The sewage lines that carry waste from your toilet and other plumbing fixtures carry gas and liquid wastes. Every home has at least one dedicated line designed to vent gases from the sewage line through vents in your roof. If these vents are clogged, the gases will back up into your home. These gases often contain hydrogen sulfide, which produces a distinctive odor reminiscent of rotting eggs.
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The sewage lines in your home use a series of vents attached to outlets above your roof to release noxious fumes from your plumbing lines and control the pressure inside the lines. When these vents become blocked at a point in the line above your toilet, the sewage gas in the line will vent through your toilet instead. Most blockages in your vents occur near the end of the vent, on the roof. Animals, debris from the wind, snow and ice are common sources of blockage in these vents.
Sewage gas contains a number of noxious chemicals that are harmful in large concentrations. Hydrogen sulfide is an eye and respiratory irritant and can cause nausea, headaches, dizziness and drowsiness in higher concentrations. Exposure to strong concentrations of this gas can interfere with your sense of smell, which makes gauging the concentration of gas in an area using your sense of smell difficult. Hydrogen sulfide is a flammable gas that presents a serious fire hazard and can cause unconsciousness or death in extreme situations.
Ventilate your bathroom to prevent a dangerous concentration of gas from building up and to limit the offensive odor. See that there are no open ignition sources in your bathroom since these may ignite the gas if it reaches a high enough concentration. If you suspect that a dangerous concentration of gas has already accumulated, extinguish any nearby ignition sources, evacuate the area and contact your local fire department.
Fixing the Problem
Most blockages occur on the roof where the vent exits your home. Clearing obstructions from the end of the vent should allow gas to vent normally, clearing up the foul odor emanating from your toilet. Covering the end of the vent with a mesh screen will help prevent future blockages. In areas that receive heavy snowfall, increasing the height of the vent can help prevent winter blockages.