Elastane

Elastane is the same fiber as spandex; the names are interchangeable, and Lycra is simply a brand of it. It is a polyurethane-based elastic yarn made by reacting diisocyanates (TDI, MDI) with polyols, so the wearer-health questions come from residual processing chemicals and from what gets blended and finished onto it, not from the inert polymer itself.

Because elastane gives garments their stretch, it lives in exactly the clothes you wear closest and sweat into: leggings, swimwear, underwear, and base layers. Studies of artificial-sweat leaching and consumer testing on polyester-elastane activewear have found bisphenol A and antimony migrating into the moisture against your skin, and durable water-repellent PFAS finishes are linked to hormonal, reproductive, and immune harm. The fiber's defining trait, a tight occlusive fit, traps heat and sweat, which both irritates skin and accelerates how much of that chemistry transfers onto and into you. The polymer is largely inert, but the company it keeps on your body is not.

Look for OEKO-TEX STANDARD 100 certification to screen out PFAS, phthalates, and excess heavy metals, and prefer pieces that use elastane only as a small stretch percentage in an otherwise natural fabric rather than head-to-toe synthetic blends.

Elastane is petroleum-based, non-biodegradable, and contaminates the recyclability of any natural fiber it is blended into.