Aamer Madhani

USA TODAY

CHICAGO—Some of America’s biggest metros have unleashed a chilling—and seemingly effective—new killer in the unending urban war against rats: Dry ice.

Sanitation officials in the nation’s third largest city told USA TODAY they recently launched a pilot program at four Chicago parks to test the effectiveness of dropping chunks of dry ice—frozen carbon dioxide—into burrows to try suffocate rats as the dry ice sublimates from a solid to a gas.

Chicago began the experiment with dry ice in late August, following Boston, which became the dry ice pioneer when it launched its pilot in March, and New York City, which launched its test program in May.

“When I first brought up the idea, people around here thought I was cuckoo for Cocoa Puffs,” said Leo Boucher, assistant commissioner for Boston’s Inspectional Services, which consulted with Chicago as it launched its dry ice pilot. “But we all became believers. It works.”

The odorless gas in solid form—commonly used by stagehands to create an artificial fog effect and by merchants trying to keep perishables from spoiling—also can be deadly to small animals at high concentrations.

Earlier this week, USA TODAY observed Chicago sanitation department workers at one of the city’s oldest parks scoop chunks of smoking dry ice into a burrow before quickly covering the entry and exit holes with dirt and newspaper to stop any rats from escaping as the -109.3-degree Fahrenheit gas dissipated. Sanitation workers say they treat burrows during morning hours, when rats are less active and most likely to be huddled inside the burrows.

The asphyxiated dead rats then decompose in place and out-of-sight of city denizens who count the disease-carrying vermin among the vilest of indignities of urban living.

While the program is only a few weeks old in Chicago, officials there say it shows promise.

“We are seeing 60% fewer burrows in areas where we are using the dry ice,” said Charles Williams, Chicago’s streets and sanitation commissioner. “It’s more environmentally friendly, and it’s very humane on the rodents as well.”

The dry ice experiment comes as many big U.S. cities are experiencing an explosion in rat complaints from residents, according to a USA TODAY analysis published earlier this year. Among the cities with the biggest spikes in complaints are Boston, Chicago, New York and Washington, D.C.

Rats! Several big U.S. cities seeing surge in rodent complaints

Rat populations grew after a relatively mild winter last year in much of the country, and sightings tend to increase in areas, such as Chicago, that have experienced a surge in building construction.

In Chicago, Mayor Rahm Emanuel boosted the number of city technicians working on rat abatement, while the city council passed rules making developers include a rat abatement proposal as part of any new construction project. The city analyzes complaints to its non-emergency hotline for 31 variables that might predict the emergence of the next rat hot spot before it becomes a problem.

In some Windy City neighborhoods, it has also become increasingly popular to adopt feral cats and let them roam the alleys and kill vermin.

City officials believe dry ice could prove to be a more efficient killer than its usual remedy: poison. Dry ice costs Chicago about 50 cents per pound. Traditional rat poison pellets cost the city about $57 per 20-pound bucket.

In Boston, where abatement crews have been using dry ice for about six months, the city spends about 25 cents per pound, and workers there pour about 2½ pounds of it into every burrow treated. At the moment, Boston uses the technique only at parks, playgrounds and cemeteries.

Officials in Boston and Chicago say that dry ice won’t totally replace the need for conventional poison—the dry ice tactic is useful only in tightly-enclosed areas. Cities will continue to rely on poison pellets to kill vermin in places like subways, which are among the most problematic for rat infestations, officials said.

But the availability of dry ice mitigates the need to use poison that could be deadly to small pets and birds that inadvertently eat it.

Wrigley Field renovation has rats 'running rampant' in Chicago

In New York, where the city’s Health Department is stuffing dry ice into burrows in Manhattan’s Columbus Park, Tomkins Square Park and other locations, Mayor Bill de Blasio has bolstered rat abatement efforts. Last year, he added $2.9 million to the Big Apple’s rat fighting program—allowing the city to hire 41 more inspectors and exterminators.

At Tomkins Square Park, city abatement workers counted 368 burrows when they started the dry ice trial. By the end of the initial trial, there were 20 active burrows, according to the city's Health Department. In Columbus Park, the city counted 60 burrows when they started applying dry ice in June, but are now down to two active burrows.

“The dry ice pilot has shown promising results in parks throughout New York City," the Health Department said in a statement. "We are evaluating what we have learned and hope to continue to use this innovative technique in our ongoing rat abatement efforts.”

In Boston, the idea to try dry ice came as the city saw rat complaints triple in the first quarter of 2016. Boucher, the Inspectional Services official, said the idea was first floated to him by John Stellberger, president of a Boston-area pest control company. Stellberger got the idea from a Massachusetts Institute of Technology (MIT) professor, Boucher said.

Academic and scientific researchers have long been using carbon dioxide to euthanize lab animals, and Boston Inspectional Services officials turned to researchers at Harvard University and MIT to help them develop their dry ice procedures. (When animals are exposed gradually to CO2, they will lose consciousness before concentrations become painful. A high concentration of CO2 can cause rats to become unconscious within one minute.)

The emerging dry ice method has been criticized by the anti-animal cruelty group People for the Ethical Treatment of Animals, which advocates for people and communities dealing with rodent infestations to take preventive action, such as keeping garbage cans sealed, and only use non-lethal traps when a rat gets in a home.

“Lethal initiatives are ineffective and actually backfire,” said Stephanie Bell, PETA’s senior director of cruelty casework. “Wild animals of any sort are attracted to places where there's a reliable food supply, and until that changes, the city will always find itself two steps back if it depends on killing.”

Boston officials say they have consulted with officials in Chicago, New York as well as Los Angeles and municipalities in Massachusetts and New Jersey that are weighing adopting dry ice in their abatement procedures.

“Quite frankly, word of mouth is how most of these things get done,” said Boucher, who added that Boston is now beyond the pilot stage and is using dry ice in its abatement procedures regularly. “We’ll take as much info as we can from people and use it.”

In Chicago, streets and sanitation laborers, who use gloves to handle the dry ice (the super frozen substance can quickly burn skin) and homemade tools to help scoop it into the burrows, say that most of the time the rats don’t know that the gas is hitting them until it’s too late.

But John Stagrowski, a sanitation department laborer who has been working on the Chicago pilot, said that he and his colleagues learned early in the pilot that it is essential to move quickly when applying dry ice to avoid any surprises.

“We put the dry ice in, and then started covering the second hole,” Stagrowski recalled of one of his team’s first attempts at using dry ice. “The son of a gun came right up through the -109 degree stuff (dry ice) like it was nothing.”

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