A bathroom sink that smells like sewer gas rather than just a general bad odor is a specific sign, and it usually points to one of two things: a dry P-trap letting gas rise up through the drain, or build-up further down the pipe that's affecting the whole line. Telling the two apart is straightforward once you know what to check.
Why It Smells Specifically Like Sewage
Every drain has a curved section of pipe underneath called a P-trap, which holds a small amount of water to form an airtight seal that blocks sewer gas from rising back up into the room. When that seal fails — or when a deeper blockage lets gas build up and escape past it — the result is that distinct sewage smell rather than a milder, mustier odor.
Cause 1: A Dry P-Trap
If the sink isn't used often — a guest bathroom, a rarely-used basin — the water in the P-trap can evaporate over time, breaking the seal. This is the most common cause of a sudden sewer smell in a sink that otherwise drains fine.
The fix: Run the tap for about a minute to refill the trap. If the smell disappears immediately, this was the cause.
Cause 2: Build-Up in the Line
Hair, soap scum, and toothpaste residue that build up over time can start to smell, and if the build-up is significant enough, it can affect how well the P-trap seals, letting more sewer gas through than usual.
The fix: Treat the drain with a deodorizing drain cleaning powder, which clears the build-up and neutralizes the odor at the same time, rather than just refilling the trap temporarily.
Cause 3: A Blocked Vent Stack (Less Common)
Vent stacks let sewer gases escape safely through the roof rather than back through your drains. If a vent is blocked, gases can be forced back down into the building. This is less common and usually affects multiple drains at once rather than a single sink.
How to Tell Which One You Have
Refill the trap by running water for a minute. If the smell is gone, it was a dry trap. If it comes right back, or if other drains in the house smell too, it's more likely a build-up or venting issue that needs treatment or a professional look.
Frequently Asked Questions
Is a sewer smell from a bathroom sink dangerous?
It's unpleasant and worth fixing, but an occasional dry P-trap isn't a health emergency. A persistent smell that doesn't resolve is worth addressing properly rather than living with.
Why does this happen more in guest bathrooms?
Sinks that aren't used daily are the most likely to have their P-trap dry out, since there's no regular water flow to keep the seal topped up.
Will running water permanently fix the smell?
If the cause is a dry trap, yes, until it dries out again from disuse. If build-up is the real cause, running water only helps temporarily — a proper treatment clears it at the source.
Related reading: What Causes Bad Drain Odours? and Why Does My Sink Drain Smell Bad? (And How to Fix It).