Rats are "plaguing part of the city," says Mayor Rahm Emanuel. View Full Caption shutterstock

CHICAGO — City crews will again start using dry ice to kill rats in city parks and along medians after the green light from federal environmental officials, City Hall leaders announced Tuesday morning.

Spurred by the soaring number of complaints about rats, city officials launched a pilot program in August 2016 that used dry ice to suffocate rats underground that Streets and Sanitation Commissioner Charles Williams said was "an effective and safe tool in our parks, where our residents frequent."

But that effort was stopped in December 2016 after city officials learned that it had not been approved by federal Environmental Protection Agency officials as a way to safely control the rat population, officials said.

This is how you use dry ice to kill rats. The city will test out the new method at North Side parks and downtown planters. pic.twitter.com/3jtzbtChj1 — Joe Ward (@JayDubWard) September 20, 2016

Now that EPA leaders have given the city the green light, the effort — criticized by People for the Ethical Treatment of Animals as inhumane — will resume, according to a statement from Mayor Rahm Emanuel's office.

"The dry ice method serves as a safe and quick approach that essentially puts rats to sleep before they perish," Williams said.

Emanuel announced the new initiative with great fanfare, calling it "smarter and more effective."

Crews put dry ice — frozen carbon dioxide — into rat burrows and sealed the holes with dirt and newspaper. As the dry ice warmed into a gas, it suffocates the trapped rats, leaving them to decompose in the holes — away from people.

City crews will also test the impact of a poison dubbed Contrapest that makes rodents infertile, officials said.

The city has replaced these signs with ones that have a more realistic depiction of a rat. [DNAinfo/Heather Cherone]

After months of feeding on the bait, rodents become infertile and unable to breed, official said.

The pilot will be conducted at the city's garbage transfer station at 34th Street and Lawndale Avenue. Twenty-five bait boxes containing the non-toxic and environmentally-safe Contrapest will be monitored for six months, officials said.

If effective, the bait could become a regular tool used enclosed or contained areas where rodents breed, officials said.

The number of complaints about rats are down 2.5 percent from January through July this year compared with the same period a year ago, Streets and Sanitation spokeswoman Sara McGann said.

The best way to reduce the number of rats is to keep garbage covered in alleys and keep yards free of pet waste, city officials said.

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