What causes this to happen???

Question

What causes the yellow stains in the arm pit area on t-shirts?

Answer 1

Those are called sweat stains.  It's a combination of sweat, the bacteria on your skin that eat the sweat, dirt, and millions of flakes of dead skin which you shed constantly.  When doing laundry, first wash in cold water with detergent, then wash again in hot water with detergent and bleach.  Or use a non-chlorine bleach on sensitive clothes.  If you wash in hot water first, the heat can make the stains permanent, but it helps the bleach work, so use hot second.

Alternate explanation:  It might be some residue from your deodorant or antiperspirant.  Mine leaves white powdery residue that's hard to wash out.

Answer 2

your sweat