Dust created by polishing iPad cases was probably responsible for causing an explosion at a Pegatron plant in Shanghai that injured 61 workers.

That's according to China Labor Watch, an organisation that campaigns for workers' rights in China's industrial areas. The organisation linked the blast to another accident at one of Apple's component suppliers earlier this year.

"According to the preliminary analysis, the blast in Riteng is a result of aluminum dust in the workshop where iPad cases are polished. This is the second reported blast in Apple’s supply chain this year. Seven months ago, a similar blast happened in the workshop of a Foxconn factory in Chengdu, killing three workers and hurting another 15," the group said in a statement.

The blast occurred on Saturday afternoon, leaving 23 workers needing hospital treatment. None of the injuries was life threatening.

"We are working closely with Pegatron to understand the cause of this accident," Carolyn Wu, a spokeswoman for Apple, told Bloomberg. "Our hearts go out to the people who were hurt in Songjiang."

But China Labor Watch pointed the finger of blame directly at Apple. "There is a sharp contrast between the strong performance of Apple and the poor condition of the workers working on its assembly line in China... Apple cannot sacrifice the Chinese workers to get high profit and Apple should be held liable for the all the accidents happening in its suppliers’ factory in China."