If you look below your kitchen or bathroom sink, you will notice a U- or S-shaped curved pipe coming down from the drain opening. This is the drain trap. That may seem like a strange name for a plumbing part until you understand the function of this part.
A plumbing drain trap is designed to retain a small amount of water each time the sink drains, and this water standing in the bottom of the curved portion of the trap seals the drain and keeps sewer gasses from escaping the drain and entering your home. There are traps in every drain because any connection that leads to the drain system is also a possible outlet for sewer gas. Even your toilet has an internal trap shape to its porcelain configuration that serves exactly the same function.
Whenever you encounter a strange odor in any room where there is a drain, always make sure that the trap is not dry. If a drain trap is dry, the sewer gas can escape and cause of the smell. This is usually a quick fix that can be remedied by running water down the drain and filling the trap back up with water. Sewer gas is hydrogen sulfide, which is created as organic waste decays. The smell is mostly an annoyance, and although it's possible that a very high concentration could cause health problems, it is not very likely.
Drain Trap Location
The location of the drain trap is often obvious. The standing water in your toilet indicates the presence of a drain trap, for example. If you look to the backside of the toilet, you can see the curved shape of the drain through which the water exits.
Other plumbing fixtures have the drain trap in an out-of-the-way location, such as the kitchen or bathroom sink, where the trap is usually hidden in a cabinet under the sink. When you look at the sink, you cannot see the standing water, but if you follow the drain lines, you can see the required U or S shape where the water is held to block the sewer gas.
Sink traps have the added benefit of trapping small objects dropped into the drain, plus they are fairly easy to remove. Don't worry if you drop a ring or something valuable down the drain--it is likely safe in the trap. Traps also collect hair, sand, and other detritus, and also limit the size of objects that can pass through into the rest of the plumbing. You can take apart most traps for cleaning, or they may have their own cleanout feature.
Large plumbing fixtures such as showers, tubs, and washing machine drains also have drain traps, but they are not as easy to see because they are under floor level or behind walls. Tubs and showers have traps that are harder to get to and either require crawling under the house or cutting a hole behind the tub or shower and digging out the area where to trap is located. Washing machine drain traps are mostly in the wall, and to access them you have to usually cut into the wall, as well.
Drain Trap Maintenance
All drains should be used at least once every couple of weeks to keep water in the traps. This includes showers, toilets, tubs, showers, bathroom sinks, kitchen sinks, washing machine drains and more. It won't hurt anything to let the trap dry out, but you may get some unwanted smells creeping into your house.
When going on vacation or traveling away from the house for long periods, some people stuff drain openings with plastic wrap to keep sewer gases from invading the house while the fixtures are not in use. If you choose to go this route, a toilet can be shut off and emptied and the toilet bowl can be loosely stuffed with a plastic bag. Don't forget floor drains and shower drains; placing a sheet of plastic over the drain and weighing it down will seal the drain if you are gone long enough for the standing water to evaporate out of the trap.