Sewer gas is harmful to human health because it contains methane gas, which contains a mixture of toxic gases. One of these is hydrogen sulfide, which can affect the functioning of the central nervous system, according to the International Association of Certified Home Inspectors. It is important to call a plumber as soon as possible, to investigate any potential sewer gas issues.
Sewer gas in closed spaces dispels oxygen and eventually causes suffocation. Anyone who smells sewer gas should watch for signs of oxygen deficiency which may include headaches, dizziness or shortness of breath, as the International Association of Certified Home Inspectors explains. Another component of sewer gas, ammonia, can aggravate membranes in the body along with the respiratory tract. It can be very toxic at high levels. Build-ups of sewer gas in a place at high concentrations can also be explosive.