The nation’s southern border has long been a hot spot for illegal smuggling of drugs, weapons and migrants. But what about illicitly transporting greenhouse gases that cause climate change?

A California man was arrested Monday on charges of doing exactly that and is facing the first prosecution under a 2020 U.S. law aimed at phasing out some of the most potent greenhouse gases on the planet.

Federal prosecutors allege that Michael Hart, 58, of San Diego, purchased the gases in Mexico and transported them across the border in the back of his truck, concealed under a tarp and tools. He then allegedly sold them online, including on sites such as Facebook Marketplace and OfferUp.

The 2020 law cracked down on chemicals called hydrofluorocarbons, or HFCs, that are widely used in air conditioning and refrigeration. The chemicals are considered climate super-pollutants because they can be thousands of times more powerful than carbon dioxide in warming Earth’s atmosphere.

Hart pleaded not guilty to 13 charges, including conspiracy, during his first appearance in court Monday. The charges carry potential prison sentences ranging from five to 20 years.

“This is the first criminal case for illegal smuggling of HFCs. It won’t be the last,” David Uhlmann, who leads the Environmental Protection Agency’s enforcement office, said in an interview Tuesday.

“We expect there to be many cases along the southern border,” he said. “There may be cases along the northern border. And because we may not be able to intercept every illegal shipment, there may be cases in cities and towns across America, until we make it clear that we have a zero-tolerance policy.”

Uhlmann added that the EPA would continue working with the Justice Department and the Department of Homeland Security to maintain an “active presence” at the border and at ports across the country.

Tara McGrath, U.S. attorney for the Southern District of California, said in a statement that “we are using every means possible to protect our planet from the harm caused by toxic pollutants, including bringing criminal charges.”

Monday’s indictment sends “a very strong signal” to would-be illegal importers of HFCs, said Avipsa Mahapatra, climate campaign lead at the Environmental Investigation Agency, a watchdog group.

“In my opinion, it is a climate crime if you are smuggling HFCs,” Mahapatra said. “It obviously undermines our global efforts to phase down the use of these chemicals.”

Companies initially introduced HFCs as substitutes for chlorofluorocarbons, or CFCs, which wreaked havoc on the ozone layer and were phased out under a 1987 treaty known as the Montreal Protocol.

But while that effort helped to mitigate damage to the ozone later, HFCs have since become used widely around the world and are hundreds to thousands of times more potent in causing global warming than carbon dioxide.

In 2016, under an international agreement known as the Kigali Amendment, countries vowed to phase down production and consumption of HFCs by 80 to 85 percent by the late 2040s.

Monday’s indictment also alleges that Hart imported HCFC-22, “an ozone-depleting substance regulated under the Clean Air Act.” According to the EPA, U.S. production and import of the substance – commonly referred to as R-22 – ended on Jan. 1, 2020.

David Doniger, senior strategic director of the climate and clean energy program at the Natural Resources Defense Council, applauded Hart’s arrest but said more action is needed.

“It’s a very good thing that they’re catching and holding to account this smuggler,” Doniger said. “[But] there is a significant smuggling problem. At one point, CFC smuggling was the second-biggest problem after illegal drugs.”

Mahapatra agreed.

“Back in the 1990s, we saw a lethal black market in CFCs,” she said. “At that time, the street value of a cylinder of CFCs was nearly matching that of cocaine. It is critical that we stop the illegal trade of HFCs before we reach that state again. Our climate can’t afford that.”

Hart is scheduled for his next hearing before U.S. District Judge Jeffrey T. Miller on March 25, according to the Justice Department.