Why does my mattress smell?

The short answer

A mattress can smell for several different reasons, and the smell alone doesn't identify which one. The most useful first step is working out when it started. A new mattress may have an odour after unpacking; if the smell appeared after the mattress got wet or was cleaned, retained moisture or incomplete drying is one possibility; and a smell that developed gradually may relate to general use, heat and sweat. If it followed a known urine accident, that's a more specific problem with its own guide. A musty or damp smell deserves attention, but it doesn't confirm mould on its own. Check your mattress maker's care guidance before treating it further.

What to check before you start

When the smell started: new, after getting wet or cleaned, or gradually over time
Whether the mattress is new, and whether it's still within the first few weeks of use
Your mattress's care label and the maker's instructions
Whether a removable cover or protector is involved, and whether it has its own separate care label
Whether the smell is musty or damp, or something else
Whether anything has already been tried on it, and what

We compared care guidance published by mattress manufacturers including Simba, Hypnos and Tempur, alongside general damp-and-mould health information from the NHS and GOV.UK. Manufacturer instructions vary by material and brand, so this guide describes what's commonly documented rather than a single universal method, and we haven't tested any mattresses ourselves. This covers cleaning and general causes only — it isn't personal health advice.

When did the smell start?

The most useful first distinction isn't what the mattress is made from, it's when you noticed the smell. This is a starting point for what's worth checking next, not a diagnosis.

New, or soon after unpacking. Manufacturer-described new-mattress odour may fit here — several makers acknowledge this after a mattress is unwrapped.
After getting wet or being cleaned. Retained moisture or incomplete drying is one possibility worth checking, since mattress layers can hold moisture for longer than the surface suggests.
Gradually, over general use. Heat retention and sweat are commonly discussed contributors to a mattress becoming less fresh over time.
Following a known urine accident. That's a more specific problem, and treating it as one tends to help more than treating it as a general odour.
Musty or damp. This is worth taking seriously and checking further, but a smell alone doesn't confirm mould on its own.

If it followed a known urine accident, our guide on how to get urine out of a mattress covers that specifically. If it followed a known vomit incident, our guide on how to clean vomit from a mattress covers that instead.

If the mattress is new

Several mattress manufacturers acknowledge that a new mattress can have a smell after it's unpacked. Simba addresses this directly in its own help guidance, and Hypnos's care guide recommends leaving a new mattress uncovered for a few hours after unwrapping so it can air. Tempur's own guidance describes a slight odour after manufacturing that it says fades over a week or two — that's Tempur's own timeframe for its own material, not a rule for every mattress.

We can't independently confirm whether this kind of odour is harmless or otherwise; that's not something we're able to verify ourselves. What manufacturers consistently recommend is airing the mattress in a well-ventilated room, ideally before making the bed.

If it smells after getting wet or being cleaned

Check the mattress maker's care instructionsLook at the care label and any manufacturer documentation before doing anything else — some makers restrict liquids or particular methods altogether.
Avoid adding unnecessary moistureIf a known, treatable issue is involved, use only what the maker's guidance permits, and no more than that.
Ventilate the room and leave the mattress uncovered where appropriateAirflow is a widely recommended low-risk step, whether the mattress is new or has recently got wet.
Reassess only once the mattress is fully dryJudge the smell once it's completely dry, not while it's still damp. This sequence doesn't guarantee the smell will go, and it isn't a substitute for checking what's actually causing it.

What a musty smell can — and cannot — tell you

A musty smell is worth taking seriously, but on its own it doesn't confirm mould. The NHS and GOV.UK both publish general guidance on damp and mould in the home, and note that a musty smell can sometimes point to mould you can't see, even without a visible patch. That guidance is about damp and mould in a home generally — it isn't a diagnosis of your specific mattress, and it isn't personal health advice.

If you can see mould, or a musty smell persists despite the mattress being properly dry and the room well ventilated, that's a reasonable point to stop guessing and look more closely, rather than continuing to treat the mattress alone.

What about bicarbonate, vinegar and other home remedies?

Bicarbonate of soda (baking soda) is commonly suggested for a smelly mattress, usually as something to sprinkle on and vacuum off again. We don't recommend it as a default step: we haven't found good evidence that it draws odour out or removes whatever is causing it, and we don't give a dwell time.

White vinegar is also widely suggested online, but the evidence for using it on a mattress specifically isn't something we can point to with confidence, and mattress materials vary. We don't give a mixture or a ratio. Whatever your mattress's care guidance says takes priority over general advice like this.

Do you need a machine or steam cleaner?

Not automatically. A carpet cleaner, spot cleaner or wet extractor isn't a default step for a mattress — these add liquid as well as removing it, and mattress makers may restrict liquids altogether.

Steam depends on the maker and the material: some mattress makers and materials restrict or prohibit it, so check your specific guidance rather than assuming it's fine.

We don't say a machine cleans more deeply, extracts odour, dries faster or works better than the steps above. If you're trying to work out what different machines actually do, our equipment guide covers that separately.

When to stop

Stop, or pause and reassess, if:

The mattress remains wet despite airing.
Your mattress's care guidance rules out the method you were about to try.
Visible mould appears. This is a reason for caution on our part, not a diagnosis.
The material or care instructions are unclear.
A strong smell persists despite reasonable drying and ventilation.
Whatever you've tried seems to be making the smell worse rather than better.

Judge the result once the mattress is completely dry rather than while it's still damp. This guide is about working out what's likely going on, not a guaranteed fix.

If you're unsure what your mattress allows, your maker's customer support is a reasonable first step — they know the construction and their own guidance.

For a persistent or strong smell, or suspected mould, a professional may be worth considering. We can't say a professional guarantees a result, and check your warranty terms first, as some makers restrict liquids, steam and chemicals.

Mistakes to avoid

Adding more liquid before the mattress is fully dry. Extra moisture can make a lingering smell more likely, not less, and some makers restrict liquids altogether.
Assuming a musty smell proves mould. A musty smell is worth investigating, but it doesn't confirm mould on its own.
Using steam without checking manufacturer guidance. Some mattress makers and materials restrict or prohibit steam. Check before you use it.
Treating vinegar, peroxide or other chemistry as a universal recipe. What's commonly suggested online isn't the same as your maker's instructions, and we don't give a mixture or ratio.
Washing a removable cover without checking its label. Removable isn't automatically the same as machine washable. Check the cover or protector's own care label.
Masking the smell without investigating its source. A fragrance or deodorising spray hides the smell rather than addressing it, and can make it harder to notice if things are actually getting worse underneath.

If you already know the cause is urine, our guide on how to get urine out of a mattress covers the method. If you're facing a similar problem on a carpet or a sofa rather than a mattress, our guides on why carpets can still smell after cleaning and why sofas can smell after cleaning cover those surfaces separately. If you're trying to work out whether a machine is actually worth considering, our guide on carpet cleaners vs spot cleaners covers what different types do.