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Building Science by Norman Aceron Garcia

Sewer gases in the home

by Norman Aceron Garcia

shower drainDecaying waste materials in public and private sewer and septic systems generate sewer gases. Methane is the major single component of sewer gas that includes a mixture of non-toxic and toxic gases, such as sulphur dioxide, nitrogen oxides, ammonia, carbon dioxide and hydrogen sulphide. Inappropriately disposed flammable liquids and gasoline can also contribute to sewer gases.

Sewer gases can cause many health hazards to the building occupants. Hydrogen sulphide is a volatile and very lethal gas that can damage several systems in the body at once, most particularly the nervous system. Hydrogen sulphide can be smelled at 0.47 parts per billion by half of human adults. The gas will start to cause eye soreness at 10 parts per million (ppm) and eye injury at 50 ppm. It can also cause other low-level symptoms like drowsiness, headache, nausea, dizziness and nervousness. High concentration exposures can induce pulmonary edema, and still higher levels (at 800-1,000 ppm) will cause almost abrupt loss of consciousness and death.

Hydrogen sulphide is accountable for the distinctive rotten-egg smell of sewer gas that can be overbearing even at very small concentrations. However, the gas’s odour is a safeguard as it warns building dwellers of the leak way before they are in any severe danger. It is essential to be reminded that at approximately 100 ppm, the olfactory nerve becomes incapacitated, eliminating the victim’s sense of smell and, consequently, their responsiveness of the peril. Ammonia also emits another warning smell, which will scorch the nostrils and gradually annoy the respiratory tract and mucous membranes. Ammonia, unlike hydrogen sulphide, is very irritating, so building dwellers are likely to evacuate before its concentration increases to toxic levels.

Aside from hydrogen sulphide, methane is also another explosive component of sewer gas. Vapours from inappropriately disposed fuel products can further amplify the risk of explosion or fire. When sewer gases disperse into household air, they slowly displace oxygen and suffocate dwellers. The effects of deficiency of oxygen consist of dizziness, nausea, headache and eventually unconsciousness. At extremely low oxygen concentrations, or less than 12 per cent, unconsciousness and death will take place rapidly and without forewarning. Oxygen will be at its lowest concentrations in the basement and crawlspace, which are where heavy sewer gases and methane are likely to accumulate.

Immediately contact a qualified plumber if you suspect that sewer gases might be the cause of odours in your home. The design of the plumbing system depends on a connection between the sewer system and household fixtures. This is the primary reason why a huge amount of effort is spent to make sure that waste products and resultant gases flow in one direction.

Many failures in the plumbing system may permit sewer gases to flow back into a home. Plumbing vents should not be installed too close to windows and air intakes in homes equipped with HVAC air handlers that draw in outside air for ventilation. Air and wind surge around the building can let sewer gases enter the home even where air intakes and plumbing vents are properly located. Homeowners can alter the height of vents and install vent pipe filters to lessen the problem.

Sewer gases can also diffuse through cracks in a building’s foundation when the leach field septic system leaks.

Vent pipe cracks are more difficult to diagnose than cracks in the drain line, which is typically accompanied by visible water leaks. Vent pipe cracks can allow a significant amount of sewer gases to enter the home. Plumbers can find these cracks by using a special device that emits artificial smoke and propels it into the plumbing drain system. The smoke pressurizes the drain and vent systems and escapes through cracks and loose fittings.

In most cases, obtrusive sewer gases are produced by a loss of the water barricade where traps have become dry. More particularly in dry weather, infrequent use of a floor drain, water closet, or shower can allow for fast evaporation and admission of sewer gases into the living spaces. The locations in which floor drains are most likely to dry out are near furnaces and water heaters, in mechanical rooms, in workshop areas and in a janitor’s closets. Homeowners can maintain the water barricade by pouring water down the drains and using the fixtures regularly. Automatic drain-trap primers can also be installed to periodically deliver small quantities of water to the traps.

Norman Aceron Garcia is a registered Professional Engineer and a Certified Property Inspector of Mr. Peg Property Inspections Inc. Please visit for more information on home inspection, building science and home maintenance tips.

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