Those yellow underarm and collar marks aren't simply “old sweat”. Here's what's really happening — and what you can do about it.
Those yellow marks aren't just sweat
You wash a white shirt.
It comes out clean.
But somehow, those yellow marks around the underarms and collar are still there.
Sometimes they're barely noticeable when a garment is new. Then, after months of wear and washing, they can become darker, more obvious and surprisingly difficult to remove.
So what is actually happening?
The answer is a little more complicated than simply “sweat stains.”
🌿 What is actually causing the yellowing?
Sweat is mostly water and salts, but the areas around our underarms and collars also come into contact with skin oils (sebum), skin cells, deodorant or antiperspirant residues and other organic matter.
Over time, these materials can build up in the fibres.
One particularly interesting component is squalene, a natural component of sebum. Research into sebum and textiles has found that oxidation of sebum can contribute to yellowing and can make these deposits more difficult to remove.
So when you see that familiar yellowing around the underarms or collar, you're often looking at a mixture of different residues that have changed over time, rather than simply dried sweat.
🧺 Why are underarms and collars affected so badly?
It makes sense when you think about where these areas sit.
Underarms are exposed to:
- perspiration
- skin oils
- deodorant and antiperspirant products
- friction
- warmth and moisture
Collars come into contact with:
- sweat
- skin oils
- moisturisers and personal-care products
- dirt and everyday grime
Washing removes a lot of this soil, but old deposits can be much more stubborn than fresh ones.
Research into sebum and textiles shows that oxidation can create compounds that are harder to remove, while sebum can also interact differently with different types of fibres.
This helps explain why a garment can be washed many times and still develop increasingly noticeable yellowing.
🔬 Why doesn't washing always remove it?
Because washing isn't always the same thing as completely removing every type of soil.
Laundry soils can be a mixture of water-soluble and oily materials, and different cleaning ingredients work on different types of soil.
That's why pretreating a stubborn stain before washing can make such a difference.
The American Cleaning Institute recommends pretreating collar and cuff soil and notes that stubborn perspiration stains may respond to enzyme-containing products or oxygen bleach when used appropriately for the fabric.
And there's another important point:
Old stains need patience.
Once a stain has been through repeated washing and drying, it may take more than one treatment to shift it.
That doesn't necessarily mean the garment is ruined.
It means the stain has had time to become much more difficult to remove.
🌿 So how do you tackle an old, washed-in sweat stain?
This is where we put the theory to the test.
We had a garment with old, set-in yellow sweat staining that had already been washed several times.
For this particularly stubborn stain, we used:
Boerseep Laundry Cream + Suiwer Oxygen Bleach
And this is exactly what we did.
1. Check the fabric
Before treating any garment, check the care label and make sure the fabric is suitable for the treatment.
If you're unsure, test the treatment on an inconspicuous area first.
2. Apply Boerseep Cream
We dampened the stained area and worked Boerseep Laundry Cream directly into the yellowed area, making sure the stain was well covered.
3. Make the Oxygen Bleach paste
For our test, we mixed Suiwer Oxygen Bleach with boiling water to create a thick paste.
We then rubbed this oxygen-bleach paste over the area where the Boerseep Cream had already been applied.
This created a Boerseep Cream + Oxygen Bleach treatment paste directly on the stain.
Important: This describes the method we used in our own test. Always follow the instructions supplied with the oxygen bleach product you are using. If your product specifies a different water temperature or preparation method, follow those instructions instead.
4. Let the treatment work
Once the stained area had been thoroughly treated, we allowed the treatment time to work.
We then hung the garment outside in the sunshine.
The treatment time and hanging the garment outside formed part of our real-life test.
5. Wash as usual
After the treatment, we washed the garment according to its normal washing instructions, using a suitable wash temperature for the fabric.
6. Check the result
And this is where the satisfying part comes in...
The old yellow staining was dramatically reduced and the garment looked noticeably brighter and cleaner.
These are the actual before-and-after photographs from our test — not a staged demonstration.
☀️ What about putting it in the sun?
Sun-drying is something many of us grew up doing, and sunlight can contribute to the brightening of some white laundry.
We treated the stain, allowed the treatment time to work, and then hung the garment outside in the sunshine as part of our real-life test.
👕 Why can old stains be so stubborn?
The yellow colour isn't necessarily evidence that the garment is permanently damaged.
It can be the visible result of body oils and other residues changing and oxidising over time.
That is why a garment that has been washed again and again can still look dirty around the underarms or collar.
Sometimes you simply need to approach the stain differently.
Clean doesn't always mean “one more wash.”
Sometimes it means:
Pretreat → Treat → Wash → Inspect → Repeat if necessary.
💚 What about harsh stain removers and your skin?
This is something we think is worth talking about honestly.
There are some very powerful stain-removing products available, and stronger does not automatically mean better.
Some laundry chemicals and bleaching products can irritate skin if they come into contact with it. Always follow the product instructions, avoid unnecessary skin contact and rinse treated garments thoroughly before wearing them.
The goal isn't to be afraid of cleaning products.
It's to use the right product, in the right amount, for the right job.
That's what smarter cleaning is all about.
🌼 Can you prevent the yellowing?
You can't always prevent it completely, but you can make stubborn build-up less likely.
Treat marks sooner rather than later.
Fresh stains are generally easier to deal with than old, oxidised deposits.
Don't ignore collars and underarms.
If you can see or feel build-up beginning, pretreat it before the next wash.
Don't overload your washing machine.
Clothes need enough movement for soil and detergent to be carried away effectively.
Follow garment-care instructions.
The right temperature and wash cycle matter.
And don't give up on a garment too quickly.
An old stain can look permanent when it isn't.
🧪 The Suiwer Truth Test
We don't just want to tell you that something should work.
We want to test it.
That's why this week's Truth Test Thursday is particularly satisfying.
We took a real garment with old, washed-in yellow sweat staining and treated it with Boerseep Cream + Suiwer Oxygen Bleach.
The photographs in this article are from that real test.
Before.
Old yellowing around the affected area.
After.
A visibly cleaner and brighter result.
And that's exactly why we love testing cleaning tips in real life.
Because sometimes the best cleaning advice isn't found on a label.
It's found by asking:
“What actually works?”
🌿 The Suiwer takeaway
Those stubborn yellow underarm and collar marks aren't simply “old sweat”.
They're the result of multiple things coming together over time — perspiration, skin oils, personal-care residues, the fabric itself and chemical changes such as oxidation.
And once they've become established, another ordinary wash may not be enough.
So before you throw away that favourite white shirt or think it's permanently stained...
Try treating the problem rather than simply washing it again.
Check the fabric.
Pretreat the stain.
Use the right laundry treatment.
Give it time.
Wash it properly.
And give that garment another chance.
Refresh. Restore. Re-wear. ♡
Suiwer | Natural By Nature
Smarter cleaning for a cleaner home — and a kinder tomorrow.
🔬 Further Reading & Research
American Cleaning Institute — Laundry Basics
Covers stain pretreatment, garment-care labels, oxygen bleach and safe laundry practices.
Read the American Cleaning Institute — Laundry Basics
American Cleaning Institute — Stain Removal Guide
Includes guidance for perspiration, deodorant and collar/cuff soil.
Read the American Cleaning Institute — Stain Removal Guide
American Cleaning Institute — How to Use Bleach
Explains oxygen bleach, fabric suitability, colourfastness testing and safe use.
Read the American Cleaning Institute — How to Use Bleach
Review: Artificial Sebum Formulations and Textile Staining
Reviews the composition of sebum and research into oxidation, yellow staining, odour and textile interactions.
Read the peer-reviewed research on sebum and textile staining
Research: Deodorants and Antiperspirants
Reviews antiperspirant ingredients and their relationship with skin and clothing.
Read the research on deodorants and antiperspirants