Yellow marks under the arms and where they come from

Short answer: The yellow is not sweat alone but aluminium from antiperspirant bonding with protein in sweat. Switch to a deodorant without aluminium and treat the area before it dries in.

What actually stains the fabric

Sweat on its own is close to colourless. The yellow appears when aluminium salts from antiperspirant react with proteins in sweat and with heat. That is why the marks get worse after drying somewhere warm, and why they set harder with every wash they survive.

How to get the marks out

Soak the garment overnight in lukewarm water with oxygen bleach. Rub gently with a soft brush before washing. Avoid chlorine bleach, which makes yellow marks darker rather than lighter. Repeat the treatment rather than raising the temperature.

How to avoid them

Let the deodorant dry completely before you dress, use less of it, and choose an aluminium free version where you can. A thin undershirt between skin and shirt absorbs most of it and is easy to change. See bamboo undershirts for men and bamboo undershirts for women.

When the mark will not go

If the fabric has gone stiff and deep yellow, the aluminium has bonded permanently to the fibres. The garment is usually finished as smart clothing but still works at home. See bamboo t-shirts for men.

Frequently asked questions

Are yellow marks a sign of poor hygiene

No. It is a chemical reaction between antiperspirant and sweat, not a question of washing enough.

Does bicarbonate of soda work

Often yes on fresh marks. Make a paste with water, leave it for an hour and then wash.

Does the tumble dryer make marks worse

Yes. Heat bakes the mark in, so dry without heat until the stain has gone.