BOSTON (CBS/AP) — The feds have given the cold shoulder to Boston’s use of dry ice to control the city’s rat population.

The U.S. Environmental Protection Agency says the city has not registered dry ice as a pesticide and issued a cease and desist order through the state.

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The city started using dry ice — frozen carbon dioxide — earlier this year. The ice is packed into the exits of rat lairs, melts into carbon dioxide gas, and suffocates the rodents–and Boston’s head inspector says it’s cheaper than poison.

City Inspectional Services Commissioner William Christopher told WBZ NewsRadio 1030’s Lana Jones that, in fields, parks, and cemeteries, the method suffocates the rats with no other ill effects.

“We ran a couple of very controlled tests on some rat borrows, and the results were perfect, they were exactly what we were hoping for,” Christopher said. “There was no issue with the gases or anything, because by the time they reach the surface, they’ve dissipated to the point where they’re not an issue.”

The city is working to register dry ice as a pesticide with the state and federal governments. If they can convince the EPA, they could get approval to resume.

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In the meantime, they’ve returned to traps and poison.

“We go back to our standard procedure,” Christopher said. “We still use these other procedures in closed environments or areas where there’s a lot of activity. This would just be another tool at our disposal to help deal with the situation.”

Christopher hopes to have approval to start using it again by the spring, when rats become more active.

WBZ NewsRadio 1030’s Lana Jones reports

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