A Californian man pleaded not guilty on Monday after being arrested and charged for allegedly smuggling greenhouse gases from Mexico into the U.S. and selling them for profit, prosecutors said.

Why it matters: Michael Hart, 58, of San Diego, faces the first-ever prosecution in the U.S. to include charges related to the American Innovation and Manufacturing Act of 2020 (AIM Act) for allegedly smuggling hydrofluorocarbons (HFCs), a potent greenhouse gas used in refrigeration and air conditioning.

• That's according to the Department of Justice and Environmental Protection Agency, which note HFCs have a high global warming potential. Some are higher than 1,000 times the warming potential of CO2, according to EPA data.
• The illegal smuggling of hydrofluorocarbons "undermines international efforts to combat climate change" under an amendment to the landmark Montreal Protocol, said David Uhlmann, EPA assistant administrator for the Office of Enforcement and Compliance Assurance, in a statement.

Details: The Department of Justice alleges that Hart bought refrigerants in Mexico and smuggled them into the U.S. in his vehicle, "concealed under a tarp and tools."

• He allegedly posted them for sale on sites including OfferUp and Facebook Marketplace and "sold them for a profit."
• In addition to greenhouse gases, Hart is accused of importing HCFC 22, an ozone-depleting substance regulated under the Clean Air Act. It became illegal to import HCFC 22 in 2020 except under certain circumstances.

State of play: Hart was arraigned and entered his plea during his first appearance in federal court on Monday afternoon, per a statement from the U.S. Attorney's Office in the Southern District of California.

• He faces charges of conspiracy, importation contrary to law and sale of merchandise imported contrary to law.
• His next hearing before U.S. District Judge Jeffrey Miller will be for a motion hearing or trial, scheduled for March 25.

What they're saying: Assistant Attorney General Todd Kim, of the DOJ's Environment and Natural Resources Division, noted in a statement that it's illegal to import certain refrigerants into the U.S. "because of their documented and significantly greater contribution to climate change."

• U.S. Attorney Tara McGrath for the Southern District of California said in a statement the DOJ was "using every means possible to protect our planet from the harm caused by toxic pollutants."
• This included bringing criminal charges. "This is the first time the Department of Justice is prosecuting someone for illegally importing greenhouse gases, and it will not be the last," McGrath said.

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Editor's note: Andrew Freedman contributed reporting.