United Win (China) Technology Ltd Co. [File photo]

He also said one of his colleagues, Li Liang, was one of the four victims, but the company told them Li had died of congenital heart disease.

"That was impossible because we never heard him having such a disease. It must have to do with the toxicant because there was a strong smell at the factory," he said.

Jiao Tan, Li's college roommate, also told China Daily that "Li was very healthy and never had any heart problem before."

Hexane is known to create extensive peripheral nervous system failure in humans. The initial symptoms are tingling and cramps in the arms and legs, followed by general muscular weakness. In severe cases, atrophy of the skeletal muscles is observed, along with a loss of coordination and problems of vision.

Zhu also complained of work overload and low pay at the factory, which he believed had driven many migrant workers like him to suffer from poor health and poverty.

"We had long been dissatisfied with the management, pay and even food provided by the company," he said.

"We had complained to the local government before, but nothing came out of that. There even seems to be an apparent rush from the government to try to play down the consequences after the protest broke out," he said.

China has witnessed an increasing number of mass protests over labor disputes in recent years. On July 24 last year, around 1,000 people launched a 10-hour riot and beat a company executive to death after being told of possible mass layoffs in the wake of a takeover deal at Tonghua Steel plant in Northeast China's Jilin province.

Sun Suiqin, a Shanghai-based lawyer, said an efficient channel on the government level for people to voice their complaints and grievances is lacking, which has given rise to growing public discontent and protests.

"In most cases workers are forced to resort to violence in order to gain public attention since we do not have an efficient legal system. So more channels need to be created to address their needs," he said.