中文摘要：

介绍了评定液压和润滑系统油液污染度等级的新旧两种国际标准ISO4406：1999和ISO4406：1987，其中ISO4406：1999污染度等级标准是对ISO4406：1987污染度等级标准的修订。新标准将等级代码增加到了30个，并且对报告形式和报告采用的污染颗粒尺寸都进行了修订。标准修订的根本原因是由于试验粉尘的改变（由ISOMTD粉末取代了ACFTD）以及相应的颗粒计数器校准方法的改变。

英文摘要：

The new and the old international standards for the evaluation of oil pollution levels, namely the ISO 4406：1999 and ISO 4406 ： 1987, were briefly introduced. Among them, the ISO 4406 ： 1999 is the revision to the ISO 4406 ： 1987, which increases the level codes from 26 to 30. Compared with the old standard, tbe report forms of the new one are also changed since different pollution particle sizes were used. The reasons are that the changes of standard test dust （ ACI;TD has been replaced by ISO Medium Test Dust） and calibration method for particle counter.