How a drying product becomes part of a stain
Antiperspirants use aluminum salts to reduce sweating. When the salts meet moisture at the skin’s surface, they form a temporary plug near the opening of a sweat duct. That same product can also remain on skin and clothing, especially when it is applied shortly before dressing.
As you move, damp fabric presses against the underarm and picks up a mix of product and sweat. The mark is therefore not simply sweat soaking into a shirt: aluminum-containing residue can become part of what accumulates in the fabric. Sweat’s proteins and oils contribute to the mixture, too.
Why the mark can turn yellow
Repeated wear can leave more residue behind. Washing may not remove all of it, and heat from drying or ironing can make a stubborn buildup harder to lift. Over time, the fabric’s underarm area can take on a yellow cast, particularly where product and sweat repeatedly meet.
That is why a shirt can stain even when the antiperspirant is doing its intended job. The aluminum salts help limit moisture at the skin, but residue transferred to cloth has a different fate: it stays among the fibers and builds up with each wear. The visible stain is the accumulated result, not a measure of how much someone sweated on one occasion.
