The Wax Ring
The flange is the top part of the plumbing pipe running under the floor to the toilet location. The flange sits at floor level and has slots for bolts to hold the toilet stool in place above the flange. Creating a waterproof and airtight seal between the toilet stool and the flange is a wax gasket or ring. This ring is commonly pushed onto the toilet first and then fits to the flange as the toilet is set in place. If the wax ring is damaged or doesn't fit properly sewer gasses can exit the plumbing system and escape around the base of the toilet.
Built into the toilet stool is a trap. This P-shaped bend in the pipes holds water preventing gas backing up through the trap area. If the water in the trap evaporates due to lack of use, gas can back up through the toilet and enter the living quarters of the home. Flush the toilet every two to three weeks to maintain adequate water in the trap.
The home sewer system contains a vent pipe through the roof of the home. This vents sewer gases away from the living quarters. Clogged vents can force sewer gases back into the home. Clogs commonly occur in the winter when ice and snow build up around the vent pipe on the roof. Summer vent problems can occur if birds or squirrels nest in the vent pipe. Keep the vent clear from any obstructions for best home sewer operations.
The odor of sewer gas comes from the ammonia and hydrogen sulfides present. Both these gasses have a disagreeable scent. Methane and other odorless gasses are also often present. Sewer gas is not only unpleasant but can pose a health risk, with headaches, nausea and dizziness all common early symptoms.