Sewer Gases in the Home
by Nick Gromicko, CMI®
Decomposing waste materials in public and private sewer and septic systems create sewer gases. Methane is the largest single constituent of sewer gas, which includes an assortment of toxic and non-toxic gases, such as hydrogen sulfide, carbon dioxide, ammonia, nitrogen oxides, and sulfur dioxide. Improperly disposed gasoline and mineral spirits may also contribute to sewer gases. 
Sewer gases pose the following risks to building occupants:
- hydrogen sulfide poisoning. Hydrogen sulfide is an explosive and extremely toxic gas that can impair several different systems in the body at once, most notably the nervous system. So potent that it can be smelled at 0.47 parts per billion by half of human adults, the gas will begin to cause eye irritation at 10 parts per million (ppm) and eye damage at 50 ppm. Other low-level symptoms include nervousness, dizziness, nausea, headache and drowsiness. Exposure to higher concentrations can lead to pulmonary edema, and still higher levels (800 to 1,000 ppm) will cause almost immediate loss of consciousness and death;
- asphyxiation. When sewer gases diffuse into household air, they gradually displace oxygen and suffocate occupants. The effects of oxygen deficiency include headache, nausea, dizziness and unconsciousness. At very low oxygen concentrations (less than 12%), unconsciousness and death will occur quickly and without warning. Oxygen will be at its lowest concentrations in the basement, which is where heavy sewer gases, principally methane, are likely to collect;
- fire or explosion. Methane and hydrogen sulfide are explosive components of sewer gas. Vapors from improperly disposed fuel can further increase the risk of fire or explosion; and
- odor. Hydrogen sulfide is responsible for sewer gas’s characteristic rotten-egg smell, which can be overbearing even at extremely low concentrations. The gas’s odor is a safeguard, however, because it alerts building occupants to the leak long before they’re in any serious danger. It is important to note that at roughly 100 ppm, the olfactory nerve becomes paralyzed, removing the victim’s sense of smell and, subsequently, their awareness of the danger. Another "warning smell" comes from ammonia, which will sear the nostrils and progressively irritate the mucous membranes and respiratory tract. This gas, unlike hydrogen sulfide, is sufficiently irritating that building occupants are likely to vacate before its concentration rises to toxic levels.
If you suspect that any odors might be caused by sewer gases, contact a qualified plumber. Be sure to mention the smell to an InterNACHI inspector during your next scheduled inspection.
The design of the plumbing system relies on a connection between household fixtures and the sewer system, which is why a great deal of effort is spent to ensure that waste products -- and the gases that result from their decay -- flow in one direction.
The following failures in the plumbing system may allow sewer gases to flow back into a building:
- dried-out piping and plumbing fixtures. In most cases, intruding sewer gases are caused by a loss of the water barrier where traps have gone dry. Especially in dry weather, infrequent use of a toilet, shower or floor drain can allow for rapid evaporation and entry of sewer gases into the living space. Particularly common culprits are floor drains placed in locations where they are likely to dry out, such as near water heaters or furnaces, as well as seldom-used drains, such as those in janitor’s closets, workshop areas and mechanical rooms. Homeowners can maintain the water barriers by using the fixtures more often or by pouring water down the drains. Automatic drain-trap primers may also be installed so that a small amount of water is periodically delivered;
- cracks in the plumbing drain line or vent pipes. A water leak typically accompanies a crack in the drain line, but vent pipe cracks are more difficult to diagnose, and they can vent a large quantity of sewer gases into the home. Plumbers can locate these cracks by using a special machine that generates artificial smoke and pumps it into the plumbing drain system. The smoke pressurizes the system and exits through any cracks or loose fittings;
- diffusion from a leach field septic system;
- through cracks in a building’s foundation; and
- plumbing vents installed too close to air intakes or windows in homes equipped with HVAC air handlers that admit outside air for ventilation. Wind and air flow around the building can allow for sewer gas to enter the building even where plumbing vents and air intakes are appropriately placed. Homeowners can add vent pipe filters or alter the height of vents to alleviate the problem.
In summary, the intrusion of sewer gases into the living space should be discovered and fixed before occupants suffer ill health.
