Most clothing labels list fiber content. Almost none explain what that fiber content means for your health.
This is a practical guide to the fabrics that show up most frequently in activewear and what the research says about each one. For women building a toxin-free activewear wardrobe, this is the reference. Not a scare piece. A framework.
How to Use This
These are not ranked by how dangerous they are in absolute terms, because toxicity depends on concentration, exposure duration, and individual sensitivity. They are ranked by how much chemical and microplastic exposure they carry compared to each other, based on the available research. The goal is to give you a framework for reading a label, not a verdict on any single brand or garment.
Fabrics to Approach with Caution
Polyester
Polyester is polyethylene terephthalate, the same polymer as a plastic bottle, spun into fiber. It sheds microplastic microfibers during both washing and wear. A single wash can release hundreds of thousands of fibers, many of which pass through wastewater treatment and enter waterways. Research by Napper and Thompson (2016) in Marine Pollution Bulletin documented consistent and substantial microfiber shedding from polyester under standard domestic washing conditions.
Beyond microplastics, polyester is hydrophobic, meaning it repels water rather than absorbing it. Sweat sits on the surface, creating warm, moist conditions that support bacterial growth. Polyester garments develop permanent odor over time because the bacteria that cause that odor embed into the fiber's structure in ways that washing cannot fully reverse.
Polyester is also frequently treated with performance finishes, including some that historically involved PFAS chemistry, though many brands have moved toward PFAS-free alternatives in recent years.
Nylon
Nylon (polyamide) shares the core problem with polyester: it is a synthetic polymer that sheds microplastic fibers. De Falco et al. (2019) in Science of the Total Environment found nylon among the consistent microfiber shedders alongside polyester.
Nylon is often used in activewear for its strength, abrasion resistance, and stretch recovery. Those are real performance advantages. They come alongside the microplastic shedding profile common to all synthetic polymer fibers.
Like polyester, nylon garments are frequently treated with DWR coatings or stain-resistance finishes that have historically used PFAS chemistry. The baseline fabric itself does not require PFAS, but the performance finishes common in the activewear category often have.
Acrylic
Acrylic is a synthetic fiber made from acrylonitrile, a petroleum-derived compound. The International Agency for Research on Cancer (IARC) classifies acrylonitrile as possibly carcinogenic to humans. Acrylic is less common in performance activewear than polyester or nylon, but it appears in some fleece garments, base layers, and knit activewear.
Acrylic sheds microfibers at higher rates than polyester, according to Napper and Thompson's 2016 Marine Pollution Bulletin research. It is the highest-shedding fiber category they tested.
Rayon and Bamboo Viscose
Rayon, including bamboo viscose and modal, is produced by dissolving cellulosic plant material in chemical solvents, particularly carbon disulfide for conventional viscose. The National Institute for Occupational Safety and Health (NIOSH) has documented carbon disulfide's links to neurological and cardiovascular damage in workers with chronic occupational exposure.
The finished fabric is soft and has reasonable moisture absorption, but retains little of the plant's original chemistry. Claims about bamboo's antimicrobial properties are largely stripped out during viscose processing, as documented by Shen et al. (2010) in Fibers.
Rayon is not a plastic microfiber shedder in the way polyester and nylon are, because its fiber fragments are biodegradable rather than petroleum-derived. But the chemical manufacturing process and the gap between the plant's reputation and the fiber's reality make it worth understanding before buying.
Conventional Cotton
Conventional cotton is not synthetic and does not shed plastic microfibers. That matters, and it should not be dismissed.
The complication with conventional cotton in activewear is agricultural. Conventional cotton production is among the most pesticide-intensive in agriculture globally. The WHO estimates that cotton accounts for approximately 16% of global insecticide use. The potential for pesticide residue in conventional cotton garments has been studied, though the research on dermal absorption during exercise remains limited.
OEKO-TEX Standard 100 certified cotton is tested for residues and provides more assurance than uncertified conventional cotton.
For activewear performance, conventional cotton also absorbs moisture and holds onto it, which means it stays wet during and after exercise. This is less a safety issue and more a practical one, but it is relevant for people choosing fabric for active use.
Fabrics With a Better Profile
TENCEL Lyocell
TENCEL Lyocell is a cellulosic fiber produced from sustainably harvested wood pulp using a closed-loop solvent process. The solvent used, NMMO, is non-toxic and recovered and reused at a rate of over 99%, according to Lenzing AG's published production documentation.
TENCEL does not shed petroleum-derived plastic microfibers. Its moisture management is structural: the fiber absorbs moisture into its core rather than wicking it along the surface, which supports thermoregulation and inhibits bacterial growth. Kaplan et al. (2014), in Fibers and Polymers, documented TENCEL's performance advantages in moisture management and thermal comfort in active wear settings. The fiber carries OEKO-TEX Standard 100 and EU Ecolabel certification.
Organic Cotton (Certified)
Certified organic cotton, particularly GOTS-certified, is produced without synthetic pesticides or fertilizers. It does not shed plastic microfibers. It is a reasonable option for lower-intensity activewear where moisture management is less critical.
The performance limitation compared to TENCEL remains: it absorbs water but does not regulate it as effectively. For a yoga class, that matters less than for high-intensity interval training.
What to Actually Look For
Three things on a label are worth checking before anything else.
First, the fiber name. Polyester, nylon, acrylic, and rayon are the fibers that warrant the most scrutiny. Lyocell, TENCEL, and certified organic cotton carry a lower-risk profile based on available evidence.
Second, the certifications. OEKO-TEX Standard 100 means the finished fabric has been tested for harmful substances by an independent body. EU Ecolabel and Bluesign cover environmental production standards. These do not eliminate all risk, but they set a verified baseline that uncertified claims do not.
Third, the performance finishes. A fabric listed as polyester can carry very different risk profiles depending on whether it has a DWR coating, odor-control treatment, or stain resistance finish. Brands that disclose their chemical finishing processes are worth paying attention to.
Bellissima's Sempre Leggings are 92% TENCEL Lyocell and 8% spandex. The spandex is disclosed because full transparency on composition is the standard Bellissima holds, not a partial-ingredient list. TENCEL Lyocell is the primary fiber, and it is the reason the fabric performs the way it does without the microplastic shedding profile of polyester or nylon.
No fabric is zero-impact. The question is which trade-offs you are making, and whether you are making them with accurate information. For a deeper dive into what chemical treatments are applied on top of the fiber, toxic chemicals in workout clothes covers each compound in detail. For how to identify whether your current wardrobe has these issues, how to tell if your activewear is toxic works through the process. And for a complete evaluation framework, the non-toxic activewear buying guide and how to read activewear fabric labels cover those tools in full.
Sources
Napper, I.E., and Thompson, R.C. (2016). Release of synthetic microplastic plastic fibres from domestic washing machines. Marine Pollution Bulletin, 112(1-2).
De Falco, F., et al. (2019). Microfiber release to water, via laundering, and to air, via everyday use. Science of the Total Environment.
International Agency for Research on Cancer. (1999). Acrylonitrile. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Vol. 71.
National Institute for Occupational Safety and Health. (2023). Carbon Disulfide. CDC/NIOSH Pocket Guide to Chemical Hazards.
Shen, L., et al. (2010). A life cycle assessment of the production and use of bamboo vs. cotton textiles in China. Fibers.
Lenzing AG. (2023). TENCEL Lyocell: Production Process. Lenzing.com.
Kaplan, S., et al. (2014). Thermal comfort of lyocell and other fibers in active wear. Fibers and Polymers, 15(6).
World Health Organization. (2012). The State of the World's Land and Water Resources for Food and Agriculture. FAO/WHO.
