This parameter has been specified in IEC 1010. You may translate it as "Pollution Degree (in the micro-environment)" or "microenvironmental pollution degree"
http://www.nolatron.com/6010.html
http://www.plcopen.org/pages/tc1_standards/iec_61131_2/


Degrees of Pollution

IEC 1010 specifies different types of pollution environments. Harsher environments require more insulation. As an alternative to increased insulation, the designer can create a cleaner microenvironment for the circuit. This microenvironment can be created by using enclosures, encapsulation, or hermetic sealing.

Pollution Degree 3 -- Conductive pollution occurs, or dry nonconductive pollution occurs that becomes conductive due to condensation that is to be expected.

Example: Circuitry that is exposed to outside air, but will not be in contact with precipitation. A garage door opener would fall under this category.

Note: Although the IEC has defined this pollution degree in other documents, IEC 1010 does not cover Pollution Degree 3.
http://zone.ni.com/devzone/cda/tut/p/id/2827



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Note added at 1 hr (2008-09-17 10:34:03 GMT)
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You may refer to this glossary as well.
http://www.fsxinya.com/zh-CN/displaynews.html?newsID=20660

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Note added at 1 hr (2008-09-17 11:14:29 GMT)
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Strictly speaking, the safest way to translate this term is to translate it as Pollution Degree (in the micro-environment).

http://zone.ni.com/devzone/cda/tut/p/id/2871#toc3