Sewer Gas

Overview

Sewer gas is a common term for a complex mixture of chemicals that are formed during the decay or processing of waste.  Two common, highly toxic components of sewer gas include hydrogen sulfide and ammonia.

In addition, sewer gas typically contains methane, carbon dioxide, sulfur dioxide, nitrous oxides, and biological agents.  Chlorine bleach, cleaners and solvents, and gasoline are frequently present in both municipal and private sewage treatment systems.
 

• Explosion and Fire: Methane and hydrogen sulfide are flammable and highly explosive at levels above 4-5% in air. 

• Hydrogen Sulfide Poisoning: Exposure to low levels of hydrogen sulfide can cause irritation of the eyes and respiratory tract.  Other symptoms include nervousness, dizziness, nausea, headache, and drowsiness.  At extremely high levels, hydrogen sulfide can cause immediate loss of consciousness and death.

• Asphyxiation: Sewer gas is heavier than air and will accumulate at low elevations of the structure where it can displace oxygen thereby resulting in suffocation.  

• Biological Pathogens: Inhalation of airborne biological agents (bacteria, viruses, etc.) can cause disease.

How does it get into the structure?
Sewer gas typically enters the structure because of problems with the plumbing drain or vent systems.  Dry drain traps, improper plumbing connections, and blocked vent lines are the most common problems.  

Accumulation in low lying areas can also occur.  Basements, underground vaults, excavations, pits, etc. in proximity to sewer lines, septic systems, landfills, wetlands/swamps are susceptible.  
 

• Hydrogen Sulfide: At non-toxic levels, hydrogen sulfide has the familiar odor of 'rotten eggs', however, at toxic levels, it can paralyze an individual's ability to detect its odor and can rapidly render the victim unconscious.  
• Carbon Dioxide and Methane: Little or no odor, primarily displace oxygen (asphyxiation).  
• Ammonia: A strong odor with good warning characteristics which are present below toxic levels (toxic levels of ammonia are typically not produced from common sewage). 

The American Conference of Governmental Industrial Hygienists (ACGIH) establishes Time Weighted Average (TWA) and Short Term Exposure Limits (STEL) levels for many types of chemicals under their Threshold Limit Value (TLV^®) program.  Note that these values typically apply to occupational settings and are not always valid when considering non-occupational environments where low-dose/long-term exposures may be present.
 

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