Sewer gas is the gaseous byproduct formed by the anaerobic breakdown of organic waste within sewage systems. This complex mixture contains compounds that can significantly affect human health. When these gases escape the plumbing system and enter an occupied space, they can cause a range of symptoms. The severity of the health effects is directly related to the concentration of the gases and the duration of exposure.
Key Components of Sewer Gas
The mixture is primarily composed of gases produced by the decay of biological material. The most recognized component is hydrogen sulfide (\(\text{H}_2\text{S}\)), which is responsible for the distinct, unpleasant, rotten egg odor and is the main toxic agent. Sewer gas also contains methane (\(\text{CH}_4\)), ammonia (\(\text{NH}_3\)), carbon dioxide (\(\text{CO}_2\)), and various nitrogen and sulfur oxides.
Hydrogen sulfide is immediately noticeable, even at concentrations as low as 0.5 parts per billion (ppb), serving as an early warning. Methane and carbon dioxide are colorless, odorless, and considered asphyxiants because they displace oxygen in high concentrations.
Common Symptoms of Low-Level Exposure
In residential settings, exposure is most often to low concentrations of hydrogen sulfide, which can lead to a variety of reversible symptoms. Frequent complaints include headaches, nausea, and general fatigue, which resolve once the individual leaves the affected area. Low-level exposure can also cause irritation to the mucous membranes, respiratory system, and eyes.
Symptoms manifest as watery eyes, a sore throat, or a cough. Low concentrations may also lead to difficulty concentrating, dizziness, and lightheadedness. Since the gas is detectable by smell at minute concentrations, the odor typically alerts residents to the problem long before the gas reaches acutely toxic levels.
Mechanisms of High Concentration Toxicity
The severe dangers of sewer gas are linked to high-concentration exposure, typically occurring in confined spaces like industrial tanks or sewer manholes. Hydrogen sulfide is classified as a broad-spectrum poison because it inhibits the function of cytochrome c oxidase, a protein involved in cellular respiration. By binding to iron in mitochondrial enzymes, \(\text{H}_2\text{S}\) prevents the body’s cells, especially those in the nervous system and heart, from utilizing oxygen.
Exposure above 100 parts per million (ppm) of \(\text{H}_2\text{S}\) can cause olfactory fatigue, rapidly paralyzing the sense of smell and removing the warning odor. At extremely high concentrations, such as 1000 ppm or higher, a single breath can cause immediate collapse, respiratory paralysis, and death. Methane becomes a threat in high concentrations by significantly reducing available oxygen, leading to asphyxiation.
Locating and Eliminating the Source
The entry of sewer gas into a structure is nearly always a result of a breach in the home’s plumbing system. The most common and easiest issue to resolve is a dry P-trap, the U-shaped section of pipe beneath every drain that holds water to create a seal against sewer gases. If a fixture, such as a basement floor drain, goes unused for several weeks, the water can evaporate, allowing gas to pass freely. Pouring water down these drains regularly restores the protective water barrier.
Another frequent cause is a faulty wax ring seal at the base of a toilet, which allows gas to escape from the drain pipe. Blockages or damage to the plumbing vent stack, which vents gases outdoors through the roof, can also force odors back inside the home. If simple measures like watering drains do not resolve the odor, a professional plumber must inspect for cracked pipes, loose connections, or a blocked vent.