Water seal is the measure using a certain degree of hydrostatic pressure to resist the air pressure in drain pipe and avoid gas in pipe flowing into room. Water seal is usually achieved by trap and arranged under sanitary ware outlet. The common tubular trap are P-trap and S-trap. The water seal height is related to pipe air pressure change, water evaporation rate, water loss, impurity content and proportion and cannot be too high or too low. The too high height will cause the deposition of solid impurities in sewage at the trap bottom and pipe blocking. While if the height is too low, the gas in the pipe will easily overcome the hydrostatic pressure in water seal, flow into room and contaminate indoor environment. When connected to the sanitary ware and sanitary sewer without trap or the drainage pipeline that may produce harmful gas, there must be trap and the water seal depth of trap must be more than 50mm. Strictly prohibit the use of moving mechanical seal instead of water seal. No share of trap of the sanitary ware in the outpatient section, ward, laboratory, test room and other different rooms of medical institutions.