The mercury concentration of oeal gas was determinded.Comparison were made on 2 groups consisting of silver amalgam covered cases(n=15) and uncovered cases (n=29) immediately by vaseline after filling silver amalgam,and the concentration of Hg before polishing old silver amalgam and after polishing (n=10),The result showed there are very significant difference (P<0.01).The surface area of filling mercury amalgam did not relate to Hg concentration.The larger volume of polishing old mercury amalgam was,the higher concentration of Hg was.In addition,mercury concentrations of air in four consulting rooms before ventilation were higher than 3.1-6.6 times national hyienic standard,however,it was reduced to 1.2-2.1 times after ventilation.