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Why do armpit stains turn yellow, and why does the shirt still smell?
The yellow most likely builds up slowly, as sweat and skin oil left in the fabric age over many wears and washes. The smell stays because skin bacteria can survive the wash on the fibers. A fresh sweat mark is invisible. According to a University of Kentucky extension guide, it "can oxidize and turn yellow with time" (source). While you wear a shirt, sweat and skin oil pass into it along with bacteria from your skin (source). In a lab test, fabric soiled with squalene, a common skin fat (source),... Read more...
Why does my gym bag stink when I wash everything?
The smell often starts before the wash, while your kit sits in the bag. A shirt you've trained in carries bacteria from your skin along with your sweat (source). Seal it in a bag, damp and in the dark, and those bacteria keep growing. Odor researchers actually rely on this. In one study, shirts worn for an hour of spinning were sealed in plastic bags and left before a trained panel smelled them (source). That was a small study of 26 people, and the bags were a lab method rather... Read more...
I showered this morning. Why do I smell again by lunch?
The smell comes from bacteria, and a shower doesn't remove all of them. Underarm odor is made by bacteria feeding on secretions from your armpit glands, and those secretions have almost no smell when they come out (source). Researchers showed this decades ago by putting underarm bacteria on fresh secretions, and the familiar smell appeared (source). A shower washes away the bacteria sitting on the surface of your skin. Not all of them live there, though. Some live deeper, in hair roots and pores (source). Studies of surgical skin cleaning... Read more...
Why sweat doesn't smell (and what actually does)
Fresh underarm sweat has almost no smell. The odor shows up when bacteria on the skin break it down into smaller, smellier molecules (source). Researchers showed this as early as 1953: sweat kept free of bacteria stayed odorless for two weeks, and the same sweat with skin bacteria smelled strong within six hours (study, summary). The main culprit molecule has been described as smelling of "sweat and onion" (source). A 2020 study traced the enzyme that makes it to underarm staph bacteria, and found those bacteria picked up that ability... Read more...