Sewage Odour: Start With the Drains

What this article contains:

1. Does This Sound Like Your Odour?

2. Sewage Smell in House: What Can Cause This Odour?

3. Why a Persistent Sewage Smell Matters

4. How to Investigate Safely

5. What Not to Do

6. When Professional Help Is Appropriate

1. Does This Sound Like Your Odour?

A sulfur or sewer odour can take several forms. It may smell like rotten eggs, sulfur, sewage, fecal matter, sour residue, grease or wastewater. These descriptions are useful clues, but they do not identify the source on their own. Different problems can produce similar odours, and air movement can carry the smell away from where it actually originates. [1]

Rotten Egg, Sulfur or Cooked-Cabbage-Like

This odour is usually sharp, sulfur-like and unpleasant. It may be more noticeable when water runs, near a drain or water fixture, or in an enclosed area. Hydrogen sulfide can contribute to this type of odour and may be associated with sewers, wastewater, well water or hot-water systems. [2]

The smell may remind you of:

• Rotten eggs

• Cooked cabbage, broccoli or cauliflower

• Garlic or onion

• A swampy or marshy smell

• Skunk

• Sulfur or burnt matches

Sewer, Wastewater or Toilet-Like

This odour is more fecal or wastewater-like than sulfur-like. It may seem heavier, dirtier or more stagnant than a simple rotten-egg smell.

The smell may remind you of:

• Raw sewage or wastewater

• A toilet or latrine

• A septic tank

• Manure or a barnyard

• A floor drain in a neglected utility room

• Stagnant wastewater

Sour, Greasy or Decaying-Drain-Like

Some drain odours do not smell distinctly like sewage. They may be sour, greasy or food-like, especially near a kitchen sink, garbage disposal or overflow opening. [3]

The smell may remind you of:

• Sour milk

• Vinegar

• Grease

• Rotting food

• Garbage

• Old dishwater

Safety, Rule Out a Natural-Gas Leak

A rotten-egg smell is not always sewer gas. Natural gas has no natural odour, so utilities add mercaptan, a sulfur-smelling odorant, to make leaks detectable. If an unexplained rotten-egg odour appears suddenly, particularly near a gas appliance, meter or service line, do not try to identify the source yourself. Treat it as a possible fuel-gas leak. Énergir [4] specifically advises avoiding flames, sparks, phones and switches near a suspected leak, leaving the building and calling 9-1-1 from a safe location. Once a fuel-gas leak has been excluded, a plumbing, water or wastewater source becomes more likely.

2. Sewage Smell in House: What Can Cause This Odour?

A sewer-like or rotten-egg odour does not always mean that the main sewer line has failed. It can come from a lost water seal, a plumbing pathway, drain buildup, a wastewater-system issue, or hydrogen sulfide in the water supply.

• Dry or missing water seals: Water in a sink, tub, shower, laundry or floor-drain trap normally blocks sewer gases. When a rarely used drain dries out, the odour can enter the room. [5][6]

• Failed plumbing seals, connections or vents: A failed toilet wax ring, loose plumbing connection, damaged vent or malfunctioning air-admittance valve can create a pathway for sewer gases to escape indoors. Plumbing traps and vents are intended to protect the indoor space from those gases. [6][7]

• Drain, sump or utility-area sources: Organic buildup in a sink drain, garbage disposal or overflow opening can create a sour, greasy or sulfur-like odour. A dry, unsealed or improperly covered sump pit or sewage ejector can also allow sewer-like odours into a utility area. [3][6]

• Sewer, septic or water-system problems: A damaged sewer line, wastewater backup, failing septic component or drainfield problem can create persistent odours. Hydrogen sulfide may also be present in well water or develop in a hot-water system, particularly when the odour is noticeable only at a tap. [2][8]

3. Why a Persistent Sewage Smell in the House Matters

A brief odour after a drain has sat unused may have a straightforward explanation. A recurring, localized or spreading sewer-like odour deserves closer attention because it can indicate that a water seal, plumbing connection or wastewater-system component is no longer working as intended.

Odour strength does not measure health risk. The actual concern depends on the substances present, their concentration and the duration of exposure. Hydrogen sulfide is one gas that can be present in sewage- and wastewater-related conditions. [2]

Possible Health Effects

At higher concentrations, hydrogen sulfide exposure may cause: [2]

• Eye, nose or throat irritation

• Coughing or shortness of breath

• Headache

• Nausea

• Dizziness

These effects are not specific to sewer gas and should not be used to diagnose an exposure. An odour alone cannot identify the gas present or determine its concentration. [2]

If sewage has backed up or flooded an indoor area, the concern is not limited to odour. Contaminated water can contain organisms that may cause illness through contact. [9]

The practical priority is to identify and correct the pathway rather than mask the odour with fragrances. Anyone with health concerns they believe may be related to indoor air should speak with a health professional.

If anyone has significant breathing difficulty or loses consciousness, leave the building and call 9-1-1 from a safe location. [2]

An odour is a reason to investigate the source. It is not a measure of exposure or a medical diagnosis.

5. How to Investigate Safely

Start with observations that do not require dismantling plumbing, removing finishes or opening walls. Note the odour pattern before making changes.

Note When and Where the Odour Is Strongest

Record whether the smell is strongest:

• In one room, near one drain or throughout the home

• In a seldom-used bathroom, basement, laundry room or utility area

• After water drains, after a toilet is flushed, or when a fixture is used

• When an exhaust fan, heating or air conditioning system starts

• After rain, snowmelt, high winds or changes in outdoor temperature

Look for Signs of a Hidden Leak

An odour that appears and disappears can also come from a cracked drain pipe, a leaking connection or a plumbing-vent problem. The odour may be more noticeable when water drains or when pressure changes in the plumbing system. [7]

Look for these signs without taking anything apart:

• An odour that returns after using a sink, toilet, shower or washing machine

• An odour localized near a wall, floor or ceiling, with no clear link to an accessible drain

• Dampness, staining, swelling or discolouration near accessible plumbing

• An odour or signs of moisture near a crawlspace access point, without entering the crawlspace

• A persistent odour with no visible source after accessible drains have been checked

A pipe that allows sewer gases to escape can create an odour without visible contamination. If wastewater is leaking into a crawlspace, wall or floor, however, materials may and will be contaminated.

Check Accessible Drains, Fixtures and Utility Areas

Inspect accessible floor drains, sinks, tubs, showers, laundry drains, exposed plumbing and utility areas without taking anything apart.

Look for:

• A drain that has not been used for a long time

• Slow drainage, gurgling, bubbling or wastewater backup

• An odour that is strongest at a sink drain, overflow opening or garbage disposal

• Water, staining or an odour around the base of a toilet

• Dampness, staining or corrosion near exposed plumbing connections

• An open, damaged or poorly sealed sump basin or sewage-ejector cover

Water in plumbing traps (P-trap) normally blocks sewer gases from entering the home. A dry trap can allow those gases to enter a room. [5][6]

Check Whether the Water Supply May Be Involved

If the odour occurs at a tap, note whether it appears with hot water, cold water or both. Also note whether it occurs at one fixture or throughout the home.

Fill a clean glass and move away from the sink. If the odour disappears, the drain may be more relevant to investigate than the water itself. [3]

Hydrogen sulfide can occur in well water and hot-water systems, as well as in sewer- and wastewater-related conditions. [2]

Check Outdoor Areas From Ground Level

From ground level, note whether the odour is stronger near a sewer cleanout, septic area, exterior drain, window, door or outdoor-air intake.

Look for:

• Wet ground or wastewater surfacing near a septic area

• A persistent odour around an exterior drain or sewer cleanout

• An odour that seems to enter through a nearby window, door or outdoor-air intake

• A failing septic system may be associated with slow drainage, wastewater backup, wet or spongy ground, or odours near the tank or drainfield. [8]

These observations do not diagnose the source, but they can give a plumber, septic professional or qualified indoor air quality investigator a more useful starting point.

6. What Not to Do

• Do not use flames, matches, lighters or electrical switches to investigate a suspected gas smell.

• Do not climb onto the roof to inspect a vent, dismantle an HVAC unit, remove a toilet or open walls in an attempt to trace the odour.

• Do not mix household chemicals or pour unapproved products into drains. Health Canada [10] warns that some household-chemical combinations can produce harmful gases.

• Masking the odour with fragrance, air fresheners or strong cleaners can also make the pattern harder to identify.

If sewage has backed up into the home or materials may be contaminated with sewage, avoid contact and do not remove or clean them without appropriate precautions.

Handling sewage-contaminated water or materials can create an infection risk, and cleanup may require professional assistance. [9]

7. When Professional Help Is Appropriate

Choose the professional that fits the pattern:

• Licensed plumber: The odour returns after seldom-used drains have been refilled, is strongest near a toilet, floor drain, cleanout or sink cabinet, or occurs with slow drains, gurgling, bubbling or leaks. [5][7]

• Septic professional: The home uses a septic system and there is wet or spongy ground, unusually slow drainage, wastewater backup, or odours near the tank or drainfield. [8]

• Water-damage restoration company: If there has been a Category 3 water loss, such as sewage backup, contaminated floodwater or other grossly contaminated water, a qualified water-damage restoration company should handle extraction, removal of contaminated materials, cleaning, disinfection and drying. An IICRC-certified firm is preferable.

• HVAC professional: The odour spreads, changes or becomes more noticeable when the heating or air-conditioning system runs, or seems strongest near the furnace, air handler or condensate drain.

• Qualified indoor air quality investigator: Immediate plumbing, gas and sewage-backup hazards have been ruled out, but the odour persists, returns after repairs, affects several rooms or has no clear source.

If sewage has backed up into the home, avoid contact with contaminated materials and seek appropriate cleanup guidance. [9] If the odour may be natural gas, leave the building and call 9-1-1 or your gas utility from a safe location. [4]

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Questions about a persistent sewage or sulfur odour?

If the source remains unclear or appears to be concealed, an indoor air quality assessment can help determine a safe and practical next step.

Contact Puritas

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References

[1] California Air Resources Board. White Paper: Odor Complaints, Health Impacts and Monitoring Methods, Appendix A: Odor Wheels. 2019. https://ww2.arb.ca.gov/sites/default/files/classic/research/apr/past/18rd010a.pdf

[2] Agency for Toxic Substances and Disease Registry. Public Health Statement for Hydrogen Sulfide. U.S. Department of Health and Human Services. https://wwwn.cdc.gov/Tsp/PHS/PHS.aspx?phsid=387&toxid=67

[3] Ville de Montréal. Why Does My Water Smell or Taste Weird? https://montreal.ca/en/articles/why-does-my-water-smell-or-taste-weird-52117

[4] Énergir. Contact Us: Emergency. https://energir.com/en/contact-us-emergency

[5] City of Ottawa. Sewer Odours. https://ottawa.ca/en/living-ottawa/drinking-water-stormwater-and-wastewater/wastewater-and-sewers/wastewater-collection-and-treatment/sewer-odours

[6] National Research Council Canada. Quebec Construction Code, Chapter III: Plumbing, and National Plumbing Code of Canada 2020 (Amended). 2025. https://publications-cnrc.canada.ca/eng/view/ft/?dsl=en&id=4931b15f-9344-43b6-a0f3-446b7b25c410

[7] North Dakota Department of Environmental Quality. Sewer Gas. https://deq.nd.gov/WM/indoor/SewerGas.aspx

[8] U.S. Environmental Protection Agency. Frequent Questions on Septic Systems. https://www.epa.gov/septic/frequent-questions-septic-systems

[9] Health Canada. Flood Cleanup: Keep in Mind Indoor Air Quality. Government of Canada, 2014. https://www.canada.ca/en/health-canada/services/publications/healthy-living/flood-cleanup-keep-mind-indoor-air-quality.html

[10] Health Canada. Use Household Chemicals Safely. Government of Canada, 2024. https://www.canada.ca/en/health-canada/services/home-safety/household-chemical-safety.html

About the author

John Slavich is the founder of Puritas, an indoor air quality investigation service in Greater Montréal. He holds a B.Sc. in Physics, a master’s degree in engineering and several IICRC certifications. His mold investigations draw on the BNQ 3009-600 standard. He holds RBQ general contractor licence 5795-5080-01.