Spandex is the fiber that makes activewear work. Without it, leggings sag, sports bras lose compression, and waistbands roll. It is functionally necessary for fitted performance wear in a way that no natural fiber has yet replicated. The question is not whether to include it, but how much is appropriate and what that amount means for the garment against your skin.
For women building a non toxic gym clothes wardrobe, this is the question that determines whether a natural fiber garment still delivers real performance. Here is the answer.
What spandex is
Spandex, also called elastane or Lycra depending on the brand, is a synthetic polyurethane fiber. It is derived from petroleum, like polyester and nylon, and it shares the category of synthetic polymer fiber. Unlike polyester and nylon, which shed microplastic fibers at significant rates during washing, spandex is used in relatively small percentages, and its contribution to microplastic shedding is proportionally lower, though not zero.
The stretch and recovery properties of spandex are unmatched by natural fibers. A spandex fiber can be stretched to five to eight times its resting length and return to its original dimension without degradation, according to fiber property data from Invista, the manufacturer of Lycra brand spandex. This property is what makes compression activewear possible.
What percentage of spandex is typical
The percentage of spandex in activewear varies by intended function. Light compression and casual activewear typically contain 5% to 10% spandex. Performance leggings and sports bras designed for high-output training typically contain 10% to 20% spandex for consistent compression and shape recovery. Above 20%, the garment becomes primarily synthetic in its material profile, and the performance benefits of natural fiber components are diluted.
Bellissima's Sempre Leggings and bras use 8% spandex, which sits at the lower end of the performance range. At 8%, the spandex provides four-way stretch and consistent shape recovery through a workout while keeping the fiber composition predominantly TENCEL Lyocell at 92%. The skin contact surface is primarily natural fiber-derived rather than synthetic polymer.
Does the spandex percentage matter for health?
The health concern most associated with spandex in activewear is not the fiber itself but the chemical treatments sometimes applied to synthetic garments that contain spandex. PFAS-based finishes, antimicrobial chemical treatments, and other post-processing treatments are applied to the fabric as a whole rather than to the spandex fiber specifically. A garment with 8% spandex in a TENCEL Lyocell blend without chemical finishing treatments carries a meaningfully different profile than an 88% polyester, 12% spandex garment with PFAS water repellency treatment.
The spandex fiber itself, in finished garment form and at typical activewear percentages, is not documented as a primary health concern in the peer-reviewed literature. The International Journal of Environmental Research and Public Health has published research on synthetic fiber exposure during exercise, with PFAS treatments and microplastic shedding from high-percentage synthetic fabrics identified as the primary concerns rather than spandex at low percentages specifically.
The microplastic question for spandex
Spandex does shed microfibers during washing, as all fibers do to some degree. At low percentages in a blend, the absolute quantity of spandex microfibers shed is small relative to the total fiber shedding. Research by Browne et al. published in Environmental Science and Technology (2011) documented microplastic accumulation from synthetic garments, with the volume correlated to synthetic fiber content percentage. A garment that is 8% spandex and 92% TENCEL Lyocell sheds primarily cellulose-based fibers, which break down in the environment differently than synthetic polymer fibers.
What too much spandex looks like
Above 20% to 25% spandex, the garment's primary material profile becomes synthetic regardless of what the remaining fiber is. At this percentage, the skin contact surface during exercise is substantially synthetic polymer, the microplastic shedding volume during washing is higher, and the garment is more likely to carry the chemical finishing treatments associated with predominantly synthetic activewear.
A label reading 75% nylon, 25% spandex is a predominantly synthetic garment. A label reading 92% TENCEL Lyocell, 8% spandex is a predominantly natural fiber garment. Both contain spandex. The context of that spandex content is materially different.
The practical answer
For women who want non-toxic activewear with real performance capability, 5% to 15% spandex in a primarily natural or naturally derived fiber blend is the range that delivers functional compression and shape recovery while keeping the garment's primary material profile outside the synthetic category. Below 5%, stretch recovery may be insufficient for fitted performance wear. Above 15% to 20%, the spandex contribution to the material profile becomes significant enough to revisit the health and environmental considerations that motivated looking for non-toxic alternatives in the first place.
Eight percent, as used in Bellissima's Sempre line, provides performance without making the synthetic component the defining characteristic of the fabric. That is the balance the formula was designed to achieve. For more on the health profile of elastane specifically, is elastane toxic covers the peer-reviewed research. The question of whether elastane sheds microplastics is also documented in detail. And if the terminology is causing confusion, whether elastane and spandex are the same thing clarifies that. For the broader buying framework, the non-toxic activewear buying guide covers how spandex percentage fits into the full evaluation.
Sources
Invista. (2022). Lycra fiber technical properties and performance data. Invista Technical Documentation.
Browne, M.A., et al. (2011). Accumulation of microplastic on shorelines worldwide: Sources and sinks. Environmental Science and Technology, 45(21).
Lenzing AG. (2023). TENCEL Lyocell fiber sustainability and production data. Lenzing Sustainability Report.
Classifying textile fibers: synthetic elastomers. International Journal of Environmental Research and Public Health, 18(4), 2021.
