Why your washing-up sponge smells even after rinsing – and the 10‑second trick microbiologists recommend instead of bleach

The smell usually shows up when you’re already done for the day.

The plates are stacked, the worktop’s wiped, the dishwasher is humming away doing the last of the heavy lifting. You squeeze out the sponge, give it a virtuous rinse under the hot tap, maybe a quick blast of washing-up liquid for good measure, and drop it back by the sink. Job done. Kitchen: reset.

Then, a few minutes later, you walk back in to make a cup of tea and it hits you. That sour, damp‑cloth, vaguely “wet dog at a swimming pool” smell. You sniff the bin, you check the plughole, you lift the tea towel. Eventually you pick up the sponge, bring it reluctantly to your nose, and there it is. The culprit, holding its nose in the air like butter wouldn’t melt.

There’s a tiny, irrational shame in that moment. You know you just cleaned everything. You ran the hot tap until the water steamed. You rinsed the sponge like an advert. How is it still gross? And why, when you Google “how to disinfect a sponge”, does it feel like the whole internet is shouting “BLEACH IT” at you?

Microbiologists will tell you this is one of those household battles you can actually win. Not with harsher chemicals or industrial sprays, but with ten calm seconds and something you already use every single day: your kettle.

Why your “clean” sponge still smells

The bit that smells isn’t the foam itself. It’s the life that’s moved in.

A washing-up sponge is basically a block of cave systems: warm, damp, full of tiny pockets that stay wet long after you’ve finished the dishes. Food particles drift in, washing-up liquid adds its own film, and bacteria settle down like they’ve found a lovely little seaside town with year‑round tourists.

Most of the culprits are ordinary environmental bacteria, the sort that live happily in soil and water. Given warmth, moisture and a bit of old gravy, they multiply fast and start to produce volatile compounds – tiny smelly molecules that drift up into the air. That “sour‑swampy” scent is what microbiologists recognise from lab cultures of sponge samples.

Rinsing under the tap mostly deals with crumbs and the very top layer of grime. It does almost nothing to the deeper biofilm – the slimy layer clinging to the internal structure of the sponge. Imagine trying to clean a carpet by waving water over it for three seconds instead of getting the dirt out of the fibres. That’s what your sponge rinse is doing.

To make things worse, many of us put the sponge down in the one place it’s guaranteed to struggle: a puddle. The corner of the sink that never quite drains, the rim of a metal holder that gathers drips, the saucer under the tap. Constant damp means the bacteria never get a chance to dry out and slow down. They just keep quietly partying while you sleep.

Why bleach isn’t the fix you think it is

Bleach feels like a power move. It smells harsh, it stings if you splash it, it’s on every emergency cleaning list. It must be the answer, surely.

Not quite.

In lab tests, a long soak in properly diluted bleach can indeed kill a lot of bacteria in a sponge. The problem is how we actually use it in real kitchens. A quick slosh in a washing-up bowl, a hurried squeeze while you’re doing five other things, and then back by the sink. That short contact time rarely reaches the microbes buried deep in the foam.

Bleach is also fussy. If there’s lots of organic matter around – bits of food, soap scum, old coffee – it gets “used up” neutralising that, leaving less active chemical to deal with the bacteria. By the time it reaches the middle of the sponge, the strength is often far lower than the label promises.

Microbiologists have another concern. Repeated, half‑hearted disinfecting can leave behind the hardier bacteria that cope better with stress. Over time, you end up selecting for the stinkiest, toughest residents. You’re not sterilising the neighbourhood; you’re quietly curating a very specific, very smelly population.

Add in the practical downsides – fumes in a small kitchen, bleached tea towels by accident, extra plastic bottles and harsh chemicals down the drain – and bleach starts to look like a lot of drama for not much payoff.

Heat, on the other hand, is gloriously simple. Hot enough, for long enough, and most everyday bacteria give up. Which is where that ten‑second trick comes in.

The 10‑second kettle trick microbiologists actually use at home

Ask people who study microbes what they do with their own sponges and you hear a similar pattern: they use heat, and they keep it boringly simple.

One easy, bleach‑free method you can bolt onto a habit you already have is the “kettle dunk”. It takes about ten seconds of your attention and makes use of water you were boiling anyway.

How to do it safely

You’ll need:

• Your usual washing-up sponge (no metal scourers attached)
• A heat‑resistant mug, bowl or the empty side of your sink with a metal base
• Recently boiled water from the kettle
• Tongs or the handle of a wooden spoon

Step‑by‑step:

1. Rinse and wring first. After you’ve finished the washing up, rinse the sponge under the tap to get rid of visible bits, then squeeze it as flat as you can. Less gunk means the hot water can reach the bacteria instead of wasting heat on soup residue.

2. Put the sponge in a safe spot. Lay it in a heat‑proof bowl, mug or in the metal part of the sink, away from plastic fittings. Make sure it’s lying flat.

3. Pour just‑boiled water over it. As the kettle clicks off, slowly pour enough boiling water over the sponge to completely cover it. Start at one end and work across so you don’t splash yourself.

4. Hold it under for 10 seconds. Use tongs or a spoon to gently press the sponge under the surface for a slow count of ten. That time with water above 90°C is what does the heavy lifting on bacteria.

5. Let it sit while you get on with your life. You don’t need to hover. Leave the sponge in the hot water until it’s cool enough to handle, then fish it out, wring it well, and stand it upright somewhere it can actually dry.

Those ten conscious seconds – the pour and the press – turn your sponge from a warm, damp incubator into a much less friendly place for microbes. You’re not chasing total sterility; you’re dramatically knocking back the population and the smell‑making compounds they churn out.

A few safety notes that microbiologists and fire officers alike would insist on:

• Don’t try this with wire or metal‑backed pads; they’re better off cleaned another way or simply replaced.
• Keep your hands and wrists out of the pour path. Use long handles or tongs to press.
• Don’t hand this job to a child; boiling water burns in a second.
• If your sponge looks damaged, melted or crumbly after repeated hot soaks, it’s time to bin it.

Do this once a day in “heavy use” weeks, or every couple of days in quieter spells, and you’ll usually notice the smell fade and stay gone. The bonus? You’re using heat you already paid for when you boiled the kettle for tea.

The quiet power of letting the sponge actually dry

Heat is half the story. Dryness is the bit most of us skip.

Bacteria adore a permanently damp environment. Knock them back with hot water and then drop the sponge back into a little swamp, and the survivors simply regroup. If you want the 10‑second trick to really stick, you need one more unglamorous habit: give the poor thing a chance to breathe.

In practice, that looks like:

• A serious final squeeze. Not a token pat. Wring it until it’s as close to dry as you can manage without redecorating your top.
• Storing it upright, not flat. Perch it on its side, on a small rack, or in a holder with plenty of holes so air can circulate.
• Keeping it out of puddles. If your sponge holder fills with water, it’s working against you. Empty it; wipe the rim of the sink; don’t let it sit in the run‑off from the tap.
• Rotating sponges. Have two on the go and alternate daily. Each one gets a proper drying day between shifts.

None of this is Instagrammable. It’s not a new gadget or a glossy caddy. It’s the kind of tiny, repeatable behaviour shift that microbes actually notice, even if you barely do.

Here’s how the key tweaks stack up:

Habit tweak	What you do	Why it helps
Daily kettle dunk	Cover sponge in just‑boiled water and hold under for 10 seconds	Uses heat to rapidly cut bacterial numbers without bleach
Wring + air‑dry upright	Squeeze hard, then store where air can circulate	Starves remaining microbes of the constant moisture they love
Rotate and replace	Use two sponges in rotation; replace every 1–3 weeks	Limits long‑term build‑up and embedded smells

Let’s be realistic: nobody is going to perform a full lab routine on a 99p sponge every single night. There will be days when you dump it by the sink and walk away. But in those weeks when the smell starts to creep in and the shame‑sniff is back, three or four evenings of kettle dunks and proper drying are often enough to pull things back from the swamp.

When to stop rescuing and just throw it away

There is a point where no amount of boiling, rinsing or airing is worth your time.

If the sponge:

• Still smells within minutes of drying
• Has gone grey, slimy or oddly stiff
• Is shedding little crumbs of foam into the sink
• Has visible mould or black spots deep inside

…that’s your sign. Thank it for its service and put it in the bin.

Microbiologists – the people most capable of sanitising anything – are also the quickest to say “just replace it”. A sponge is a cheap tool designed to be sacrificed regularly, not a family heirloom. Using the kettle trick stretches its sweet spot; it doesn’t sentence you to keep it forever.

FAQ:

• Can I just microwave my sponge instead? Microwaving a very wet, microwave‑safe sponge can kill a lot of bacteria, but it comes with more “ifs” and “buts” – it must be soaking, totally metal‑free and watched carefully to avoid fire. The boiling‑water method is simpler and safer for most households.
• Does this work with scourers and cloths too? Plain microfibre cloths and non‑metal scourers tolerate brief boiling water well. Metal pads, glued‑on sponges and anything with plastic trim are better cleaned in hot soapy water and replaced more often.
• Is vinegar or lemon juice as good as bleach? Acids like vinegar can freshen smells a bit, but they’re far less effective than proper heat at penetrating the sponge and hitting bacteria. They’re fine as an extra, not a replacement for the kettle dunk.
• How often should I replace my sponge if I use this trick? In a busy kitchen, aim for every 1–3 weeks. The hotter soaks and better drying will keep it fresher within that window, but they don’t make it immortal.
• What if I hate sponges altogether? That’s valid. Many microbiologists quietly prefer dish brushes and reusable cloths; they dry faster and harbour fewer smells. The same principles apply: hot water regularly, good airflow, and a low‑drama willingness to replace them when they’re past their best.