Drains are designed to let water out while keeping air from the waste pipe from coming back in. That barrier is usually a small amount of standing water sitting in the trap under the fitting. When that seal works and the pipework flows freely, you generally don't notice the drain at all.

• A dry or rarely used trap. In an unused guest bathroom or a floor drain that never sees water, the water sitting in the trap can evaporate, and once it's gone there's nothing holding the smell back.
• Build-up inside the fitting or pipe. Hair, soap residue and other everyday household material collect on the inside of the pipe and can produce a smell of their own.
• A partial blockage or restricted flow. When water sits rather than draining away cleanly, smells tend to follow.
• Slow or poor drainage generally, which keeps waste water in contact with the pipe for longer than it should be.
• Sealing or plumbing issues - for example a fitting, joint or seal that isn't doing its job, or a trap arrangement that isn't holding water as intended.

Those causes look identical from above the drain, which is why guessing rarely helps.

A smell that appears once after a bathroom has been shut up for a fortnight, then disappears after normal use, usually tells you very little. A smell that returns week after week is far more useful, because something is repeatedly recreating the conditions that cause it.

That's also why masking the smell tends to be a short-lived fix. An air freshener or bleach down the drain changes what you can smell for a while, but it doesn't change whether the trap is holding water or whether the pipe is flowing properly.

These are all safe observations - nothing here involves opening up plumbing.

• Has the drain been unused for a while? Running the tap or shower for a minute or two refills the trap. If the smell goes and stays gone, that was likely all it was.
• Is water draining normally, or is it slower than it used to be? Watch how a full basin or the shower tray clears.
• Is it one drain or several? Check the basin, shower and any floor drain separately, and note which ones smell.
• Is there standing water anywhere - in the shower tray, around a floor drain, or under the vanity?
• Does the smell come and go with use, or is it constant?
• Have you noticed gurgling sounds from another fitting when you use one of them?

Please leave anything involving dismantling pipework, traps or seals alone. It's easy to disturb a joint and turn a smell into a leak.

The pattern matters more than the smell itself. It's usually worth a proper look when:

• the smell keeps returning even though the fittings are used regularly
• drainage is noticeably slow as well as smelly
• the same drain blocks repeatedly
• more than one fitting is affected
• you can see standing water that isn't clearing

Any of those suggests the smell is a symptom of how the drainage is behaving rather than a dry trap. If the same drain also keeps blocking, our guide on why a drain keeps blocking covers that pattern in more detail.

If the smell keeps returning, the useful next step isn't another air freshener - it's working out where it's actually coming from.

A plumbing visit starts with looking at the drain and how it's flowing, rather than assuming a cause. You can book a drain smell visit - booking reserves the callout fee only, and the work itself is quoted to you in writing once the plumber has seen it, before anything goes ahead.