Polyester

Polyester is PET plastic spun into fiber, and roughly 80 to 85 percent of virgin PET is made using antimony trioxide as a polymerization catalyst, leaving antimony residues of about 125 to 470 micrograms per gram in the finished fabric. It is then colored with disperse dyes that sit on, rather than bond to, the fiber.

When polyester contacts your sweat, a fraction of that antimony mobilizes into the moisture against your skin; antimony trioxide is classified by IARC as a possible human carcinogen, and exposure rises in tight, warm activewear. Polyester is also the workhorse of disperse-dye allergy, the most common cause of textile contact dermatitis, with the dye migrating onto skin worst where you sweat and rub. On top of that, polyester sheds microplastics onto your skin and into household air, and consumer testing has flagged bisphenol A (an endocrine disruptor) in polyester athletic wear at many times the safe limit. The dose from any one garment is small, but you wear it constantly and sweat into it.

Choose OEKO-TEX STANDARD 100 certified polyester to cap antimony, BPA, formaldehyde, and disperse-dye allergens, and do not be reassured by recycled polyester, which is still plastic, still sheds microplastics, often more than virgin, and can carry higher BPA.

Polyester is fossil-fuel based, effectively non-biodegradable, and a primary driver of microplastic pollution.