To figure out what might be causing that awful smell, it helps to have a basic understanding of how the plumbing in your bathroom works. (Don't worry -- this will be quick!)
At some point, you've probably looked under the bathroom sink and noticed the U-shaped pipe that runs from your sink drain (B) to a larger wastewater pipe in the wall. This pipe is called the P-trap. One end of the P-trap runs down to your sewer or septic system (E); the other end leads all the way up through the roof (D), letting fresh air in while allowing any smelly sewer gases to vent out to the sky. And although you can't see it, the same basic drain setup is at work behind or beneath your tub and shower (C).
The U-shape of the P-trap allows it to collect a small amount of water after each sink use, which acts as a barrier against sewage odors. When everything's working as it should, the water stays in the P-trap after you turn off the sink, empty the tub or flush the toilet. That little bit of water is enough to prevent gases from drifting out of the sewage system and into your house. Under normal conditions, those gases flow right past your bathroom and out through the vent pipe in your roof [source: Wm. Henderson].
"The role of the P-Trap is to keep the sewer gases out of the home," says Ray Patrick, a master plumber with Roto-Rooter. "There are misconceptions that the traps are also there to catch objects that might be dropped in the drain, which is not the case. Traps are there to prevent sewer gases from escaping into the home."
Failure to properly vent those stinky gases has the potential to be more than just annoying. "It could be dangerous because of other gases that can be present in sewer systems. Methane is the largest cause of sewer smell but there is also danger of hydrogen sulfide, carbon dioxide, ammonia, nitrogen oxides and sulfur dioxide," explains Patrick in an email interview, adding that the main culprit of such added toxins are people disposing of gasoline and other chemicals down their drains. Signs of exposure can include dizziness, headaches, nausea and drowsiness.
Clearly, it's important to figure out exactly what's causing the sewage smell and make sure it's corrected quickly and effectively. If you're lucky, it will be one of the easy fixes on the next page.