The FDA has issued warning letters to four e-liquid and hookah companies informing them that 44 of their products cannot legally be sold in the United States.

The letters to the companies note the various products are in violation “for not having a required FDA marketing authorization order in effect and/or misbranded under section 903(a) of the FD&C Act (21 U.S.C. § 387c(a)(6)) because a required notice or other information respecting the products was not provided.”

That federal code regulates the manufacturing, propagation, compounding and processing of tobacco products solely to establishments registered to do so.

The letters go on to note that failure to comply with the FD&C Act “may result” in the FDA imposing civil penalties, criminal prosecution, seizures and/or injunctions, and calls for the companies to submit written responses to the FDA within 15 days of their receipt of the letters describing their “corrective actions.”

Hookah Imports Inc., Ovo Manufacturing & Distribution, V8P Juice International and Likido Labs USA each received letters from the FDA. The letters include overviews of which of their products are in violation of the act.

In a press release, the FDA said these actions are part of their effort to take action against “illegally marketed tobacco products amid the epidemic of youth e-cigarette use in America.”

Dr. Ned Sharpless, the acting commissioner of the FDA, said in the press release they will continue to keep a “close watch” on whether companies are breaking the law.

“The marketing of illegal tobacco products is particularly concerning given the epidemic of youth vaping that we’re facing, which we know has resulted in part from irresponsible practices of manufacturers, importers and retailers who have targeted kids in their marketing of these products,” Sharpless said.

At least 2,082 U.S. college and university campuses are smoke-free as of November 2017, according to an analysis by the Centers for Disease Control and Prevention. 79.6% of those campuses explicitly prohibit e-cigarette use, and smoke-free campuses can reduce the social acceptability of tobacco use, according to the analysis.

Iowa State maintains a smoke-free campus, and has since the passage of the 2008 Smokefree Air Act, which banned smoking in most public places in Iowa. Iowa State banned e-cigarettes on campus effective January 1, 2016.