A rat looks for food at a trash can in September near Georgia Avenue NW in the Park View area of Washington. (Salwan Georges/The Washington Post)

The District now has another weapon in its war on rats: dry ice.

With complaints to the city's 311 phone line concerning rats at a four-year high, city officials are turning to dry ice — the frozen form of carbon dioxide — to suffocate rodents. But dry ice will not replace the poison that the District's rodent control division currently uses, said its program manager Gerard Brown.

"It's just going to be another tool in our toolbox," said Brown, who has been killing rats in Washington for 30 years.

His explanation for how dry ice works is concise, and a little dramatic: "The CO2 that emanates from the dry ice suffocates the rats, and their homes become their graves," he said.

City officials are encouraging residents to purchase dry ice — which is available at local grocery stores — to kill rats on their own properties. The city is working on guidelines that will instruct residents on how much dry ice to use and how to avoid burning themselves, Brown said.

Brown, Mayor Muriel E. Bowser (D) and D.C. Council member Charles Allen (D-Ward 6) oversaw a demonstration last month in which staff from the Department of Health stuffed dry ice into a rathole in an alley off the H Street corridor in Northeast Washington. The ice smoked a little, and — theoretically, at least — any rats inside the burrow suffocated.

The use of dry ice is the latest move in the city's war on rats, which has included the installation of state-of-the-art solar trash cans in "rat hot spots" at an annual cost of $85,000 and grants to businesses that lease commercial trash compactors.

"We need (DOH) to be going as aggressive as they can," said Allen, who has introduced a bill that would require restaurants to follow pest-prevention plans and gives the city's health department more funding and enforcement capability. "But it comes down to managing our trash the right way, not having food waste that is out in the alley."

The District's rat problem is not new, but it has worsened in recent years. The surging rat population is driven by a combination of factors, including Washington's booming human population, which means more trash, and recent mild winters, which means fewer rats die from frigid temperatures — although city officials are hoping the current cold snap will help reduce the population.

There have been 3,286 rat complaints to the city this fiscal year, up 64 percent from fiscal year 2015, according to data from the health department.

As rat-killing methods go, dry ice is relatively humane and cheap — only 50 cents per burrow, according to some estimates — and easy to purchase at local grocery stores, said Tommy Wells, director of the Department of Energy and Environment. He also said it is pet-friendly, with cats and dogs unlikely to ingest the compound.

One drawback is that dry ice can burn skin, so residents should use insulated gloves or tongs when handling the substance, he said.

Dry ice is "more humane than certain poisons and backbreaking or suffocating sticky glue traps," said Stephanie Bell, senior director of cruelty casework for People for the Ethical Treatment of Animals. But PETA does not give dry ice a total green light. Preventing rats from spreading by limiting their food source, rather than killing them, remains "the only truly humane policy," Bell said.

Washington is not the first city to use dry ice to fight rats. Boston began using the compound in 2016, after experiments at local universities, said William Christopher Jr., Boston's commissioner of inspectional services.

"It was wonderful — it did everything we wanted," Christopher said.

But then the Environmental Protection Agency issued a cease-and-desist order because dry ice was not registered as a method of rodent control, and Boston and other cities had to stop using dry ice, Christopher said.

Brown said Washington used dry ice for five months in 2016 before all cities had to stop.

A spokesman for the EPA said because dry ice was not registered with the EPA as a form of pest control, it was unlawful to distribute for that purpose until June last year, when the EPA registered Bell Laboratories' dry ice product for rodent control.

"After EPA issues a federal registration, states and the District of Columbia have their own registration programs that must be satisfied before the product can be sold and used within the state or D.C.," the spokesman said.

Bowser said the District had confirmed it was complying with all laws, federal and local.

"We assure you that everything we're doing is according to all regulations," she said.