We want our homes to smell fresh and clean, but sometimes we get a whiff of something that doesn’t smell right. For example, a noxious, sewer-like odor could be a sign of a problem with your plumbing system. So what does sewer gas smell like?

Sewer gas most often smells like rotten eggs. Although sewer gas can include ammonia and other gasses, hydrogen sulfide is the cause of its distinctive smell. A skunk-like smell comes from natural gas, which must be dealt with immediately.

Sewer gas can cause severe illnesses at high concentrations, but houses don’t get concentrations high enough for severe symptoms. That doesn’t mean you should ignore the smell, however. The odor could be a sign that something is not functioning correctly in your plumbing system.

What Is Sewer Gas?

Sewer gas is a mixture of poisonous and harmless gasses resulting from the chemical output of human excrement as it decomposes. We smell it when our plumbing system malfunctions and allows hydrogen sulfide to return to our homes instead of the sewage system.

Sewer gas includes several gasses, such as ammonia, methane, carbon dioxide, and hydrogen sulfide. 

Ammonia is a common ingredient in cleaning products like Windex. Although it has a distinct odor, hydrogen sulfide overpowers it. At low concentrations, ammonia exposure can irritate the eyes, nose, and throat. However, ammonia is flammable and hazardous to humans at greater concentrations.

Methane and carbon dioxide are two nontoxic greenhouse gasses. However, in large quantities, methane gas is combustible. When combined with the flammability of ammonia, significant volumes of sewage gas become a fire threat.

Hydrogen sulfide is the easiest to smell of all the gasses in sewer gas. Hydrogen sulfide has been proven in studies to seriously affect the central nervous system at concentrations above 150 ppm, so it can be dangerous.

Most people can detect hydrogen sulfide at levels of .01 ppm, and household sewers do not produce enough gas to reach those harmful levels. Therefore, the sewer level could never reach that level without a neighbor reporting the smell.

Luckily, you won’t often find levels that could cause combustibility or adverse health effects in houses.

Does Sewer Gas Smell Like Poop?

Sewer gas doesn’t necessarily smell like poop. Most people describe the smell of sewer gas as like rotten eggs or moldy cheese. That’s because the smelliest chemicals released from sewer gas are usually from decomposition, giving the air a sour, musty, decaying scent.

So, if you smell poop, you probably don’t have a sewer gas issue. A “poop” smell could be caused by a dead or living animal in an attic, the walls, crawl space, or garage. If a rat or possum has taken residence in your house, you will eventually smell its poop.

What Causes Sewer Gas Leaks in Homes?

Sewer gas leaks are usually caused by P-traps that have become clogged or dried out. All plumbing fixtures should have a P-trap, including sinks and bathtubs, and the gas can come from any plumbing fixture with a malfunctioning P-trap.

These traps serve two purposes.

First, they help reduce the risk of clogging the drainage system. Bulky items are more likely to get stuck in the P-trap instead of traveling down the drainage system and into the sewer line.

Getting something out of the P-trap is much easier than the sewer line, but gas could rise from your home plumbing system if you don’t remove it.

Also, a clogged system can cause drainage problems throughout the house, which is usually an excellent way to tell if the P-trap is the issue.

But more importantly, a P-trap keeps sewer gas from rising up the drainpipe. Some water will stay in the curved area of the p-trap, sealing the pipe, and gravity will keep this water in place.

This water plug acts as a block against the gas that tries to re-enter the house through the sewer pipe.

Is Sewer Gas Harmful to Breathe?

Sewer gas can be harmful, but it usually needs to be at higher levels than those found in homes. Toxic sewer gas usually occurs in industrial workplaces since there is a high volume of sewage lines in bigger buildings, and overexposure can severely affect a person’s health.

The hydrogen sulfide in the gas is the main culprit in harmful exposure. The sulfide inhibits enzymes that regulate the use of oxygen in our cells, and the resulting anaerobic (without oxygen) metabolism causes an acid-base imbalance.

This imbalance affects our nervous system and our heart tissues. 

If an individual is not removed from exposure to hydrogen sulfide, they might die from respiratory arrest. However, this is highly unlikely to occur in your home. Instead, you might suffer from minor symptoms such as headaches, dizziness, or nausea.

What Are the Symptoms of Sewer Gas Poisoning?

The symptoms of sewer gas poisoning in the levels homeowners will get are mild. These can include fatigue, headaches, or dizziness. Signs of higher exposure include a loss of smell and lung irritation.

The eyes and respiratory tract get irritated when exposed to low levels of hydrogen sulfide. Nervousness, dizziness, nausea, headache, and drowsiness are other symptoms that might occur. 

Because excessive quantities can impair one’s sense of smell, people might not notice the rotten egg smell after the first few minutes of exposure. Hydrogen sulfide can cause immediate loss of consciousness and death at extremely high concentrations, but again, a homeowner shouldn’t have to worry about this.

Suffocation can occur in confined locations with high methane concentrations– another sewer gas– because excessive levels of methane reduce the oxygen level of an area. Headache, nausea, dizziness, and unconsciousness are symptoms of not getting enough oxygen.

Still, the chances of methane causing deadly low oxygen levels (under 12%) are unlikely in a home. 

Warning: If you detect sewer gas and experience trouble breathing, nausea, or dizziness, contact your doctor. The chemicals from the sewer gas could be exacerbating underlying medical issues.

How to Find the Sewer Smell in Your House

To find where the sewer smell is coming from, you first need to understand what can be causing the smell. It could be from clogged or dry drains, blocked air vents, leaking pipes, or cracked sewer pipes. Knowing this can help you sniff out the source of the odor.


The first thing to check is the drains in your home. If the P-trap in a sink dries out, it no longer prevents sewer odors from entering your home. Check your sinks and showers, starting with any that don’t get much use. Run some water for thirty seconds to fill the P-trap and create the water “plug.”

Plugged drains can also cause a sewer smell if the clog lets a small amount of gas leak out. However, this is rarely the case.

If the P-trap is damaged or leaks, the seal that keeps sewer gas from coming into the house is broken. If that’s your issue, the trap needs to be replaced.

Leaky pipes can also be the culprit, and you will need to repair them before they cause severe water damage.

Air Vents

The air vent system in your plumbing is intended to allow sewer gasses to escape the plumbing system via the roof.

If an air vent becomes blocked, the gasses will become trapped in your home. Debris can block the vents, and in cold weather, condensation from sewer gas moisture can create ice that clogs them.

If you can’t locate the odor after inspecting the drains, a plugged air vent could be causing the gasses to enter your home another way. A toilet that doesn’t flush properly or gurgles is another indicator the air vents are clogged.

Sewer Pipes

Leaks or cracks in the pipes that connect to the sewer can be responsible for the odor because they prevent all the waste from reaching the sewer line. A sewer gas smell combined with slow drains and gurgling toilets is a sign you should call someone who has expertise in that kind of repair.

See also: Why Does My Toilet Smell Like Sewage When Flushed?

Is There a Sewer Gas Detector?

There is not a gas detector specific for sewer gas. However, sensors can identify the presence of gasses found in sewer gas, especially hydrogen sulfide. 

A user must know which gasses are typically found in the mixture of gasses included in sewer gas and determine whether or not those gasses are present. Then, you can get a detector that targets the most prominent gas– hydrogen sulfide.

You can purchase detectors that will alert you of hydrogen sulfide in the air at big-box home improvement stores like Home Depot or Lowes. 

A carbon monoxide detector is designed to detect the presence of carbon monoxide and nothing more. Sewer gas will not set one off, and you can’t use it to detect sewer gas.

Can I Prevent Sewer Gas From Forming?

Minimal maintenance can prevent most sewer gas problems. The first place to start is with the drains and traps. Then, depending on the climate, you may also need to take preventative measures with the vent stack.

So, to keep sewer gas from coming into your home:

  • Flush drains with water to keep the P-traps from drying out.
  • Clean out your drains regularly. Remove the stopper, clean it, and then use a wire brush to clean the pipe. Alternatively, remove the P-trap and clean it.
  • Regularly check the roof plumbing vent for leaves, bird nests, and other debris.
  • If you have ice forming in the vent stack, insulate the vent pipe in the attic. Insulated sewer vents are available at most hardware and plumbing supply stores.

Unfortunately, there is little you can do to prevent cracks in the sewer line. If they occur, you will have to call in a specialist.

What Should I Do if I Can’t Locate the Problem?

If you cannot locate the problem, you should contact a local, trusted plumber. They have more experience in troubleshooting the problem and making necessary repairs. Ignoring the problem isn’t going to make it go away.

Indeed, if the problem disappears, it could be because your olfactory senses have become desensitized to the odor. A visitor, however, will notice it and hopefully alert you to it.

What Other Odors Should I Worry About?

Before you buy room deodorizers to mask an odor, you should be aware that some scents are a reason for concern. For example, a fishy or cat urine odor could signal a severe problem, while you should also investigate the smell of dirty socks or rotting food.

So, let’s look at some common smells that signify an issue:

  • If your house has a fish-like odor, you should contact an electrician immediately. A fishy smell is usually a sign of a damaged electrical component, including loose or frayed wires, overloaded circuits, or inadequately sized breakers or fuses. If not treated immediately and properly, any of these issues could result in a house fire.
  • A cat urine odor, especially when it rains, can signify a mold problem. Several mold strains, including black mold, can emit a smell similar to cat urine. You should contact a mold removal expert to eliminate this dangerous and potentially toxic mold.
  • A buildup of bacteria on your heating system coils could cause a “dirty sock scent.” This bacteria buildup is most common in the spring and fall when the system is constantly cycling between hot and cold. The condensation on the coils provides the ideal environment for bacterial development and the “dirty sock” odor. 
  • A wet dog, poop, or rotting food odor can be a sign that you have a dead animal, like a mouse or rat, in the walls of your home.

Bottom Line

Sewer gas is a mixture of dangerous and non-poisonous gasses produced by the decomposition of human waste. When our plumbing system malfunctions and allows hydrogen sulfide to enter our houses, we detect a rotten egg odor. 

Please don’t wait for the problem to resolve itself. Instead, investigate the most common source of the gas, which are drains and P-traps, or call a plumber to troubleshoot the problem.

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