The Chemical Composition of Perspiration
Perspiration or sweat, is secreted by glands in the skin to remove metabolic waste and regulate body temperature. While mostly water, sweat also contains several compounds that can gradually affect the color of cotton textiles: o-cresol (lit. 2-methylphenol), p-cresol (lit. 4-methylphenol) and urea.
O-Cresol and P-Cresol
Part of a class of organic aromatic compounds called phenols, o-cresol and p-cresol oxidize into yellow-brown deposits through prolonged exposure to heat. Insoluble in water, these compounds can collect in the armpits of cotton shirts to such concentrations that water-based detergents can't dissolve them. When the shirts are put in the dryer, the intense heat oxidizes the phenols left in the armpits. As these yellow compounds accumulate, the overall color of the fabric under the armpits takes on a dirty brown hue.
Thanks to the strong hydrophobic nature of their aromatic benzene groups, phenols are also used industrially as organic solvents and cleaners. Therefore, when o-cresol and p-cresol come into contact with non-polar pigments in the cotton fabric, they begin to slowly dissolve them.
Used very commonly in the dyeing process, urea helps increase the solubility of dyes in water. As sweat is almost completely water, the urea in a sweat-saturated shirt armpit amplifies the gradual fading process that normally occurs in dyes over multiple washings.