The U.S. Environmental Protection Agency is investigating chemicals in microwave popcorn that have been linked to a rare lung disease in people who work in factories that produce popcorn.

Researchers are zeroing in on vapours released when the chemical diacetyl heats up. The National Institute for Occupational Safety and Health said it suspects the chemical made workers ill in Missouri, Illinois, Iowa and Nebraska.

Health officials say people who microwave bags of popcorn at home are not at risk because they're exposed to such small quantities of the vapour.

The EPA began studying the type and amount of chemicals in about 50 brands and flavours of microwave popcorn last fall and it expects to wrap up the research this year.