Dry ice is frozen CO2. Rat nests usually have two holes, an entrance and an exit. Close one, place dry ice in the other and close the other hole to seal it.

Dry ice is frozen CO2. Rat nests usually have two holes, an entrance and an exit. Close one, place dry ice in the other and close the other hole to seal it. The dry ice melts and floods the chambers with CO2, no more rats. It's considered more humane and safe than poison. Some cities use this method.