An efficient, chilling new weapon in the urban war on rats may be swiftly meeting its demise after the government notified major cities that the use of dry ice as a rodenticide violates federal law.

The method of stuffing dry ice — frozen carbon dioxide — into burrows to suffocate rats as it sublimates from a solid to a gas proved more efficient at killing rodents and cheaper than using conventional rat poisons in cities such as Boston, Chicago and New York, which all recently launched programs to test the method.

But the Environmental Protection Agency reached out to state agencies in Illinois, Massachusetts, New York and elsewhere in recent months to make clear that federal guidelines prohibit the use of dry ice for rat abatement because the deadly treatment is not registered with the federal agency as required. The law is in place to ensure products are safe, and directions for use minimize risks to users, the public and the environment.

The revelation prompted Boston and New York to halt the use of dry ice in their rat abatement programs, while Chicago is investigating the issue. All three cities launched tests this year as urban centers around the country experienced a spike in the number of citizen complaints about rodents after a relatively warm winter.

The notifications, months after the cities first began using dry ice, highlight the complicated process of pesticide use in the USA. States enforce the federal guidelines and receive notification from the EPA if a city is in violation.

William “Buddy” Christopher, who heads Boston’s Inspectional Services Department, said he halted the city's dry ice experiment on Oct. 20 after receiving a notification from the Massachusetts Department of Agricultural Resources about the illegal use of the product. But the state agency received the notice from the EPA on Aug. 8, about two months earlier. The agency did not provide a reason for the delay.

“We didn’t see this thing as a pesticide by the classic definition, because dry ice is used everywhere,” Christopher said. “You go to a nightclub and they’ll serve cocktails in a bucket of dry ice. This is not necessarily one of those logical things, it’s a regulatory issue, and we’ll respect everything they want us to do.”

The Boston's legal team is in the process of applying for an EPA permit to restart the program, Christopher said. The city recorded as much as a 95% reduction in rodent activity in areas where it deployed dry ice after it launched the pilot in April.

Chicago launched its pilot in August and immediately recorded a 60% reduction in burrows in areas it tested. The city noted that at 50 cents per pound, dry ice is far cheaper than the rat poison pellets selling for $57 per 20-pound bucket.

Melaney Arnold, a spokeswoman for the Illinois Department of Public Health, said the agency received notification last week from the EPA that the Chicago program violated federal law.

The city is now investigating the matter. “The city of Chicago is reviewing the matter to ensure that we are in compliance with any regulations,” said Sara McGann, a Chicago Street’s and Sanitation Department spokeswoman.

In New York, the state’s Department of Environmental Conservation notified New York City officials that its use of dry ice violated federal law after the EPA reached out. In a statement, the state agency said it “does not support the use of illegal products or substances, such as dry ice, for rodent control.”

Carolina Rodriguez, a spokeswoman in New York City’s Department of Health, said
“the dry ice trial was promising,” but “it has concluded.” She declined further comment.

Widespread media attention about the cities' use of dry ice prompted several municipalities and school systems to contact the EPA about the legality of the product, the agency said.

Ruth Kerzee, executive director of the Midwest Pesticide Action Center, said her organization raised concerns with regional EPA officials and the city of Chicago about the new rat-killing method.

Kerzee, whose organization promotes minimizing the use of pesticides, said while dry ice is less toxic than some conventional pesticides it remains unclear what, if any, guidelines cities created to ensure the product is being safely handled by personnel.

“We think it could be a sea changer, a great thing to be able to use, but it does need to be vetted and go through the process, so that we don’t end up in a situation where we throw the baby out with the bathwater,” Kerzee said.

The National Pest Management Association, a trade group representing private pest control companies, also inquired with EPA and the Illinois Department of Public Health about the use of dry ice after Chicago launched its pilot and was told it could not be legally used as rodenticide, said Jim Fredericks, chief entomologist for the association. The group published a message to members in its newsletter last month that “any use of CO2/dry ice to control rodents would be a violation of federal law.”

Fredericks said the industry association is not calling for the EPA to permit dry ice as a rodenticide. “It’s not one of our priorities right now,” he said.

Loretta Mayer, CEO of the Arizona-based SenesTech, a company marketing a non-lethal product that sterilizes rats, said some products on the market are more dangerous than dry ice.

Still, even if dry ice is eventually approved by the EPA, Mayer said it won’t be a panacea for cities in their epic battle with rats, which reach sexual maturity five weeks after birth.

“In the end, it’s just another form of killing, and killing is not a sustainable rodent-pest control strategy,” Mayer said. “From that standpoint, dry ice is not particularly a big winner. We’ve been killing rats for centuries, and we still have the problem.”

Follow USA TODAY Chicago correspondent Aamer Madhani on Twitter: @AamerISmad