A health watchdog group will challenge e-cigarette manufacturers in California court over cancer-causing chemicals in their products.

The Center for Environmental Health found one or more cancer-causing chemical in the majority of the 97 products tested by its researchers. The findings, which are detailed in a new report from the group, are the basis of legal action arguing that cigarette manufacturers failed to inform users of their risk under a California consumer protection law.

“Anyone who thinks that vaping is harmless needs to know that our testing unequivocally shows that it’s not safe to vape,” said CEH Executive Director Michael Green in a press release. “Our legal action aims to force the industry to comply with the law and create pressure to end their most abusive practices.”

Read More: 4 Weird Health Effects of E-Cigarettes

The watchdog group purchased 97 e-cigarette products from major retailers and online before sending them to a laboratory for testing. Scientists found high levels of either or both formaldehyde and acetaldehyde in 50 of the products.

Recent research has warned of the health risks of e-cigarettes, though many argue that they are an improvement on cigarettes. Scientists need to conduct additional research before a full understanding of their effects can emerge.

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Write to Justin Worland at justin.worland@time.com.