For a long time, microplastics were treated primarily as an environmental problem: something that accumulated in oceans, entered the food chain, and harmed marine life. The research has moved well past that frame.
Microplastics are now documented in human blood, lung tissue, and placental tissue. And a growing body of research is examining not just their presence in the body, but what they do once they are there, including how they interact with the endocrine system. For those evaluating non-toxic athletic wear, this research is directly relevant to what touches your body during exercise.
Two Mechanisms: Particles and Chemical Leachates
The relationship between microplastics and hormones operates through two distinct mechanisms that are worth separating.
The first is the particles themselves. Plastic polymers are not biologically inert. Research has shown that microplastic particles can interact with hormone receptors and transport proteins depending on their size, shape, and surface chemistry. A review published in Environmental Research (Campanale, C., et al., 2020) examined the endocrine-disrupting potential of microplastic particles across available in vitro and in vivo studies and found evidence of hormonal interference at concentrations relevant to documented human exposure levels.
The second mechanism is chemical leaching. Plastics are not pure polymer. They contain additives, including plasticizers, stabilizers, flame retardants, and colorants, many of which are known endocrine disruptors. Phthalates, used as plasticizers in many synthetic polymers, are among the most studied. Bisphenol A (BPA) and its replacements are others. These compounds leach from plastic particles as they degrade, and they can do so within the body after absorption. The National Toxicology Program has concluded that several phthalates are reproductive toxicants. Human epidemiological studies have associated phthalate metabolite levels with disruptions to estrogen, testosterone, and thyroid hormone function.
What the Human Tissue Research Shows
The most consequential recent findings on microplastics and health are the human tissue studies. Leslie et al. (2022), in Environment International, detected microplastics in human blood in 77% of samples tested, with polyethylene terephthalate as the most common particle type. Ragusa et al. (2021), also in Environment International, found microplastic particles in human placental tissue, raising questions about fetal exposure during pregnancy.
A 2024 study published in the New England Journal of Medicine (Marfella, R., et al.) found microplastic and nanoplastic particles in carotid artery plaque samples and associated their presence with significantly higher rates of heart attack, stroke, and all-cause mortality over a follow-up period. While this study focused on cardiovascular outcomes rather than hormonal ones, it established that microplastics in human tissue have measurable pathological associations, not just presence.
The hormonal research is less settled than the presence data but is moving in a consistent direction. The endocrine system is sensitive to chemical signals at very low concentrations, which makes it a plausible target for the kinds of interference that microplastic particles and their chemical leachates have demonstrated in laboratory settings.
Why Activewear Is a Relevant Exposure Source
The connection between activewear and microplastic hormone exposure runs through two pathways.
First, synthetic activewear sheds microplastic fibers during wear and washing. Those fibers enter the body through inhalation, skin contact, and ingestion. During exercise, when inhalation rate is elevated and skin permeability is increased by sweating and heat, both pathways are more active than at rest. The gym environment compounds this further, as covered in the research on microplastics in the gym.
Second, synthetic activewear contains the same classes of chemical additives that are associated with endocrine disruption in microplastic research. Phthalates are present in synthetic polymer manufacturing. PFAS, documented in activewear including through the Texas Attorney General's 2026 investigation into Lululemon, are endocrine disruptors with documented thyroid and immune system effects according to the EPA. The skin contact pathway during exercise, with elevated permeability, puts these compounds in contact with the body under conditions that favor absorption.
What a Lower-Exposure Choice Changes
Reducing microplastic and chemical exposure from activewear starts with the base fiber. TENCEL Lyocell is a cellulosic fiber with no petroleum-derived polymer structure, no phthalate plasticizers, and no plastic microfiber shedding profile. It does not carry the endocrine-disrupting chemical additives that are inherent to synthetic polymer manufacturing.
Kaplan et al. (2014), in Fibers and Polymers, documented TENCEL's moisture management advantages in active wear, including reduced surface moisture during exercise, which also reduces the sweat-mediated chemical migration pathway that makes synthetic activewear a higher-exposure garment category. The Sempre Leggings are 92% TENCEL Lyocell and 8% spandex. The spandex is synthetic and disclosed. It is not the primary fiber, and it does not define the chemical profile of the garment.
Where the Research Is Headed
The science on microplastics and hormones is not complete. The human studies documenting presence are stronger than the studies documenting specific hormonal effects at real-world exposure concentrations. That gap will narrow as research accumulates.
What is already established is that microplastics are in human tissue, that the chemical additives in plastic carry documented endocrine-disrupting mechanisms, and that synthetic activewear is a meaningful and modifiable source of daily exposure. The precautionary case for reducing that exposure does not require waiting for a complete mechanistic picture. For the broader health picture, the research on microplastics and women's health covers the reproductive and systemic findings most relevant to female wearers, and the evidence on microplastics and fertility examines those implications directly. For practical steps, how to reduce microplastic exposure outlines the highest-leverage changes.
Sources
Campanale, C., et al. (2020). A detailed review study on potential effects of microplastics and additives of concern on human health. International Journal of Environmental Research and Public Health, 17(4).
Leslie, H.A., et al. (2022). Discovery and quantification of plastic particle pollution in human blood. Environment International, 163.
Ragusa, A., et al. (2021). Plasticenta: First evidence of microplastics in human placenta. Environment International, 146.
Marfella, R., et al. (2024). Microplastics and nanoplastics in atheromas and cardiovascular events. New England Journal of Medicine, 390(10).
National Toxicology Program. (2012). NTP Monograph: Phthalates. US Department of Health and Human Services.
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.
U.S. Environmental Protection Agency. (2024). Our Current Understanding of the Human Health and Environmental Risks of PFAS. EPA.gov.
Kaplan, S., et al. (2014). Thermal comfort of lyocell and other fibers in active wear. Fibers and Polymers, 15(6).
