"They chose to load the dangerous goods in the evening, when they thought they were less likely to be observed. This is one of the common characteristics of such cases," said Xue Zhen, presiding judge at the criminal court of Shanghai High People's Court.

"Another thing these cases have in common is the profit-driven mentality of the violators. Some were willing to turn small profits by going against public morality and neglecting the seriousness of the aftermath," Zou said.

In another such case, a man surnamed Cao received 1,200 yuan for dumping 28 tons of industrial wastewater from a spice factory on farmland in Shanghai's Qingpu district. He was sentenced to one year and six months in jail in June 2010.

"Chemical companies should also avoid any irresponsible methods of passing on the costs of sewage and waste treatment to the community," Zou said.

Shanghai has a dense population and any environmental pollution accident would have serious consequences, he said.

The relevant charge was changed from "causing a major environmental pollution accident" in 1997 to "polluting the environment" in 2011.

"The judiciary authorities have shown their determination to severely punish crimes of environmental pollution on a larger scale," Zou said.