What is the cause of a bad foul smell from the bathroom sink?

Answer 1

Could be mold in the space inside that sink intended to prevent overflow from overfilling. The inside of the sink is actually hollow so that any water that flows into the little overflow hole runs down into the drain. If you look down into the drain you should see spaces intended to let that water into the drain. Any black color there could be mold and would need to be bleached.

Answer 2

The drop to the trap needs to be the right size so that there is enough water pressure to clear the trap of debris.

Another issue may be that the trap aka u-bend underneath the sink has no water to keep the smell out of the room by blocking the smell. If the water evaporates by any means the smell would come in. There is a reason this happens. The last plumber back pitched the drain the wrongway or not enough, so instead of water staying in the trap, it drains down, or if the room is not used alot the water evaps and the smell comes in. Try running hot water to the trap to see if the smell stops, and air out the room. There is one more thing , usually in the washroom aka laundry room has a floor drain. That drain has a trap. You need to put in water every now and then to keep it from evaporating and causing sewer smell to come in.

Answer If the trap is OK, another source could be the overflow, which, depending on how it is built, can collect material which collects bacteria, which then smells. Disassembling this should make it clear (it's filled with gunk that stinks).

Answer 3

The foul odour you describe is sulfur dioxide that is given off by sewer gas. If a fixture drain is not used for some time the P-trap may dry out. Some P-traps have a drain fitting at the bottom which may leak enough to completely drain the trap between uses. To keep sewer gas from entering the dwelling, the P-trap must remain full of water to the level of the horzontal run (where the main drain pipe attaches). Look for signs of leakage if the area under the shower is accessable and run the shower for a few seconds every week if it is not regularly used.

Answer 4

A blocked vent. The sink and tub have a small vent attached and comes out of the roof of the house. Squirrels and some bird species often use this and get stuck in the vent. You can also get odor from standing water in the pipe, try pouring a 1/2 cup of baking soda into the sink and follow that with 1 cup of white vinegar. This will bubble up and freshen the pipes. Rinse after about 10 minutes with fresh water.

Answer 5

poor venting and bacteria and trap seal loss ALL very dangerous