Sewer gas is dangerous in high concentrations, but fatal exposure is rare in homes

Sewer gas can make you very sick and, in extreme cases, cause death — but this almost never happens in a residential setting. The gas is a mixture of methane, hydrogen sulfide, ammonia, and carbon dioxide. Hydrogen sulfide is the main threat: it can knock you unconscious at high levels and stop your breathing. However, your nose detects it at concentrations far below what would kill you, and the smell usually drives you away before you reach dangerous levels.

The real risk in homes is not sudden death but chronic illness from repeated low-level exposure. People living with persistent sewer gas leaks report headaches, nausea, respiratory irritation, and fatigue that worsens over weeks or months. Children and people with asthma or heart conditions face higher risk from ongoing exposure. The good news: sewer gas problems are fixable, and most are caused by straightforward issues like a dried-out trap or a loose vent cap.

Key Takeaways

  • Hydrogen sulfide in sewer gas can cause unconsciousness and death at very high concentrations, but your sense of smell warns you at much lower, non-lethal levels.
  • Fatal sewer gas exposure in homes is extremely rare because the gas disperses quickly and the smell drives people away before concentrations become lethal.
  • Chronic low-level exposure — from a slow leak or blocked vent — causes headaches, nausea, and respiratory problems that build up over weeks or months.
  • Most sewer gas problems come from a dried P-trap, a loose or missing vent cap, or a cracked vent pipe, all of which are repairable.

How hydrogen sulfide damages your body at different levels

Hydrogen sulfide affects you in stages. At 10 parts per million (ppm), you smell it — a rotten-egg odor. At 100 ppm, you feel eye and throat irritation within minutes. At 300 ppm, you lose your sense of smell (called olfactory paralysis), which is dangerous because you no longer detect the hazard. At 500 ppm and above, you can lose consciousness within minutes, and at 1,000 ppm, death can occur within minutes.

The critical detail: your nose detects hydrogen sulfide at 0.3 ppm, which is roughly 3,000 times lower than the concentration that causes unconsciousness. This means you will smell sewer gas long before it reaches lethal levels — and the smell is strong enough that most people leave the area when ready. In a home, the gas also disperses into the air and dilutes rapidly, making it even harder to reach dangerous concentrations in a living space.

The exception is a sealed space: a basement, crawlspace, or attic where sewer gas accumulates without ventilation. Even then, you would need a major leak — a broken sewer line, a backed-up drain, or a severely blocked vent — for concentrations to climb high enough to cause unconsciousness. These situations are obvious: you smell it intensely, and you would evacuate.

Why chronic exposure is the real household concern

Most sewer gas problems in homes are not acute (sudden and severe) but chronic (ongoing and low-level). A dried P-trap under a sink, a loose vent cap on the roof, or a small crack in a vent pipe can leak sewer gas into your home for weeks or months without you realizing it. The concentration stays below the level that causes acute symptoms, but it is high enough to cause problems over time.

People exposed to chronic sewer gas report persistent headaches, nausea, dizziness, fatigue, and coughing. Some develop sinus problems or worsening asthma. Children are more sensitive than adults, and people with existing respiratory or heart conditions are at higher risk. The symptoms often feel like a lingering illness or poor air quality, and people may not connect them to sewer gas until they notice the smell or have the problem fixed.

The danger is that you adapt to the smell. After a few days or weeks of exposure, your nose becomes less sensitive to hydrogen sulfide, and you stop noticing it — even though the gas is still there and still affecting your health. This is why people sometimes do not realize they have a sewer gas problem until a visitor mentions the smell or symptoms suddenly worsen.

Common sources of sewer gas in homes

A dried P-trap is the most common cause. The P-shaped bend under your sink, toilet, or shower holds water that blocks sewer gas from rising up the drain. If you do not use a drain for weeks, the water evaporates, and gas flows freely into your home. The fix is straightforward: run water down the drain for 30 seconds to refill the trap.

A loose or missing vent cap on your roof is the second most common source. The vent cap keeps rain and animals out while letting sewer gas escape safely to the atmosphere. If the cap is loose, corroded, or missing, gas leaks down the vent pipe and into your home. A roofer or plumber can replace it in under an hour.

A cracked or separated vent pipe, a blocked vent (from leaves, bird nests, or ice), or a backed-up sewer line can also cause sewer gas to enter your home. These require a plumber to diagnose and repair. If you smell sewer gas consistently, do not assume it will go away — call a plumber to locate the source.

When to call a plumber versus when to investigate yourself

If you smell sewer gas only occasionally and it comes from a specific drain, start by running water down that drain and checking the P-trap. If the smell is under a sink, look at the pipes below — they should be wet, not dry. If they are dry, the trap has lost its seal. If the smell is in a bathroom you rarely use, run water in the sink, shower, and toilet to refill all the traps.

If the smell persists after refilling the traps, or if it comes from multiple drains or from the walls and ceiling, call a plumber. Do not ignore it. A plumber can use a smoke test (blowing harmless smoke through the drain system to find leaks) or a camera inspection to locate the problem. The cost of an inspection is far less than the cost of damage from a cracked sewer line or the health effects of months of exposure.

If you smell sewer gas in a basement, crawlspace, or attic, or if the smell is very strong anywhere in your home, ventilate when ready by opening windows and doors. Then call a plumber. Do not spend time investigating — these situations suggest a serious leak that needs professional attention.

Protecting yourself and your family from sewer gas

The best defense is prevention. Keep all drains in use. If you have a guest bathroom, basement bathroom, or rarely-used sink, run water down the drains once a week to keep the P-traps full. If you are away for more than a few weeks, ask a neighbor to run water in your drains, or pour a cup of water down each drain before you leave.

Check your roof vent caps once a year, especially after storms or heavy winds. Look for loose, corroded, or missing caps. If you see damage, call a roofer or plumber to replace it. This is a cheap fix that prevents months of sewer gas problems.

If anyone in your home has asthma, heart disease, or a weakened immune system, take sewer gas smells seriously. These people are more vulnerable to the effects of chronic exposure. Do not wait to see if the smell goes away — have it investigated promptly.

Frequently Asked Questions

Can sewer gas kill you while you sleep?

No. For sewer gas to cause unconsciousness or death, the concentration would have to be extremely high — high enough that you would smell it intensely and wake up. In a home, the gas disperses too quickly to reach lethal levels in a bedroom. A major sewer line break or backed-up drain might create dangerous conditions in a basement or crawlspace, but not in living areas.

What should I do if I smell sewer gas but do not see any obvious problem?

Start by running water down all drains to refill the P-traps. Check the vent cap on your roof for damage. If the smell persists after a few days, call a plumber. Do not assume it is harmless — chronic exposure causes real health problems, and the source is usually fixable.

Is sewer gas toxic to pets?

Yes, pets are affected similarly to humans. Hydrogen sulfide can irritate their eyes, nose, and lungs. If you notice your pet coughing, sneezing, or avoiding certain areas of your home, and you smell sewer gas, have the problem investigated. Pets cannot tell you they feel sick, so take the smell seriously.

Can sewer gas cause permanent damage to your lungs?

Chronic low-level exposure can worsen asthma and cause lasting respiratory irritation, but it does not typically cause permanent scarring or irreversible lung damage in otherwise healthy people. However, people with existing lung disease are at higher risk for lasting effects. The longer the exposure, the greater the risk — another reason to fix the problem quickly.

Do I need to leave my home if I smell sewer gas?

Not unless the smell is very strong or you feel dizzy, nauseous, or short of breath. Open windows and doors to ventilate, then call a plumber. If the smell is overwhelming or you feel ill, leave and call from outside. Do not re-enter until a professional has located and fixed the problem.