Yellowing in white shirts and linens usually comes from one of three sources: residue from antiperspirant reacting with sweat, sunscreen and body oils, or simple oxidation over years of storage.
Antiperspirant stains in the underarm area are stubborn. They contain aluminum compounds that bind with protein in sweat and react further with chlorine bleach — which is why bleaching often makes the yellow worse.
Fresh yellowing responds best to professional treatment soon after it appears. Older, set yellowing can sometimes be lifted partially with a series of treatments but rarely returns to bright white.
At Joy we inspect every shirt at intake and tell you up front if a stain is unlikely to come out.