Last Updated on October 4, 2026 by Umar Farooq
A washer sanitize cycle is heat, held for longer. That is the whole mechanism.
The cycle does not scrub differently, tumble differently, or use a special chemistry. It heats the water well beyond a normal hot wash and then holds it there long enough for the temperature to do biological work. Everything else about the cycle — the fill, the tumble, the rinses — is broadly what it would otherwise be. That is why the cycle takes so much longer than a standard hot wash: most of the extra time is the machine waiting at temperature, not washing.
Knowing that explains most of what follows. Heat is what kills the microorganisms, and heat is also what shrinks cotton, sets protein stains, destroys elastane and delaminates anything glued. The thing that makes the cycle work is the same thing that makes it dangerous to the wrong load. There is no way to have one without the other.
The 99.9% claim, and what actually stands behind it
Every page you will read about this cycle repeats the same figure — 99.9% of bacteria — usually with no source. The figure is real, and it comes from somewhere specific, which is worth knowing because it also tells you how to check your own machine.
NSF P172 is the protocol that tests it. It sets the minimum performance requirements for a residential clothes washer to demonstrate that it reduces 99.9% of microorganisms when the sanitization cycle is selected, and it also requires that there is no carryover of test organisms between loads afterwards — a detail that matters more than the headline number, because a machine that kills bacteria but then reintroduces them on the next load has not really solved anything.
Here is the part nobody mentions. Certification under that protocol is not universal, and you can look yours up. NSF publishes the certified list, and as it stands it contains 13 washer models from two manufacturers — Electrolux and GE Appliances (NSF P172 certified product listing).
Read that carefully, because it is easy to draw the wrong conclusion. It does not mean every other sanitize cycle is fake. Manufacturers choose whether to pursue certification, certifications lapse, and a machine can perform perfectly well without appearing on that list. What it does mean is this: the 99.9% figure is a tested claim for a small number of specific models, and a manufacturer claim for everything else. If that distinction matters to you — and for cloth nappies or someone immunocompromised in the house, it might — the list is the place to check rather than the marketing copy.
When it is genuinely worth using
The washer sanitize cycle earns its cost in a narrow set of situations, and they are all cases where the biological load is the actual problem rather than dirt.
Illness in the house. Bedding, towels and clothing used by someone with a stomach bug or a skin infection. This is the clearest case, and the one the cycle was designed for.
Cloth nappies and potty-training laundry. Regular, heavy biological soiling on items that go back against skin.
Pet bedding, particularly after any skin or parasite issue.
Second-hand or stored textiles coming into the house for the first time.
Dust mites are the case that does not survive checking. Bedding in an allergic house is the situation people reach for this cycle most often, and the allergy practice parameter on dust mites says the opposite: bedding “should be washed weekly to decrease dust mite numbers and mite allergen levels, and that high temperature is not necessary.” Its reasoning is that “as long as the mites are removed, it is immaterial whether the mites are scalded to death, drowned, or simply washed down the drain still alive, and there is evidence that temperatures lower than 130F are adequate” (Joint Task Force on Practice Parameters — environmental assessment and exposure control of dust mites, 2013). The same document asks that home hot water be kept below 120F because of the scalding risk. Use the cycle on bedding when there is illness or heavy soiling in the house — not for mites alone.
For ordinary laundry — everyday clothes, towels that simply smell stale, a normal week’s washing — it is the wrong tool. Odour in towels is a residue and rinsing problem far more often than a bacterial one, and it responds better to less detergent and a proper hot wash than to a sanitize cycle. See what cycle for towels.
What to keep out of it
| Keep out | Why |
|---|---|
| Anything with elastane, Lycra or spandex | Heat destroys the stretch fibres permanently |
| Technical activewear and waterproofs | Coatings and membranes degrade; see delicates and activewear |
| Wool, silk, cashmere | Felts, shrinks, or loses its hand irreversibly |
| Anything glued or bonded — trainers, printed tees, appliqué | Adhesives soften and fail |
| Dark or bright colours you care about | Sustained heat accelerates dye loss |
| Most synthetics | Distort and pill at high temperature |
| Anything labelled cold or warm wash only | The label already answered this |
In practice the cycle suits white and colourfast cotton and linen, and very little else.
What it costs
Two costs, and the second one is the one people underestimate.
Energy. Heating water is the single most energy-expensive thing a washing machine does, and this cycle heats more water, hotter, and holds it. It is the most expensive cycle on the dial by a wide margin, and running it habitually on ordinary laundry is a straightforward waste.
Fabric life. Every sustained high-temperature wash ages a garment faster than a warm one. That is not a reason to avoid the cycle when it is warranted — it is a reason not to make it a default. The clothes will simply not last as long.
Expect it to take substantially longer than a normal cycle. Most of that is the heating and hold phase, so a cycle that seems to sit at the same time reading for a long stretch is behaving correctly. If you want to know why the number on the display moves at all, that is covered in why your washer adds time mid-cycle.
Washer sanitize cycle vs a hot wash vs Tub Clean
These get confused constantly, and they do three different jobs.
| What it heats | What it cleans | When | |
|---|---|---|---|
| Hot wash | Water to a hot setting | Your laundry | Whites, heavy soil, ordinary hygiene |
| Sanitize | Water hotter still, and holds it | Your laundry, biologically | Illness, nappies, allergens |
| Tub Clean | Usually hot, drum empty | The machine itself | Monthly maintenance |
The one that catches people out is the third. Tub Clean is run with nothing in the drum — it cleans the machine, not the clothes, and it does not reach the door gasket, the detergent drawer or the pump filter. If the smell is coming from the machine rather than the laundry, a sanitize cycle full of towels is the wrong answer. See the tub clean cycle explained.
Equally, if your instinct is that hotter always cleans better, that instinct is mostly out of date for everyday washing — modern detergents are built for cold, and the trade-offs run the other way. That argument is made properly in does cold water actually clean clothes.
Using it properly
- Sort ruthlessly first. One synthetic garment in the load is one ruined garment.
- Use your normal detergent. The cycle needs no special additive, and adding bleach on top of a sanitize cycle is belt-and-braces at the cost of the fabric.
- Do not overfill. Heat has to reach the middle of the load, and a packed drum defeats that as surely as it defeats a normal wash — see how full should a washing machine be.
- Run it when you need it, not on a schedule. This is a response to a situation, not a maintenance habit.
- Empty the machine promptly. Leaving a hot, damp load sitting undoes the point of the cycle.
FAQ
Does the washer sanitize cycle actually kill germs?
On a certified model, tested to a 99.9% reduction. On an uncertified one, it is the manufacturer’s claim rather than a tested one — the mechanism is the same, the verification is not.
Is it the same as a hot wash?
No. Hot wash reaches a temperature. Sanitize reaches a higher one and holds it, which is where the time goes.
Can I use it on towels?
Only white or colourfast cotton ones, and only when there is a reason. Routine towel odour is usually residue, not bacteria.
Do I need a special detergent or additive?
No. Your normal detergent is fine.
Why does it take so long?
Most of the cycle is the machine holding temperature rather than washing. That wait is the active ingredient.
My machine has no sanitize cycle. Then a hot wash on cotton is your closest equivalent. It will not meet the same standard, but for most household situations it is adequate.
Can it clean the machine itself?
No — that is Tub Clean, run empty. Different job entirely.
