Any mists spotted rising over the swamp may just be Washington wielding its newest weapon in its never-ending war on rats: dry ice.

The District of Columbia's rodent control division's program manager, Gerard Brown, tells The Washington Post the frozen form of carbon dioxide complements the poison the city uses, as reported rat complaints reach a four-year high.

Last month, Brown and Mayor Muriel Bowser oversaw a demonstration in which health department staffers stuffed dry ice into a northeast Washington alley rathole. As the ice smoked, the emanating carbon dioxide suffocated the rats, according to Brown's explanation.

Residents are encouraged to purchase their own dry ice. The city is working on usage guidelines.

Department of Energy and Environment Director Tommy Wells says dry ice is relatively humane, cheap and pet-friendly.