How to Freshen a Smelly Sink Drain (and Why the Baking Soda Trick Is Mostly for Show)

Why the baking soda and vinegar trick is mostly for show

Baking soda is a mild base and vinegar is a mild acid. When you pour one onto the other, they react with each other, mostly turning into water, a little salt (sodium acetate) and carbon dioxide gas. That gas is the fizz. By the time the bubbling stops, much of what you poured in has neutralized itself, and what is left is mostly salty water.

The fizz is not strong enough to scrub the slimy film, called biofilm, that lines a drain and holds the smell. It can loosen a little loose debris near the top, and the hot water people pour afterwards does some rinsing. But on its own it is more satisfying to watch than it is effective. Baking soda alone is a mild deodorizer, and vinegar alone is a mild acid. Mixed together in a drain, each mostly cancels the other.

And why boiling water is a bad idea

Pouring a kettle of boiling water down the sink is another popular tip, and it carries real risk. Many homes and offices have PVC plastic drain pipes, and very hot water can soften or warp PVC and loosen the joints over time. Boiling water poured straight into a porcelain or ceramic sink can crack it through sudden temperature change, especially if the sink is cold. Hot water from the tap is hot enough to help, without those risks.

Where the smell actually comes from

• The stopper. Hair, soap, toothpaste and food wrap around the pop-up stopper and rot. This is the most common source in bathroom sinks.
• The overflow hole. The small opening near the top of many bathroom sinks collects grime and is never rinsed.
• Biofilm on the pipe walls. A slimy layer of soap, grease and bacteria builds up inside the drain.
• The P-trap. The U-shaped bend under the sink holds water to block sewer gas, and also collects gunk.
• A dry trap. If a sink is rarely used, the water in the trap evaporates and sewer gas can come up through it.
• A garbage disposal. If your sink has one, the smell often comes from under its rubber splash guard. Our guide to getting rid of garbage disposal smell covers it.

What actually works, step by step

1. Pull out and clean the stopper. Most bathroom pop-up stoppers lift out, or come out after you loosen a small nut on the rod behind the drain under the sink. Pull off the hair and gunk, then scrub the stopper with warm, soapy water and an old toothbrush.
2. Scrub inside the drain opening. Use a bottle brush or a narrow drain brush to scrub the first part of the pipe you can reach. This is where you physically remove biofilm, which the fizz cannot do.
3. Clean the overflow. Push a small bottle brush into the overflow hole and scrub, then flush it with warm, soapy water.
4. Clean the P-trap if the smell remains. Put a bucket under the trap, unscrew the two slip nuts by hand (or with a cloth around a wrench), and let the water drain into the bucket. Rinse the trap out over another sink or outside, scrub it with a bottle brush, then reattach it hand-tight. Run water and check for leaks.
5. Use an enzyme drain cleaner. These contain bacteria and enzymes that digest the organic film over hours rather than seconds. Use it as the label says, usually at night when the sink will not be used, and repeat as directed. They are slow, gentle and safe for most pipes.
6. Flush with hot tap water. Run the hottest water from your tap for a minute or two after cleaning, and then regularly afterwards.

Common mistakes

• Relying on the fizz. It looks like it is working, so people stop there. The biofilm is still in the pipe.
• Pouring boiling water down PVC pipes or into a cold porcelain sink. Hot tap water is safer.
• Covering the smell with fragrance. Air freshener and scented drain sticks hide the warning sign without removing the cause.
• Pouring bleach down the drain. It does not remove biofilm well, it is dangerous to combine with other products, and it can harm septic systems.
• Pouring grease or coffee grounds down the sink. Both build up in the trap and pipe and are among the most common causes of smells and slow drains.
• Overtightening the trap. Plastic slip nuts crack. Hand-tight is usually enough.

What professionals use, and why

Cleaning professionals and plumbers treat drain smells as a mechanical problem first: they remove the stopper, brush the drain and open the trap, because physically removing the biofilm is what works. For ongoing maintenance, many facilities use enzyme or bacterial drain treatments on a regular schedule, because they keep digesting the film without the hazards of caustic chemicals. For blockages deep in the line, plumbers use a drain snake or auger rather than pouring in more product.

The office kitchenette

Shared sinks get coffee grounds, tea leaves, milk, soup and the odd noodle, which is a recipe for a smelly drain.

• Put a mesh strainer in the drain and empty it into the bin daily.
• Post a polite note: "No coffee grounds or grease in the sink, please."
• Run hot tap water for a minute at the end of the day.
• Add an enzyme treatment to the weekly routine, on a Friday evening so it works over the weekend.
• Check rarely used sinks and floor drains. A washroom sink or floor drain that nobody uses can dry out and let sewer gas in. Pour a jug of water into it every week or two to refill the trap.

If the simple method does not work

A smell that comes back within a day or two after cleaning the stopper and trap may be deeper in the pipe. Repeat the enzyme treatment for a week, as the label allows.

A rotten-egg or sewage smell, especially from several drains, can point to a dry trap, a failed seal, a blocked vent or a sewer issue. Refill unused traps first. If that does not fix it, call a licensed plumber.

Small flies around the sink may be drain flies breeding in the biofilm. Our guide to getting rid of fruit flies and drain flies explains how to tell them apart and clear them.

A slow drain or gurgling sounds mean a partial blockage. A plunger or a hand drain snake often clears it. If not, or if more than one fixture is affected, call a plumber.

Frequently asked questions

How do you freshen a smelly sink drain naturally?

Pull out and clean the stopper, scrub inside the drain opening and the overflow with a bottle brush, and clean the P-trap under the sink if the smell remains. Then use an enzyme drain cleaner overnight as the label says, and flush regularly with hot tap water.

Does baking soda and vinegar clean a drain?

Not much. Baking soda is a mild base and vinegar is a mild acid, so when mixed they mostly neutralize each other into water, a little salt and carbon dioxide, which is the fizz. It does not scrub away the biofilm that causes most drain smells.

Is it safe to pour boiling water down the drain?

It is best avoided. Very hot water can soften or warp PVC plastic drain pipes and loosen joints over time, and boiling water poured into a cold porcelain or ceramic sink can crack it. Hot tap water is a safer choice.

Do enzyme drain cleaners work?

Yes, for organic buildup such as soap, food and hair residue. They digest the film slowly over hours, so use them as the label says, usually overnight, and repeat as directed. They are not designed to clear a full blockage.

Why does a sink that is rarely used start to smell?

The water in its trap evaporates, which lets sewer gas come up through the drain. Run water into it, or pour in a jug of water, every week or two to refill the trap.

When should I call a plumber about a smelly drain?

When the smell comes back after you have cleaned the stopper and trap, when several drains smell or there is a sewage or rotten-egg smell, or when drains are slow or gurgling and a plunger does not help.

If it is a workplace kitchenette or washroom you are looking after, you can find out more on our janitorial services page, and get more practical tips like this through our free cleaning tips sign-up.