Quick Answer:
A mildew-like smell near a sink usually comes from damp organic buildup around the drain, inside the stopper, or in the P-trap. Start by cleaning the visible areas around the drain and the stopper, then check the trap and seals. If the smell keeps returning after cleaning, there may be a leak or trapped organic matter deeper in the plumbing.
Why This Happens
- Water and food particles collect in small gaps and on rough surfaces, creating a place for mildew and bacterial film to grow.
- Stoppers, drain flanges and the inside of the tailpiece can stay damp and harbor growth.
- P-traps can hold organic sludge if they are slow to drain or if solid debris accumulates.
- If the smell shows up mainly when you use hot water, see Odor only when hot water used for a related explanation about heat releasing trapped odors.
Step-by-Step What to Do
1. Look and smell to narrow the source
Remove any dishes or items from the sink, run the water briefly, and sniff around the drain, the stopper, and the cabinet under the sink to locate where the scent is strongest.
2. Clean around the drain flange where mildew can hide.
Remove the stopper or strainer if possible. Use an old toothbrush or small scrub brush with warm soapy water to scrub the drain flange and the immediate area. For stubborn film, sprinkle baking soda, add a small amount of white vinegar, let it fizz for a few minutes, then scrub and rinse.
3. Clean or remove the stopper
Many stoppers collect hair and gunk. Remove the stopper according to its design, wipe it clean, and scrub with a brush. Rinse thoroughly before reinstalling.
4. Check and clean the P-trap
Place a bucket under the P-trap, loosen the slip nuts, and remove the trap. Empty any trapped water and sludge, then scrub the inside with a bottle brush. Reassemble carefully and check for leaks.
5. Flush and dry
Run hot water for a minute to flush loosened debris, then dry the area with a cloth. Leave cabinet doors open for a while to help drying and reduce moisture.
6. Improve drainage and ventilation
Make sure the sink drains freely and that the area under the sink stays dry. Use a fan or keep the room ventilated to reduce humidity.
7. Monitor and repeat if needed
Check the area after 24–48 hours. If the smell fades, cleaning worked. If it returns, clean again or proceed to the next section for when to call a pro.
What Not to Do
- Do not spray air freshener instead of addressing the source.
- Do not mix cleaning chemicals (for example, bleach and ammonia) — mixing can create toxic gases.
- Do not force or overtighten drain fittings; cross-threading can cause leaks.
When to Call a Professional
- If the smell persists after thorough cleaning and drying.
- If you find active leaks, loose seals, or damaged drain parts that you cannot safely repair.
- If you notice the smell is sewage-like rather than mildew, or if Odor returns every few weeks despite cleaning — this can indicate a deeper plumbing issue or venting problem.
Safety Notes
- Wear gloves and eye protection when handling cleaners or removing gunk.
- Work in a well-ventilated area when using chemical cleaners. Prefer mild cleaners like soap, baking soda, and vinegar first.
- Have a bucket and towels ready for water when you open the P-trap. Shut off under-sink water valves only if you are confident doing so.
Common Homeowner Questions
- Why can I still smell mildew after cleaning? Answer: Some areas may still hold moisture or trapped organic matter. Re-clean the flange, stopper, and P-trap and allow the area to fully dry.
- Will running hot water eliminate the smell? Answer: Hot water can help flush loosened residue, but it won’t remove biofilm stuck to surfaces — you need to scrub and clean those parts.
- Can I use bleach in the drain? Answer: A diluted bleach solution can disinfect surfaces, but use it sparingly, rinse well, and never mix it with other cleaners; consider gentler options first.