What are the dangers of sewer gases?

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Sewer gas is a mixture of toxic and nontoxic gases. Toxic gases include hydrogen sulphide and ammonia. Sewer gas also contains various levels of methane, carbon dioxide, sulphur dioxide and nitrous oxide. Sewer gases enter your home through drains, leaky plumbing and clogged sewer vents on the roof.

Fumes come from municipal sewers, storage tanks and septic tanks. Exposure to sewer gases is dangerous and can be life threatening.

Hydrogen sulfide

Low levels of hydrogen cause allergy-like symptoms in humans, such as burning eyes and coughing. Symptoms that are more serious include nervousness, dizziness, nausea, headaches and drowsiness. Hydrogen sulphide smells much like rotten eggs. Exposure to extremely high levels of this gas can cause unconsciousness and death.


Although not toxic, exposure to high levels of methane in enclosed areas can cause asphyxiation. Methane decreases the amount of oxygen in the air, causing headache, nausea, dizziness and unconsciousness. Death can occur quickly and without warning. Methane is also highly flammable; a build-up of this gas becomes extremely explosive.

Carbon monoxide

Carbon monoxide is a byproduct of methane. It is odourless, tasteless, nonirritating and deadly. The NHS reports that about 50 people a year in the UK die from inhaling the gas and a furyther 200 a year are left seriously ill by it. Those who survive often suffer from long-term neurological problems. Symptoms may take the form of a migraine, depression, flu or chronic fatigue syndrome when exposed to low levels on a daily basis. As exposure becomes more evident, you may experience confusion, convulsions and unconsciousness. You may also experience hallucinations or become overly emotional; you may even see ghosts or experience supernatural happenings. Installing a carbon monoxide detector could save your life.

Septic tank

A septic tank provides its own source of gaseous fumes that are especially dangerous for children and pets. Toxic gases trapped within the tank may escape through the opening located on the ground level. A much smaller volume of gas can quickly harm a child or pet who inhales the gas from the opening. Septic tanks can back up into living spaces, exposing families to bacteria, viral infections and toxic fumes.

Safety tips

Regularly add water to plumbing fixtures that you don't use often to prevent evaporation of water in the water-seal trap. An evaporated water seal allows sewer gases to enter your home.

Do not allow grease to enter the sewer system by not flushing it down the drain. Grease solidifies in the pipes and sewer system, trapping debris. Over time, a build-up of grease can obstruct water flow and cause sewage to back up into your home.

Do not pour chemicals such as antifreeze, insecticides, pesticides, cleaners, fertilisers paint thinners or fuels down the drain. Chemicals enter the sewer system and travel to the water treatment plant, making it difficult to purify the water; they can also damage expensive equipment at the plant. Additional expenses at the treatment plant can raise your water and sewage bill.