Researchers using high-tech air monitoring equipment rolled through an industrialized stretch of southeast Louisiana in mobile labs and found levels of a carcinogen in concentrations as much as 10 times higher than previously estimated, according to a paper published Tuesday in the journal Environmental Science & Technology.

The study by researchers at Johns Hopkins University raises new health concerns for communities that sit among the chemical plants lining a stretch of the Mississippi River between Baton Rouge and New Orleans — dubbed “cancer alley” by environmentalists.

The Environmental Protection Agency considers long-term exposure to inhaled ethylene oxide gas a cancer risk — a stance challenged by the chemical industry. The state of California, which has its own environmental health agency, also lists the chemical as “known to cause cancer and reproductive toxicity” in men and women.