The rotten egg smell in the mechanical room. The headaches in the ground-floor office. The nausea complaints from staff in the basement storage area. These are not random building issues. They are symptoms of sewer gas entering the occupied space, and they indicate a failure in the building's drain system that affects air quality, occupant health, and regulatory compliance.
This article covers what sewer gas actually is, the specific health risks at measurable concentrations, how it enters buildings, and the methods that reliably prevent it.
What sewer gas is made of
Sewer gas is not a single substance. It is a complex mixture of gases produced by the decomposition of organic matter in the wastewater system. The composition varies depending on the waste stream, temperature, and microbial activity, but the primary components are consistent across all sewer systems.
Hydrogen sulfide (H2S)
Hydrogen sulfide is the most dangerous component of sewer gas and the one responsible for the characteristic rotten egg odor. It is produced by anaerobic bacteria that break down sulfur-containing organic matter in wastewater. H2S is heavier than air, meaning it accumulates in low-lying areas such as basements, mechanical rooms, and floor-level spaces.
At low concentrations, hydrogen sulfide is a nuisance. At high concentrations, it is lethal. The critical danger is that H2S paralyzes the olfactory nerve at concentrations above approximately 100 ppm. This means the smell disappears just as the gas becomes most dangerous. Occupants may believe the problem has resolved when in fact the concentration has increased to a life-threatening level.
Methane (CH4)
Methane is produced by the same anaerobic decomposition process. It is colorless, odorless, and lighter than air. Methane itself is not toxic, but it is highly flammable. In enclosed spaces, methane can accumulate to explosive concentrations. The lower explosive limit (LEL) for methane is 5% concentration in air; the upper explosive limit (UEL) is 15%. Within this range, any ignition source (electrical spark, pilot light, static discharge) can trigger an explosion.
Methane also displaces oxygen. In poorly ventilated spaces, methane accumulation can reduce oxygen levels below the threshold for safe breathing (19.5% O2), creating an asphyxiation risk.
Ammonia (NH3)
Ammonia is present in sewer gas from the breakdown of nitrogen-containing waste, including urine and protein-based organic matter. It has a sharp, pungent odor that is distinct from hydrogen sulfide. Ammonia is a respiratory irritant at low concentrations and can cause chemical burns to the eyes, skin, and respiratory tract at higher concentrations. OSHA sets the permissible exposure limit for ammonia at 50 ppm as an 8-hour time-weighted average.
Other components
Sewer gas also contains carbon dioxide (CO2), which displaces oxygen; volatile organic compounds (VOCs) from industrial and household chemicals; and sulfur dioxide (SO2), another respiratory irritant. In healthcare facility wastewater systems, sewer gas may also carry aerosolized bacteria and other pathogens from the drain biofilm.
Health effects by concentration
The health impact of sewer gas exposure depends primarily on the concentration of hydrogen sulfide, which is the most acutely toxic component. OSHA and NIOSH have established specific exposure limits and health effect thresholds.
0.01 to 1.5 ppm: odor threshold
Most people can detect hydrogen sulfide at concentrations as low as 0.01 ppm. At this level, the rotten egg odor is noticeable but there are typically no immediate health effects. This is the range where most building occupants first notice a sewer gas problem. It indicates a breach in the drain system but is not immediately hazardous.
2 to 10 ppm: irritation begins
At 2 to 5 ppm, the odor becomes moderate to strong. Eye irritation begins, particularly in contact lens wearers. Some individuals experience mild headaches. At 5 to 10 ppm, the odor is strong and offensive. Nausea, tearing of the eyes, and moderate headaches are common. This is the range where occupant complaints become frequent and sustained.
10 to 50 ppm: OSHA regulatory range
OSHA's permissible exposure limit (PEL) for hydrogen sulfide is a ceiling of 20 ppm, with an acceptable maximum peak of 50 ppm for a maximum of 10 minutes, no more than once per 8-hour shift. At these concentrations, workers experience severe eye irritation, coughing, respiratory distress, and digestive upset. Prolonged exposure in this range can cause pulmonary edema (fluid in the lungs).
50 to 100 ppm: serious health risk
At 50 ppm, conjunctivitis (eye inflammation) and respiratory tract irritation become pronounced. At concentrations approaching 100 ppm, the olfactory nerve begins to fatigue and the ability to smell the gas diminishes. Workers may experience dizziness, loss of coordination, and impaired judgment. NIOSH classifies 100 ppm as the Immediately Dangerous to Life or Health (IDLH) concentration.
Above 100 ppm: life-threatening
Above 100 ppm, the olfactory nerve is paralyzed and the gas can no longer be smelled. This is the most dangerous phase because the warning sign (odor) disappears. At 200 to 300 ppm, rapid loss of consciousness occurs within minutes. At 500 to 700 ppm, death can occur within minutes from respiratory paralysis. At 1,000 ppm and above, a single breath can cause immediate collapse.
OSHA fact: Hydrogen sulfide is classified as a chemical asphyxiant and is one of the leading causes of workplace death in confined spaces. Between sewer systems, manure pits, and industrial settings, H2S exposure accounts for multiple fatalities every year in the United States. While building interiors rarely reach the highest concentrations, chronic low-level exposure in the 2 to 20 ppm range is far more common than most facility managers realize.
How sewer gas enters buildings
Sewer gas does not appear randomly. It enters through specific, identifiable pathways in the building's plumbing system. Understanding these pathways is the key to prevention.
Dry P-traps (the most common cause)
The P-trap water seal is the primary barrier between the building interior and the sewer system. When the water in a P-trap evaporates, the seal is broken and sewer gas flows freely into the building. A typical P-trap dries out in 2 to 3 weeks without water flow. In hot, dry climates or spaces with strong HVAC airflow, it can happen in under a week.
Floor drains are the most common culprits because they often receive no regular water flow. Mechanical rooms, storage areas, vacant tenant spaces, school buildings during breaks, and hotel rooms during low season all have floor drains that sit unused long enough for the trap to dry out. If you are troubleshooting a specific drain issue such as odor, slow drainage, or gurgling, our guide to common floor drain problems covers root causes and DIY fixes for each.
Cracked or broken drain pipes
Physical damage to drain pipes below or within the building allows sewer gas to escape into wall cavities, floor spaces, and occupied areas. Common causes include ground settling, root intrusion, corrosion in older cast iron systems, and construction damage. Gas from cracked pipes may enter the building in locations far from any visible drain, making the source difficult to identify without a smoke test.
Failed wax rings on toilets
The wax ring that seals a toilet to the floor flange can deteriorate over time, creating a gap that allows sewer gas to leak into the restroom. Symptoms include a persistent sewer smell near the base of the toilet, particularly noticeable when the building is quiet (evenings, weekends) and HVAC airflow is reduced.
Missing or loose cleanout caps
Cleanout access points in the drain system are sealed with threaded caps. If a cap is missing, loose, or improperly seated after a maintenance visit, sewer gas escapes directly into the building. This is a common issue in mechanical rooms and utility areas where cleanouts are accessed for maintenance and not always properly resealed.
Deteriorated pipe joints and connections
In older buildings with cast iron, clay, or early-generation PVC piping, joints can separate or deteriorate over time. Hub-and-spigot joints packed with oakum and lead in cast iron systems are particularly vulnerable to degradation. Each failed joint becomes a gas leak point.
Negative building pressure
HVAC systems that create negative pressure inside the building relative to the sewer system can actively draw sewer gas into the building through any available opening. This is common in large commercial buildings, especially during heating season when exhaust fans, kitchen hoods, and air handling units create sustained negative pressure. The effect amplifies any existing breach in the drain system.
Showing dry traps, cracked pipes, missing cleanouts, and failed joints
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Finding the source
When occupants report sewer gas odor, the first step is identifying which pathway the gas is using. A systematic approach saves time and prevents the common mistake of treating the wrong area.
Step 1: Check all floor drains
Walk every floor of the building and pour water (at least one gallon) into every floor drain, particularly those in low-traffic areas. If the odor stops or reduces in a specific zone after pouring water, the source is a dry P-trap. This is the cause in the majority of cases.
Step 2: Inspect cleanout caps
Check every cleanout access point in the building for missing, loose, or damaged caps. Pay particular attention to cleanouts in mechanical rooms, utility corridors, and areas recently accessed for plumbing service.
Step 3: Smoke test
For persistent or hard-to-locate sewer gas issues, a plumbing smoke test is the definitive diagnostic tool. A plumber introduces non-toxic, artificially generated smoke into the drain system through a roof vent or cleanout. The smoke fills the entire drain system and escapes through any breach: dry traps, cracked pipes, failed joints, missing caps, and deteriorated seals. Observers inside the building watch for smoke emerging into occupied spaces. Every smoke exit point is a sewer gas entry point.
Step 4: Electronic detection
Portable H2S meters can measure hydrogen sulfide concentrations in real time. Walking the building with a calibrated meter identifies concentration gradients that point toward the source. This is particularly useful for gas entering through concealed pathways (inside walls, above ceilings) where the smoke test may not produce visible results in occupied spaces.
Prevention methods
Once the source is identified, prevention falls into two categories: fixing structural issues (cracked pipes, failed joints, missing caps) and maintaining drain seals.
Structural repairs
Cracked pipes, separated joints, and failed connections require physical repair or replacement. There is no maintenance shortcut for structural plumbing defects. These repairs are typically one-time expenses that permanently resolve the gas entry pathway.
Manual trap maintenance
The lowest-tech approach: assign staff to pour water down every floor drain on a regular schedule. This keeps P-traps charged and the water seal intact. The limitation is human consistency. Buildings may have dozens or hundreds of floor drains spread across multiple floors. Staff turnover, schedule disruptions, building closures, and simple oversight mean that drains get missed. One missed drain in one mechanical room is enough to fill an entire floor with sewer gas. Understanding why preventive maintenance outperforms reactive drain repair can help facility teams build more reliable protocols.
Trap primers
Trap primers automate the process of keeping water in P-traps. They work, but they consume water (up to 52,000 gallons per year per drain for continuous flow models), require maintenance, and fail silently when mechanical components break down. When a trap primer fails, the building has no warning until the odor returns.
Waterless trap seals
A waterless trap seal eliminates the evaporation problem entirely by replacing the water-based seal with a mechanical one-way valve. Green Drain drops into the existing floor drain body in 30 seconds, requires no tools or plumbing modification, and creates a physical barrier that blocks sewer gas regardless of whether the P-trap has water in it or not.
Because the seal is a physical silicone valve rather than a pool of water, it cannot evaporate. It works during building closures, summer breaks, overnight, and through extended vacancy periods. There is no water consumption, no mechanical maintenance, and no silent failure mode.
Use the Water Savings Calculator to see the annual water and cost impact of replacing trap primers with waterless seals in your building, or explore Green Drain products for your specific drain sizes.
Building types most at risk
While sewer gas can affect any building, certain facility types face elevated risk due to their drain configurations and usage patterns:
- Commercial office buildings -- Vacant tenant spaces, reduced weekend occupancy, and mechanical rooms with infrequently used floor drains create multiple dry trap opportunities.
- Healthcare facilities -- Sewer gas in patient care areas compounds with the pathogen risk from drain biofilm. Hospitals face both air quality and infection control consequences from failed drain seals.
- Schools and universities -- Extended breaks (summer, winter, spring) leave hundreds of floor drains unattended for weeks, guaranteeing trap dry-out across the facility.
- Hotels and resorts -- Seasonal occupancy fluctuations mean entire wings or floors may sit unused for months, with every bathroom and floor drain trap drying out.
- Industrial and warehouse facilities -- Large floor areas with widely spaced floor drains that receive infrequent water flow. Often combined with poor ventilation that allows gas to concentrate.
Frequently asked questions
What does sewer gas smell like?
Sewer gas has a characteristic rotten egg smell caused by hydrogen sulfide (H2S). At low concentrations (0.01 to 1.5 ppm), the smell is noticeable but tolerable. At higher concentrations (above 2 ppm), the odor becomes strong and offensive. At very high concentrations (above 100 ppm), hydrogen sulfide paralyzes the olfactory nerve, causing you to lose the ability to smell it entirely. This makes high-concentration exposure especially dangerous because the warning sign disappears.
Is sewer gas dangerous?
Yes. Sewer gas contains hydrogen sulfide, which is toxic at concentrations above 10 ppm and can be fatal above 500 ppm. It also contains methane, which is flammable and explosive at concentrations between 5% and 15% in air. Even at the low concentrations common in buildings with dry traps, hydrogen sulfide causes headaches, nausea, eye irritation, and respiratory issues. OSHA sets the permissible exposure limit at 20 ppm ceiling, with a peak of 50 ppm for 10 minutes.
What causes sewer gas smell in buildings?
The most common cause is a dried-out P-trap in a floor drain. P-traps use water to create a seal between the building and the sewer system. When the water evaporates (which takes 2 to 3 weeks in an unused drain), the seal is gone and sewer gas flows freely into the building. Other causes include cracked or broken drain pipes, failed wax rings on toilets, missing cleanout caps, and deteriorated pipe joints.
How do I find a sewer gas leak?
Start by checking every floor drain in the building, especially in low-traffic areas like mechanical rooms, storage closets, and vacant spaces. Pour water down each drain. If the smell stops temporarily, the cause is a dry P-trap. For more systematic detection, use a smoke test: a plumber introduces non-toxic smoke into the drain system and observes where it exits into the building. This reveals dry traps, cracked pipes, and failed seals simultaneously.