Sweating during exercise does two things simultaneously. It cools the body through evaporation, and it creates conditions that accelerate the release of microplastic fibers and chemical compounds from synthetic clothing toward the skin.
The second part is less discussed. It should not be.
How Sweat Changes the Fabric-Skin Interface
When you sweat, the microclimate between your skin and your clothing shifts. Skin temperature rises. Pores open. A thin layer of moisture accumulates at the skin surface, held in place by the fabric pressed against it. For synthetic workout clothes, which are hydrophobic and do not absorb that moisture into the fiber, the sweat stays at the interface rather than being drawn away.
That warm, moist interface is a more favorable environment for both microplastic fiber adhesion and chemical leaching than dry, cool skin. Research published in the journal Environmental Science and Technology Letters documented that elevated temperature and humidity accelerate the release of chemical compounds from synthetic textiles, with sweat-simulating solutions extracting higher concentrations of chemical residues from fabric than dry contact alone.
The implication is that the period of highest chemical and microplastic exposure from workout clothes is not before or after exercise. It is during it, when sweat is active and the fabric-skin interface is at its warmest and most humid.
Sweat as a Vector for Chemical Absorption
Synthetic fabrics carry chemical residues from manufacturing and finishing processes, including PFAS from durable water repellent treatments, phthalates from polymer production, and formaldehyde from anti-wrinkle finishes. These compounds are not bonded permanently into the fabric. They migrate out over time, and the rate of migration increases with heat and moisture.
A 2021 analysis in Sustainability examining transdermal absorption of microplastics and chemical compounds from synthetic textiles noted that exercise-specific conditions, sweating and elevated skin temperature, create more favorable conditions for transdermal absorption than resting conditions. The skin's permeability is not fixed. It is higher when the body is warm and the skin surface is moist.
This is the same mechanism that makes the Lululemon PFAS investigation particularly relevant to people who actually wear activewear to exercise rather than just own it. The Texas Attorney General's 2026 investigation into Lululemon cited potential PFAS presence in activewear marketed to health-conscious consumers. The exposure pathway that makes PFAS in workout clothes more than a theoretical concern is exactly what happens during exercise: sweat-mediated contact between PFAS-treated synthetic fabric and permeable skin.
Microfiber Release During Sweating
Sweating also affects microplastic fiber release. Mechanical friction between fabric and skin during exercise is a documented shedding pathway for synthetic microfibers, as established in research by Napper and Thompson (2016) in Marine Pollution Bulletin. The addition of moisture at the skin-fabric interface changes the adhesion dynamics between shed fibers and the skin surface, potentially increasing the fraction of released fibers that remain in contact with skin rather than falling away.
Pirc et al. (2016), writing in Environmental Science and Pollution Research, documented microfiber release from synthetic fabrics during mechanical abrasion that simulates wear. In exercise conditions, that mechanical abrasion is constant and occurs against a skin surface that is more receptive to fiber adhesion than dry skin.
What the Body Does with What It Absorbs
Leslie et al. (2022), in Environment International, detected microplastic particles in human blood, confirming that systemic absorption of plastic particles is occurring across the general population. The specific contribution of exercise-related clothing exposure to that body burden has not been isolated in research. What has been established is that the absorption is real and systemic, and that the conditions of exercise, elevated temperature, open pores, active sweating, are among the most favorable conditions for transdermal absorption of both particles and chemical compounds.
A Different Starting Point
The sweat-mediated release problem is specific to synthetic fabrics with chemical finishing histories and plastic polymer fiber structures. TENCEL Lyocell does not have a petroleum-derived polymer base. It is a cellulosic fiber that absorbs moisture into its core rather than leaving it at the fabric surface. The conditions that drive sweat-mediated chemical leaching from synthetic fabrics, sustained surface moisture against warm skin, are reduced by TENCEL's structural moisture management.
Kaplan et al. (2014), in Fibers and Polymers, documented TENCEL's superior moisture regulation in active wear settings. The Sempre Leggings are 92% TENCEL Lyocell. The fiber draws sweat away from the skin rather than allowing it to accumulate at the interface where chemical and microplastic transfer is most likely to occur.
The Takeaway
Sweat does not just make your workout clothes wet. It actively changes the conditions at the skin-fabric interface in ways that increase the transfer of chemicals and microplastic fibers from synthetic fabric to skin. That process is most active during the part of the day most people assume they are doing something good for their bodies.
Choosing what your workout clothes are made of is the most direct lever available for reducing that exposure. The fiber is not a minor detail. It is the whole variable.
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).
Pirc, U., et al. (2016). Emissions of microplastic fibers from microfiber fleece during domestic washing. Environmental Science and Pollution Research.
Leslie, H.A., et al. (2022). Discovery and quantification of plastic particle pollution in human blood. Environment International, 163.
Texas Attorney General. (2026, April 13). Attorney General Ken Paxton Launches Investigation into Lululemon Over Potential Presence of Toxic "Forever Chemicals" in Activewear. texasattorneygeneral.gov.
Kaplan, S., et al. (2014). Thermal comfort of lyocell and other fibers in active wear. Fibers and Polymers, 15(6).
