For the past several weeks, workers from the agency have been using picnic coolers to pick up dry ice from a local company.

The City of Boston is experimenting with a chilling new way to kill rats: dry ice.

They take the supercold substance to known trouble spots. The workers use steel scoops and wear gloves as they place the dry ice into the multiple exits of each burrow. They use their regular work boots to pack the dry ice in.

Dry ice is the solid form of carbon dioxide. As it melts, it turns into carbon dioxide gas, which fills the burrow, suffocating any rats inside.


Christopher said it is a more humane way of killing the rodents — and significantly cheaper than using rat poison.

He said his staff has used more than 400 pounds of dry ice over the past six weeks, and that altogether it cost just $225.

“The stuff is dirt cheap,” he said.

“The simplicity of this process is one of the things that most intrigues me,” he added. “And the success is what has me very excited.”

Using dry ice reduces the risk to other animals and children that poison can pose, he said.

Dry ice can burn if it comes into direct contact with skin, but workers monitor the substance after it’s placed in the burrows, Christopher said.

“It’s simple science,” said Christopher. “It has not hurt anyone or any other wildlife or plant life. Based on everything we’ve seen so far, it’s been excellent.”

Christopher said officials from other cities have inquired about the dry ice method because they are interested in adopting it. He said Boston officials got the idea from local colleges that use carbon dioxide to euthanize lab rats.


John Meaney, the department’s assistant commissioner, said the dry ice method would be demonstrated for attendees at a rodent control summit the city is hosting this week for local officials and other interested parties.

A rat looked out from a burrow hole. Keith Bedford/Globe staff

One of the first places the city tested out the new pest control method was in the Public Garden. The work was done in the early morning hours, and Christopher said the burrows there were picked because they are away from both homes and the garden’s walking paths.

He said the method would not work to eradicate rats in spaces that aren’t confined like burrows. In those settings, bait, traps, and poison will continue to be used. And the city will continue to try to educate residents, landlords, and business owners about rodent control, including how to properly seal trash and to contact the city if you see signs of rats or other pests.

“The dry ice is just one tool in our toolbox,” he said.

The method does have at least one opponent.

Stephanie Bell, an activist with the People for the Ethical Treatment of Animals, said in a statement that killing rats is ineffective.

She agreed with the city officials on one point: Areas should be made less attractive to the rodents, including by disposing of trash properly and sealing buildings.

“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,” Bell said.


Matt Rocheleau can be reached at matthew.rocheleau@globe.com. Follow him on Twitter @mrochele