Sewer gas is unlikely to set off a standard home carbon monoxide detector, but it’s not impossible. The answer depends on the type of sensor inside your detector and the specific gases present in the sewer gas. In most cases, a CO alarm going off alongside a rotten-egg smell points to a real problem worth investigating, even if the detector isn’t responding to actual carbon monoxide.
Why CO Detectors Usually Ignore Sewer Gas
Home carbon monoxide detectors use electrochemical sensors tuned specifically to carbon monoxide molecules. When CO reaches the sensor’s electrode, it triggers a chemical reaction that produces a measurable electrical signal. Other gases can sometimes cause a similar reaction, a phenomenon called cross-sensitivity, but the effect varies widely depending on the gas.
Hydrogen sulfide is the signature component of sewer gas, the one responsible for that unmistakable rotten-egg smell. According to cross-sensitivity testing data from Forensics Detectors, when an electrochemical CO sensor is exposed to 15 ppm of hydrogen sulfide, it produces a reading of roughly negative 0.1 ppm. That’s essentially nothing. The sensor barely registers it, and the tiny signal it does produce actually goes in the negative direction, meaning it would suppress a reading rather than trigger a false alarm.
This means that under normal conditions, the hydrogen sulfide in sewer gas won’t fool an electrochemical CO detector into sounding an alarm.
When Sewer Gas Could Trigger an Alarm
Not all carbon monoxide detectors use the same technology. Some older or cheaper models rely on tin oxide (semiconductor) sensors instead of electrochemical ones. These sensors are significantly less selective. They respond to a broader range of gases, including volatile organic compounds like alcohols, hydrocarbons, and other fumes. Research on the UL 2034 safety standard, which governs CO alarm certification, found that tin oxide sensors are highly sensitive to these compounds. Methane, which is a major component of sewer gas, was specifically identified as a gas that might cause false alarms at lower alarm thresholds around 15 ppm CO.
Sewer gas is a complex mixture. Beyond hydrogen sulfide and methane, it can contain carbon dioxide, ammonia, sulfur dioxide, and nitrous oxides. In homes connected to municipal sewer systems, traces of industrial solvents, gasoline vapors, and chlorine bleaches can also make their way into the gas mix. If enough of these compounds reach a less-selective sensor, a false alarm becomes more plausible.
Age matters too. Internal filters in CO detectors degrade over time, which is one reason manufacturers recommend replacing them every five to seven years. A detector past its expiration date may have reduced ability to filter out interfering gases.
What’s Actually in Sewer Gas
Sewer gas forms when household and industrial waste decays inside drain pipes, septic systems, and sewer lines. Its composition shifts depending on what’s decomposing and where, but the core ingredients are consistent: hydrogen sulfide, methane, ammonia, carbon dioxide, sulfur dioxide, and nitrous oxides. Both methane and hydrogen sulfide are flammable and potentially explosive in high concentrations.
Hydrogen sulfide is the most immediately dangerous component for your health. You can smell it at remarkably low concentrations, with odor detection starting between 0.008 and 0.13 ppm. Below 10 ppm, it causes minor eye and respiratory irritation. At higher levels, the danger escalates quickly. Concentrations between 41 and 76 ppm can cause serious health effects within minutes, and levels of 500 to 1,000 ppm are typically fatal.
Here’s the unsettling part: at around 100 ppm, hydrogen sulfide causes olfactory fatigue, meaning your nose stops detecting the smell. By 150 ppm, the gas can paralyze the nerve responsible for your sense of smell entirely. So the absence of a rotten-egg odor doesn’t guarantee the gas is gone.
Sewer Gas and CO Poisoning Feel Different
If your CO detector goes off and you also smell rotten eggs, the problem is almost certainly sewer gas rather than carbon monoxide. Carbon monoxide is completely odorless and colorless. You cannot smell it at any concentration. Its symptoms, headache, dizziness, nausea, and confusion, build gradually and mimic the flu.
Hydrogen sulfide, by contrast, announces itself with that sulfur smell at very low levels. It irritates the eyes and throat early on. At moderate concentrations it can cause coughing, shortness of breath, and fluid buildup in the lungs. The overlap in symptoms (headache, nausea, dizziness) means both gases can make you feel sick in similar ways, but the presence or absence of smell is the clearest initial distinction.
Methane poses a different kind of risk. It isn’t toxic in the traditional sense, but in enclosed spaces like a basement or crawl space, high concentrations displace oxygen. This can cause suffocation without any warning smell, since methane is also odorless in its pure form.
Better Tools for Detecting Sewer Gas
A standard carbon monoxide detector is the wrong tool for finding a sewer gas leak. If you suspect gas is entering your home through a dried-out P-trap, a cracked sewer line, or a failing wax ring on a toilet, you need a detector designed for combustible gases or hydrogen sulfide specifically.
Combustible gas leak detectors are the most accessible option for homeowners. These handheld devices use sensors that respond to methane and other flammable gases, letting you trace a leak to its source by moving the probe along pipes and fixtures. Several brands make consumer-grade versions, including Klein Tools, Ridgid, and UEI. Dedicated sewer gas detectors exist as well, targeting hydrogen sulfide and ammonia in addition to combustible gases, though they tend to cost more.
The sensors inside combustible gas leak detectors and dedicated sewer gas detectors are functionally identical. Both respond to combustible gas sources, so a natural gas leak detector will work for sewer gas detection too.
What to Do if Your CO Detector Goes Off
If your carbon monoxide alarm sounds and you smell rotten eggs, take it seriously regardless of the cause. Leave the area, ventilate the space by opening windows, and call your gas utility or a plumber. Even if the CO detector is reacting to a non-CO gas, the presence of sewer gas itself is a genuine hazard: hydrogen sulfide is toxic, and methane is explosive.
If your CO alarm sounds with no smell at all, treat it as a potential carbon monoxide emergency. That’s exactly the scenario CO detectors are built for, and carbon monoxide’s invisibility is what makes it so dangerous. In either case, don’t dismiss a sounding alarm as a simple malfunction without investigating. Check your detector’s expiration date, and if the unit is more than seven years old, replace it. A fresh detector with an electrochemical sensor will give you far fewer false positives while still catching real CO threats.