Levels of benzene, a cancer-inducing chemical, in the two most affected districts of Lanzhou returned to safe levels on Sunday morning.

Levels of benzene, a cancer-inducing chemical, in the two most affected districts of Lanzhou returned to safe levels on Sunday morning.?

The initial investigation has confirmed that the contamination was caused by nearby petro-chemical factories. An oil pipeline leaked into a channel that links the city's two water treatment plants, leading to the high levels of benzene. Officials at the plants say the channel has been used for nearly 60 years, and suffered an oil leak 30 years ago.