The first thing is to make sure the smell is sewer gas—usually a rotten egg smell—and not a natural gas smell—usually smells like a skunk.
If you think it’s the natural gas smell, call the natural gas company immediately and if it’s anything but a very faint small—evacuate your home, office, or whenever it is—and wait for the gas company to give you an all clear—because a natural gas leak can lead to a violent explosion.
Getting back to sewer gas. Actually, sewer gas is mostly methane which is odorless but it’s almost always mixed with other gases, the most common of which is hydrogen sulfide (for you chemists—H2S) which is the rotten egg smell. The hydrogen sulfide comes from decomposing organic matter—either animal or vegetable.
Natural gas, for that matter, is odorless too! The skunk odor is actually a chemical—called a mercaptan that is added to warn us of the danger.
A sewer gas odor can come from a household septic system, or the sanitary sewer system. Many suburban and rural households have septic systems but most urban and commercial/business buildings are tied into a sanitary sewer system. The most common source of sewer gas odors whether residential or commercial is a “dry trap!” A dry trap occurs when a sink, floor drain, or toilet is not used for sometime and the water trap in the drain line dries out and the sewer gas can then back up into the room. Another common source of sewer gas is a broken drain line that allow sewer gas into a crawl space, basement or mechanical area, that then seeps into the office, factory, warehouse, apartment, etc. Plugged or backed up drains can also push sewer odors back into the air.
Besides the offensive odor, sewer gas can be hazardous especially over any extended period of time. Methane gas can displace the oxygen especially in a confined space and be deadly. Hydrogen sulfide is also toxic. Fortunately for us we can generally smell Hydrogen Sulfide (H2S) at very low levels and far below what could be toxic. I say generally because as the H2S gets stronger, we lose our ability to smell it. Scientifically, it’s called olfactory fatigue!
If you smell sewer gas—rotten eggs—check first for dry traps—just add water to fill the trap. This will block the path for the gas to escape. If that doesn’t work, it could be a broken line, a vent that is plugged or not vented to the outdoors, or a dead animal somewhere—or even worse some other source of hydrogen sulfide than a sewer system. It’s time to get help from someone who can inspect for possible sources—a plumber or contractor.
Another option is to contact an environmental consultant, such as Atlantic Environmental, Inc., who has the equipment to test for methane, hydrogen sulfide or other gases, and identify the source and the concentration, and give you specific guidance on correcting the problem.