What are the effects of exposure to sewer gas?
The principal risks and effects associated with exposure are:

• Hydrogen sulfide poisoning. Exposure to low levels of hydrogen sulfide causes irritation of the eyes and respiratory tract. Other symptoms include nervousness, dizziness, nausea, headache, and drowsiness. This gas smells like rotten eggs, even at extremely low concentrations.  Exposure to high concentrations can interfere with the sense of smell, making this warning signal unreliable. At extremely high levels, hydrogen sulfide can cause immediate loss of consciousness and death.
• Asphyxiation. High concentrations of methane in enclosed areas can lead to suffocation as large amounts of methane will decrease the amount of oxygen in the air. The effects of oxygen deficiency include headache, nausea, dizziness and unconsciousness. At very low oxygen concentrations (<12%), unconsciousness and death may occur very quickly and without warning.  Sewer gas diffuses and mixes with indoor air, and will be most concentrated where it is entering the home. It can accumulate in basements.
• Explosion and fire. Methane and hydrogen sulfide are flammable and highly explosive.