Sewer gas is produced by the natural decomposition of human and industrial waste within a drainage system. This gas includes a variety of components, and in high concentrations, certain elements can be lethal, making exposure while sleeping particularly dangerous. Modern plumbing systems are designed with barriers and vents to prevent this gas from entering a home. When a failure occurs, the toxic components can quickly accumulate in enclosed spaces, posing a danger that directly interferes with the body’s ability to function.
What Makes Sewer Gas Dangerous
Sewer gas is a combination of gaseous byproducts, primarily including hydrogen sulfide, methane, carbon dioxide, and ammonia. While many of these gases are relatively harmless at low levels, their presence in an enclosed indoor environment creates two distinct and serious threats. The most significant toxic threat is hydrogen sulfide (H2S), recognized by its distinct rotten egg smell at low concentrations.
Methane (CH4), another major component, is colorless and odorless. Methane is highly flammable, but more relevant to the immediate danger is its ability to displace oxygen in a confined space. When methane concentrations are high enough, they lower the oxygen level in the air, which can lead to unconsciousness and death from asphyxiation without warning.
The direct threat to life is primarily linked to the toxicity of hydrogen sulfide. Low-level H2S exposure can cause symptoms like headaches, dizziness, and respiratory irritation. The concentration of H2S in the air is the determining factor between mild irritation and a life-threatening scenario.
How Hydrogen Sulfide Affects the Body
Hydrogen sulfide is a fast-acting poison that affects multiple systems in the body. The gas is a broad-spectrum toxicant, and its mechanism of action is similar to that of hydrogen cyanide, a chemical asphyxiant. This chemical interference makes high-concentration exposure rapidly fatal, especially while a person is asleep and unable to react.
A dangerous property of H2S is its ability to quickly paralyze the sense of smell, known as olfactory fatigue. While the rotten egg odor is noticeable at low concentrations, the warning sign disappears at concentrations above 100-150 parts per million (ppm). This loss of smell creates a false sense of security, as the most severe, lethal concentrations become entirely undetectable by the nose.
The physiological process of H2S poisoning centers on its interference with cellular respiration. The gas binds to the iron in mitochondrial cytochrome enzymes within the cell’s mitochondria. This binding prevents the cells from utilizing oxygen delivered by the bloodstream, effectively arresting aerobic metabolism.
The body’s cells are starved of the energy required for basic function, leading to a rapid shutdown of the central nervous and respiratory systems. Concentrations between 530 and 1000 ppm can cause strong stimulation of the nervous system, leading to rapid breathing and eventual loss of respiration. At concentrations around 800 ppm, H2S can be lethal to 50% of humans within just five minutes of exposure. A single breath of concentrations above 1000 ppm can cause immediate collapse and loss of consciousness, leading to death in minutes.
Stopping Leaks and Ensuring Home Safety
Preventing sewer gas leaks relies on maintaining the integrity of the home’s plumbing system, which is designed to keep the gas contained. One of the most common causes of gas intrusion is a dry P-trap, the U-shaped curve in the pipe beneath a drain that holds a small water barrier. If a drain in a basement or guest bathroom goes unused for too long, the water seal can evaporate, allowing sewer gas to bypass the barrier and enter the living space.
Simple maintenance steps, such as regularly running water down all unused drains, helps ensure the P-traps remain full and the water barrier is intact. Other common entry points include cracked drain lines hidden beneath floors, loose seals around the base of toilets, and blocked or damaged sewer vent pipes. These vent pipes extend to the roof and are meant to safely release gases outside and regulate pressure in the system.
If a foul odor persists despite basic maintenance checks, it may indicate a more serious issue, such as a cracked pipe or a blocked vent pipe on the roof. Homeowners should visually inspect exposed plumbing and check for signs of moisture or damage near drains and toilets. For problems that are not immediately obvious, professional plumbers can use specialized equipment to locate and repair hidden leaks, such as those behind walls or under the foundation.