Clogged Vent
Smells in a bathroom can also be caused when the drains in a bathroom are not properly vented, or the vent is clogged. A vent can become clogged with a bird's nest, body of a bird, etc. Also, in extremely cold climates an undersized vent can close up due to frost forming on the inside of the vent walls from warm humid air from washing machines, dishwashers, showers, etc. In addition, a vent can become clogged at the fixture connections with grease, soap scum, etc. A clogged vent requires running a snake down through the vent pipe from the roof. Climbing around on a roof is tricky and best left to a professional plumber who has the proper equipment and experience.