A rare disease contracted by staff at popcorn manufacturing plants in the US is being linked to the vapours released when a bag of microwave product is opened.

Factory workers in Illinois, Iowa, Missouri and Nebraska have fallen ill with a lung disease, and the National Institute for Occupational Safety and Health has said it suspects the chemical diacetyl caused the illnesses, reported the Associated Press.

The Environmental Protection Agency is investigating the incidents, but it is thought unlikely that consumers could be at risk.