Can sewer gas explode?

Every flammable gas has a range. Below the lower explosive limit (LEL) there's too little gas to burn. Above the upper limit (UEL) there's too little oxygen. In between, a spark can set it off.

Those are big numbers. A room would need several percent of the whole air to be methane before it could ignite.

### Why hydrogen sulfide poisons you before it burns

Put hydrogen sulfide's numbers side by side. NIOSH sets 100 ppm as immediately dangerous to life or health. OSHA says 700 to 1,000 ppm can drop a person within one or two breaths. The lower explosive limit is 40,000 ppm. Long before the air could burn, it would kill anyone breathing it. So for the rotten-egg part of sewer gas, the fire risk is real in sewers and tanks, but the poison risk comes first.

OSHA's fact sheet adds two details. Hydrogen sulfide is heavier than air, so it can travel along the ground to a flame or spark and flash back. And when it burns it makes sulfur dioxide, another toxic gas.

### Why methane is the sneaky one

Methane has no smell of its own. NOAA describes it as colorless and odorless, and lighter than air, so it rises and gathers under ceilings and in the tops of closed spaces. Wisconsin adds a second risk: in an enclosed area, high methane pushes out oxygen and can cause suffocation. Wisconsin says oxygen below 12 percent can cause unconsciousness and death quickly and without warning.

Wisconsin lists methane as part of the gas mix that forms as waste decays, which is why it's a hazard inside sewers, septic tanks, sewage ejector pits and lift station wet wells. That's also why OSHA's fact sheet says air testing before entering such spaces should check whether fire or explosion precautions are needed, not just toxic gas.

### Where each gas collects in a building

The two gases split up once they're loose. Hydrogen sulfide is heavier than air, 1.19 times as dense by NIOSH's figure, so it settles toward floors, floor drains, pits and below-grade rooms. Methane is lighter, 0.55 by NOAA's figure, so it drifts up toward ceilings and the tops of closed spaces. Wisconsin's health department adds that sewer gas is most concentrated where it enters, which is often a basement or a low room.

That split explains two things. It's why a sewage ejector room or a pit can hold a dangerous layer near the floor while the air at head height seems fine. And it's why professionals testing a closed space read the air at more than one height before anyone goes in.

### Why ventilation for this gas has special rules

OSHA's guidance on controlling hydrogen sulfide says exhaust and ventilation systems for it should be non-sparking, grounded, corrosion-resistant, explosion-proof and separate from other exhaust systems. The reason given is simple: the gas is flammable and it corrodes materials that aren't protected. A standard household fan in a closed pump room isn't built with that in mind. For a building pit, the plumbing code's answer is a sealed cover and a vent pipe that carries gas out through the roof, not a fan in the room.

A faint sewer smell from a dry trap or a loose toilet seal is gas leaking in and mixing with room air. That smell tells you gas is getting in, and it deserves a fix. It is a very different situation from a sealed pit or a closed tank where gas can pile up for weeks. The places to worry about explosion are:

• Closed pits and tanks such as sewage ejector pits, septic tanks and grease interceptor vaults.
• Manholes and sewer lines, which OSHA names as low spots where gas collects.
• Small closed rooms built around sewage equipment with poor ventilation.

If you manage a building with an ejector room or lift station, read why an ejector or lift station pit smells.

### Three myths worth dropping

• "If I can't smell it, it's gone." Methane has no smell at all, and OSHA says high hydrogen sulfide levels can knock out your sense of smell. No smell is not proof of clean air.
• "Any sewer smell means an explosion risk." Most indoor sewer smells come from a dry trap or a loose seal. Those need fixing, but gas seeping into a room with doors, windows and air handling gets diluted, which is a very different situation from a sealed pit where it can build for weeks.
• "Sewer gas only matters in sewers." It matters anywhere sewage sits in a closed space inside a building, which includes ejector pits, lift station wet wells and septic tanks.

Natural gas is mostly methane too, according to the U.S. Energy Information Administration. Southwest Gas says raw natural gas is odorless, so it adds mercaptan, which smells like sulfur or rotten eggs. That means a rotten-egg smell near a stove, water heater or gas meter may be a gas leak, not sewer gas. Southwest Gas's rule: if you smell rotten eggs, leave the area, then call for help. Call 911 and Southwest Gas at 877-860-6020 from a safe place. Our guide on a house that smells like gas or rotten eggs walks through telling them apart, and a restaurant owner should see gas smell in a restaurant: leak or sewer gas.

Leave first and sort out the cause later. The steps are the same whether it turns out to be sewer gas or natural gas.

1. 1Get everyone out. Leave doors open behind you.
2. 2Make no sparks. Southwest Gas and Wisconsin both say not to flip switches, use matches or lighters, or do anything that could make a spark.
3. 3Call 911 from outside. Wisconsin says to evacuate and call the fire department if you believe sewer gas is concentrated in an enclosed space.
4. 4Stay out until you're cleared. Then have a plumber find how the gas got in.

Once you're cleared to go back in, write down where the smell was strongest, what time it started and whether a pump, drain or appliance had just run. Those details point a plumber to the leak quickly. For the full decision on when to clear a building, see when to evacuate for a sewer smell.

HQ Plumbing offers 24/7 emergency service. Once the fire department has cleared the building, we find and fix the drain, vent or seal that let the gas in.