Tan became interested in environmental issues following April 2005's violent struggles over pollution and corruption in the town of Huashui, Zhejiang, where many residents believe that releases of toxic substances from chemical plants into the water supply are destroying crops and causing birth defects.
[citation needed] Further riots in Dongyang, in Xinchang (over a pharmaceutical factory), and at a battery factory in Changxin, convinced Tan to set up an environmental monitoring group, which he did informally in the summer of that year, together with five other individuals: Mr. Lai Jinbiao, Mr. Gao Haibing, Mr. Wu Yuanming, Mr. Qi Huimin, and Mr. Yang Jianming.
Although Tan pleaded innocent and no evidence of any crime was presented (the person from whom the secrets were supposedly taken did not appear), he was convicted in a three-hour trial at the Xihu District Court in Hangzhou, on the morning of May 15, 2006, which was closed to the public.
He was sentenced to 18 months in prison for "illegally obtaining state secrets" by the Hangzhou Municipal People's Intermediate Court on August 11, 2006.
His lawyer Li Heping raised concerns about Tan's health condition in prison, as he suffers from a liver disease.