Anning is a county in Yunnan province. "PX" refers to paraxylene - a hydrocarbon based on benzene.
These screenshots were taken on March 29, 2013, and shows that both Sina Weibo and Tencent Weibo were censoring searches for "Anning px" (安宁 px).
This screenshot, also taken on March 29, shows that Baidu has banned users from establishing a PostBar (贴吧) forum under the heading "PX."
For more examples of protests against PX plants and related censorship, see: