Your plumbing vent stack can freeze up in extremely cold weather, letting harmful sewer gases build up in your home.
In extremely cold weather, water vapor in the vent can freeze on the top of the stack, and may close it off completely. When this happens, the pressure in your drain system may be disrupted, causing the water traps to empty. Then, with no vent to the roof, the gases may build up in your home. Besides smelling awful, sewer gas also can contain harmful compounds like hydrogen sulfide, ammonia, and carbon monoxide. Even if the exhaust vent doesn’t freeze shut, it may be restricted, and this may cause toilets to have trouble flushing or affect other drains in your house.