Sewer gas poisoning causes symptoms ranging from headaches and nausea at low concentrations to unconsciousness and death at high ones. The most dangerous component is hydrogen sulfide, which smells like rotten eggs at first but can paralyze your sense of smell at higher levels, making it impossible to detect just when it becomes most dangerous. Other toxic gases in the mix, including ammonia, methane, and carbon dioxide, contribute their own health effects.
What Sewer Gas Actually Contains
Sewer gas forms when household and industrial waste decays inside pipes, septic systems, and drains. It’s not a single chemical but a complex mixture. The two most toxic components are hydrogen sulfide and ammonia. The gas also contains methane, carbon dioxide, sulfur dioxide, and nitrous oxides. Each of these affects the body differently, but hydrogen sulfide is the primary threat because it interferes with your cells’ ability to use oxygen, essentially suffocating tissues from the inside even while you’re still breathing.
Symptoms at Low Concentrations
At the levels most likely in a home with a plumbing leak, symptoms tend to be mild and easy to mistake for other problems. You may notice a persistent rotten-egg smell, along with headaches, nausea, dizziness, or eye irritation. In the range of 50 to 100 parts per million (ppm) of hydrogen sulfide, mild eye inflammation and respiratory irritation develop within about an hour of exposure.
These symptoms often come and go, which makes them confusing. You might feel fine when you leave the house and sick again when you return. If the smell is faint and intermittent, it’s easy to dismiss as a minor plumbing issue. But even at these lower levels, prolonged exposure matters.
Symptoms at Dangerous Concentrations
As concentration rises, symptoms escalate quickly. At 170 to 300 ppm, hydrogen sulfide is the maximum a person can tolerate for about an hour without serious consequences. Beyond that, the timeline compresses dramatically:
- 500 to 700 ppm: Dangerous within 30 minutes to an hour. Severe respiratory distress, confusion, and loss of coordination.
- 700 to 1,000 ppm: Rapid unconsciousness, breathing stops, and death follows without immediate rescue.
- 1,000 to 2,000 ppm: Unconsciousness and death within minutes.
These extreme concentrations are rare in homes but do occur in confined spaces like basements with major sewer line breaks, or in manholes and industrial settings. The critical danger is that most people who collapse from high-level exposure never had time to register a warning, because their sense of smell was already gone.
Why You Can’t Always Smell the Danger
Hydrogen sulfide has one of the most recognizable odors in nature: rotten eggs. At low levels, your nose picks it up easily. But at around 100 ppm, a phenomenon called olfactory fatigue kicks in. The gas essentially overwhelms and temporarily paralyzes the smell receptors in your nose. The rotten-egg odor disappears, and you may assume the air has cleared when it hasn’t. This is one of the most dangerous features of hydrogen sulfide exposure. If a strong sewer smell suddenly vanishes without any plumbing fix, that’s a reason for more concern, not less.
Chronic, Low-Level Exposure
Not all sewer gas poisoning is sudden. A slow leak from a cracked pipe or dry drain can expose you to low concentrations for weeks or months. The body doesn’t accumulate hydrogen sulfide the way it does lead or mercury, but repeated exposure still causes real problems. Reported symptoms of chronic, low-level exposure include persistent headaches, nausea, loss of appetite, weight loss, low blood pressure, chronic cough, and eye inflammation.
Neurological effects are also documented. People with prolonged exposure have reported difficulty with balance and coordination, mood changes, poor concentration, and other psychological symptoms. These can be subtle enough that neither you nor your doctor connects them to a plumbing problem, especially if the smell has faded or you’ve gotten used to it.
Children face a higher risk from chronic exposure. Their smaller body size means the same concentration hits them harder, and because they may live in the same home for years, their total exposure window is longer. If a child in your household has unexplained recurring headaches or nausea, and you’ve noticed even an occasional sewer smell, it’s worth investigating the plumbing.
Other Gases in the Mix
Hydrogen sulfide gets the most attention, but it’s not working alone. Ammonia irritates the eyes, nose, and throat and can trigger coughing and breathing difficulties at moderate concentrations. Methane is primarily an asphyxiation and explosion risk. It displaces oxygen in enclosed spaces and is flammable, so a significant methane leak in a basement creates both suffocation and fire hazards. Carbon dioxide at elevated levels causes headaches, dizziness, and shortness of breath. Because methane and carbon monoxide are odorless, you won’t smell them at all.
Common Causes of Sewer Gas in Your Home
If you’re experiencing symptoms, it helps to know where the gas might be coming from. The most frequent culprit is a dry P-trap. Every drain in your home has a curved section of pipe that holds a small amount of water, creating a seal against sewer gas. When a drain goes unused for a while (a guest bathroom, a basement floor drain, a laundry sink), that water evaporates and the seal breaks. Running water through unused drains every few weeks prevents this.
Other common sources include cracked or degraded sewer pipes, which let gas seep through walls or floors. Blocked vent pipes on your roof, often clogged with leaves, bird nests, or debris, prevent toxic gases from venting safely and force them back into interior pipes. Clogged drains can cause sewage to back up and decompose in place, generating gas that leaks into living spaces. A toilet that isn’t tightly sealed to the floor creates a gap where gas escapes from the sewer line. And in some cases, a failing septic system nearby can push gas through cracks in your home’s foundation.
Workplace Safety Limits
Federal safety standards give useful context for how seriously experts take even small amounts of hydrogen sulfide. NIOSH sets a recommended ceiling of 10 ppm, meaning workers shouldn’t be exposed to more than that for even 10 minutes. OSHA’s permissible ceiling is 20 ppm, with an absolute maximum peak of 50 ppm for no more than 10 minutes. These limits exist because the margin between “mildly irritating” and “life-threatening” is narrower than most people realize. A concentration that causes mild eye irritation at one hour can become dangerous at slightly higher levels or longer durations.
What to Do if You Suspect Exposure
The most important immediate action is getting to fresh air. If you smell rotten eggs and are experiencing headaches, dizziness, nausea, or eye irritation, leave the area. Open windows if you can do so safely. Don’t re-enter an enclosed space where someone has collapsed, because the same gas that knocked them out will affect you too. Multiple-victim incidents are common in confined-space sewer gas accidents precisely because rescuers rush in without protection.
For a persistent, low-level smell in your home, start by running water through every drain you haven’t used recently. Check that toilets are firmly seated. If the smell continues, a plumber can pressure-test your drain system and inspect vent pipes. For stronger or sudden sewer odors, especially in basements or enclosed rooms, ventilate the space and avoid open flames (methane is explosive). Portable hydrogen sulfide detectors are available for around $100 to $200, and they can confirm whether you’re dealing with a real gas leak or a temporary odor.