A bathroom sink that smells fine when it sits idle but stinks the moment you turn on the tap is one of the most confusing plumbing issues homeowners deal with. It feels like the problem is coming out of nowhere, yet there is always a clear cause behind it.

This guide breaks down every common reason this happens, how to diagnose it properly, and the safest way to fix it before it becomes a bigger problem.

Common Reasons Your Sink Smells When the Tap Runs

1. Bacteria inside the tap aerator

Most taps have a small aerator at the end that mixes air with water. Over time, the aerator traps moisture and minerals which creates the perfect environment for bacteria.

You will notice:

A rotten or mouldy smell only when the water runs
Small black or brown residue on the aerator
The smell disappears once the tap stops

This is the number one cause in homes with older taps or areas with mineral heavy water.

2. Biofilm buildup inside the drain

Bathroom drains collect soap residue, toothpaste, shaving foam and hair. These all form a sticky bacterial layer called biofilm that clings to the inside of the pipes.

When water flows over it, the biofilm releases a foul smell that travels straight back up the sink.

Signs include:

Smell gets worse the longer the water runs
Slow drainage
A slimy ring inside the drain

For information about how drains behave when they start to clog, you can also read the guide here:
Blocked Drains

3. A dry or weak P trap

The P trap is the curved pipe under the sink that holds water to block sewer gas. If it dries out or does not hold enough water, gas can sneak through when running water pushes air through the system.

This happens in:

Guest bathrooms
Rarely used sinks
Homes that have been empty for a while

A simple refill of the trap can fix it, but only if the issue is caused by evaporation.

4. Contaminated hot water line

If the smell happens only when using the hot tap, the problem is often inside the hot water system. Sediment, rust, damaged anode rods or bacterial growth can create a sulphur or rotten egg smell.

To understand other hot water faults, see the guide below:
Hot Water Repairs

5. Vent pipe problems

Your plumbing system has a vent that lets air escape so wastewater can flow freely. If the vent is blocked with debris or damaged, sewer gas can be pushed back up the drain whenever you run the tap.

Common signs:

Smell appears only during water flow
Gurgling noises in other fixtures
Smell coming from multiple drains

Vent issues always require a licensed plumber.

How to Test Where the Smell Is Coming From

Use this step by step method to pinpoint the source.

Step 1. Do the glass test

Fill a clean glass with tap water and take it to another room.
If the smell is gone, the issue is not the water supply but the drain or tap hardware.

Step 2. Test hot and cold separately

If the hot water smells but cold does not, the hot water system is the cause.

Step 3. Remove and clean the aerator

Unscrew the aerator and clean it with vinegar and a toothbrush.
If the smell disappears after reinstalling it, you have found the culprit.

Step 4. Inspect the P trap

Check for:

Low water level
Leaks
Loose connections

If topping up the trap with water removes the smell temporarily, this confirms the issue.

Step 5. Check for drainage issues

Run water for thirty seconds.
If the smell gets stronger over time, the problem is further down the drain line.

Solutions That Work

Clean or replace the aerator

Aerators cost very little and take minutes to replace. Cleaning works for mild buildup, but heavily blocked aerators should be replaced.

Flush the drain properly

Use boiling water followed by a natural enzyme based cleaner or a baking soda and vinegar flush.
Avoid harsh chemicals because they can damage older pipes.

Clean out biofilm

Use an oxygen based cleaner that breaks down organic buildup inside the pipe walls.

Service the hot water system

A plumber can:

Flush the tank
Replace the anode rod
Clear sediment
Test for bacterial growth

If your hot water system is old or producing rusty water, servicing is essential.

Check for deeper plumbing issues

If the smell keeps returning, you may have:

A cracked pipe
A blocked vent
A partial blockage deeper in the system

For complex faults, this guide may help:
Plumbing Repairs

Is a Smelly Bathroom Sink Dangerous?

Sometimes, yes.

Sewer gas can contain:

Hydrogen sulphide
Methane
Bacteria

Exposure can cause migraines, dizziness or nausea. If the smell is strong or affects multiple rooms, stop using the sink until it is inspected.

If you ever suspect a gas smell, read this essential guide immediately:
Water and Gas Leaks

When to Call a Plumber

Call a professional if:

The smell comes from more than one sink
The drain gurgles when the tap runs
There is water pooling under the vanity
You notice slow drainage across the home
The smell gets stronger over days instead of disappearing

If you want a licensed plumber to diagnose it properly, you can reach out here:
Contact Plumb Well Solutions

Final Thoughts

A bathroom sink that smells only when the tap runs is usually caused by bacteria, a blocked aerator, biofilm, a dry P trap or hot water contamination. These issues are often simple to fix, but ignoring them can lead to bigger plumbing problems. A clear diagnosis is the key to getting rid of the smell for good.