indicators of early renal damage (such as N-acetyl β-D-glucosaminidase, microalbumin and β2-microglobulin). The one week before intervention and the second week after intervention, the volunteers were asked to collect their urine samples for one consecutive week during mornings and afternoons. Blood samples were taken on Fridays morning, workplace air was sampled for 5 consecutive days. Then the investigators continued to trace the melamine concentration change of volunteers and workplace every three-month intervals after intervention, and blood samples were taken on 12th and 24th month.