Acrylic

Acrylic is a synthetic fiber polymerized mostly from acrylonitrile, a petroleum-derived monomer; polymerization is never perfectly complete, so trace residual acrylonitrile remains in the fiber, historically estimated at under 1 part per million in acrylic and modacrylic.

The residual monomer is the concern: acrylonitrile is classified by IARC as Group 1, carcinogenic to humans, and the EPA treats it as a probable human carcinogen, with textile-plant workers showing raised lung and colon cancer risk after long exposure. Acrylonitrile can be absorbed through the skin, so warm, sweaty contact with a soft acrylic sweater or fleece is a real, if low-level, exposure route over time. Acrylic also sheds microplastics heavily, putting plastic fibers onto your skin and into the air at home, and like other synthetics it carries disperse dyes and finishing chemistry that can irritate sensitive skin.

Look for OEKO-TEX STANDARD 100 certification, which sets strict limits on residual acrylonitrile and other monomers, and favor natural insulating fibers like wool or cotton for next-to-skin knitwear where possible.

Acrylic is fossil-derived, does not biodegrade, and is one of the worst microplastic-shedding fibers in the wash.