Short exposure to sewer gas is usually harmless, but prolonged exposure causes serious health effects
A few minutes of sewer gas exposure — from an open manhole, a backed-up drain, or a broken pipe — will not harm a healthy adult. Your nose detects it long before the concentration reaches dangerous levels, and you will naturally move away. The real risk comes from sustained exposure over hours or days, which can cause respiratory damage, chemical burns to your lungs, and in extreme cases, loss of consciousness or death.
The danger depends on three things: how concentrated the gas is, how long you breathe it, and your own health. A plumber working in a basement for two hours with poor ventilation faces a different risk than someone living above a cracked sewer line for a month. Sewer gas contains hydrogen sulfide, methane, ammonia, and carbon dioxide — each with its own threshold for harm.
Key Takeaways
- Exposure under 15 minutes in normal conditions causes no lasting harm, but you should leave the area as soon as you smell sewer gas.
- Hydrogen sulfide, the main toxic component, can cause respiratory damage after 30 minutes of exposure at moderate concentrations.
- Exposure lasting hours or days — from a backed-up drain or broken pipe in your home — can cause persistent cough, throat irritation, and chemical pneumonia.
- People with asthma, COPD, or heart disease face serious risk from even brief exposure and should leave when ready and seek fresh air.
- Unconsciousness and death can occur within minutes at very high concentrations, typically only in confined spaces like sewers or septic tanks.
What happens in the first 15 minutes
Your sense of smell is your first line of defense. Hydrogen sulfide — the rotten-egg smell — becomes noticeable at concentrations as low as 0.0047 parts per million (ppm). At this level, you feel no physical effect; you straightforward know something is wrong and leave. This is why sewer gas smells so bad: the odor itself is a warning system.
If you stay in the area for 5 to 15 minutes at low to moderate concentrations, you may feel mild irritation in your eyes, nose, or throat. Your eyes may water. You might cough. These symptoms disappear within an hour of breathing fresh air and cause no permanent damage. This is the typical exposure for a homeowner discovering a backed-up drain or a plumber opening a cleanout.
The problem is that your sense of smell can fail you. At very high concentrations — above 100 ppm — hydrogen sulfide actually deadens your olfactory nerve, so you stop smelling it even though the gas is still there. This is called olfactory fatigue, and it is why people in confined spaces like septic tanks or sewers can lose consciousness without warning.
Exposure lasting 30 minutes to a few hours
Once you pass the 30-minute mark at moderate concentrations, your lungs begin to absorb the gas more deeply. Hydrogen sulfide at 10 to 15 ppm causes noticeable irritation to the respiratory tract — coughing, chest tightness, and difficulty breathing. Your throat feels raw. If you continue breathing it, the irritation worsens and can develop into chemical bronchitis, a temporary but painful inflammation of the airways.
Exposure for one to three hours at these concentrations can cause symptoms that last for days: a persistent dry cough, hoarseness, and a sensation of burning in your chest when you breathe deeply. You may feel fatigued and short of breath even after leaving the area. These symptoms usually resolve within a week without medical treatment, but they indicate real damage to your respiratory lining.
People with asthma or chronic obstructive pulmonary disease (COPD) can develop severe bronchospasm — a sudden tightening of the airways — within minutes of exposure at concentrations that a healthy person might tolerate for an hour. If you have a respiratory condition and smell sewer gas in your home, leave when ready and do not return until a professional has found and fixed the source.
Exposure lasting a full day or longer
Living or working in a space with continuous low-level sewer gas exposure — from a cracked sewer line under your basement, a malfunctioning septic system, or a backed-up drain you have not yet repaired — causes cumulative damage. After 8 to 12 hours of exposure at concentrations of 5 to 10 ppm, you develop a persistent cough, nasal congestion, and throat irritation that does not fully resolve even when you leave the space.
After 24 hours or more, you may develop chemical pneumonia, a condition in which the gas irritates your lung tissue so severely that it swells and fills with fluid. Symptoms include a deep cough, chest pain, shortness of breath, and sometimes fever. Chemical pneumonia requires medical attention and can take weeks to resolve. It also increases your risk of a secondary bacterial infection.
Long-term exposure also affects your nervous system. Hydrogen sulfide can cross the blood-brain barrier and damage nerve cells. People exposed for days or weeks report headaches, dizziness, memory problems, and mood changes. These symptoms may persist even after the exposure ends, and in severe cases, they can become permanent.
Dangerous concentrations and when ready health threats
The risk jumps sharply at higher concentrations. At 100 ppm, hydrogen sulfide causes when ready eye and respiratory irritation, and exposure for more than a few minutes can trigger severe bronchospasm or pulmonary edema — fluid in the lungs. At 300 ppm, loss of consciousness can occur within 15 minutes. At 500 ppm or higher, death can occur within minutes, even with a single breath in some cases.
These extreme concentrations are rare in homes but common in confined spaces: the inside of a septic tank, a deep sewer line, a manhole, or a basement that has been sealed for months with a major backup. This is why you should never enter a confined space with sewer gas exposure without professional rescue equipment and a trained team standing by. Even a few seconds of breathing air at 500+ ppm can be fatal.
Methane, another component of sewer gas, is not toxic at the concentrations found in homes, but it is explosive. If methane accumulates in a confined space and meets an ignition source, it can explode. This is a secondary hazard that adds to the danger of working in sewers or septic tanks.
Who is at highest risk
Healthy adults tolerate brief sewer gas exposure better than vulnerable populations. Children, elderly people, and anyone with asthma, COPD, emphysema, heart disease, or a weakened immune system should leave the area when ready if they smell sewer gas. For these groups, even 10 to 15 minutes of exposure can trigger serious symptoms.
Pregnant women should also avoid sewer gas exposure. While the gas itself does not directly harm a fetus, the respiratory distress it causes can reduce oxygen flow to the placenta. If you are pregnant and discover sewer gas in your home, call a plumber and stay elsewhere until the problem is fixed.
Workers in sewers, septic systems, or wastewater treatment plants face cumulative exposure over their careers. Even with ventilation and safety equipment, they are at higher risk for chronic respiratory disease, and they require regular health monitoring and proper protective equipment on every job.
What to do if you have been exposed
If you smell sewer gas in your home, leave the area and go outside when ready. Open windows and doors to ventilate the space. Do not try to find the source yourself — call a licensed plumber. If you have symptoms like coughing, chest pain, or difficulty breathing that do not resolve within an hour of leaving the area, seek medical attention and tell the doctor you were exposed to sewer gas.
If you work in an industry with sewer gas exposure, wear a respirator rated for hydrogen sulfide and methane, and never work alone in a confined space. Your employer is required to provide training, equipment, and atmospheric monitoring. If you develop respiratory symptoms that persist after work, report them to your occupational health provider.
If someone loses consciousness in a space with sewer gas, do not enter to rescue them. Call 911 when ready and tell the dispatcher the location and the suspected hazard. Professional rescue teams have the equipment and training to recover someone safely.
Frequently Asked Questions
Can sewer gas cause permanent lung damage?
Yes, but only after prolonged exposure. A single incident of a few hours or less typically causes temporary irritation that resolves within days. Exposure lasting days or weeks, or repeated exposures over months, can cause permanent scarring of the airways and chronic cough. Chemical pneumonia from sewer gas can also leave lasting inflammation.
Is it safe to sleep in a room that smells like sewer gas?
No. If your bedroom smells like sewer gas, there is a source — a cracked pipe, a backed-up drain, or a septic system problem — and you are being exposed while you sleep. Call a plumber when ready. Sleeping in that room for even one night can cause respiratory irritation; sleeping there for weeks can cause chemical pneumonia.
How quickly does olfactory fatigue happen?
Olfactory fatigue to hydrogen sulfide typically occurs within 15 to 30 minutes at very high concentrations (above 100 ppm). At lower concentrations, your sense of smell remains reliable. This is why people in confined spaces are at such high risk — they stop smelling the gas and do not realize they are in danger.
What is the difference between sewer gas exposure and carbon monoxide exposure?
Sewer gas causes when ready irritation to your eyes, nose, and throat, so you know something is wrong. Carbon monoxide is odorless and causes no irritation, so you can be exposed for hours without noticing. Both are dangerous, but sewer gas gives you a warning signal that carbon monoxide does not.
Can sewer gas exposure cause death in a home?
Death from sewer gas in a home is extremely rare because the concentrations are usually too low. Death occurs in confined spaces like septic tanks or sewers where gas accumulates to very high levels. In a home, the main risk is chemical pneumonia and respiratory damage from prolonged exposure, not sudden death.