research

When we say sweat can increase how much your skin absorbs PFAS from fabric by up to 3,252×, this is what we mean.

A 2025 study published in Science of the Total Environment modelled dermal exposure to PFAS (per- and polyfluoroalkyl substances) and OPEs (organophosphate esters) in textiles, comparing dry contact against sweat-mediated contact. The exposure model found sweat increased PFAS absorption by up to 3,252× and OPE absorption by up to 835× compared to dry contact. Concentrations were highest in functional garments — sportswear and water-repellent clothing.

The study focused on children’s textiles, where the risk is highest due to body-weight-to-surface-area ratios. The mechanism — sweat as a solvent that mobilises textile-bound chemicals into the body — applies to adult activewear too, which is why we treat it as relevant.

Supporting research on dermal PFAS uptake

When we say PFAS exposure has been linked to around 2× the risk of PCOS, this is the primary source.

A 2021 Swedish cohort study followed 29,106 women in Ronneby, where one of the municipal water supplies was contaminated by firefighting foam (PFAS levels above 10,000 ng/L). Women in the highest-exposure group had approximately twice the risk of developing polycystic ovary syndrome (PCOS) compared to the unexposed group.

We use the word “around” deliberately. The 2× figure is from a population exposed to very high concentrations of PFAS via drinking water. Wearing PFAS-treated clothing is not the same exposure pathway, and we are not claiming it is. The point we are making is narrower: PFAS exposure, by any route, is associated with elevated PCOS risk, and we’d rather not contribute to it through what we make.

Supporting studies linking PFAS exposure to PCOS at lower background exposure levels

Independent testing has found PFAS in roughly 25% of activewear samples tested across major brands. PFAS are commonly applied to athletic fabrics as durable water-repellent (DWR) finishes, or appear as residues from upstream textile processing.

Mamavation is an advocacy publication, not a peer-reviewed journal. We cite it for the lab-tested PFAS content findings, not for the surrounding commentary. For a regulatory-side overview of PFAS dermal absorption from clothing, the US Environmental Protection Agency’s 2024 review covers the same ground:

When we say polyester running shorts release about 180,000 microplastic particles into the air during an hour of running, this is what we mean.

A 2020 study published in Environmental Science & Technology by researchers at the National Research Council of Italy (IPCB-CNR) and the University of Plymouth measured microfibre release from four polyester garments under two conditions: a standard 40°C wash, and a wear simulation in which volunteers in a clean lab performed a sequence of movements approximating normal activity. The wash test released up to 4,000 fibres per gram of fabric. The wear test released up to 400 fibres per gram of fabric in just 20 minutes.

Scaled to a 150g pair of running shorts worn for one hour, the wear figure works out to roughly 180,000 microplastic particles released into the air around the wearer. We use the word “roughly” deliberately. The 400-fibres-per-gram number is the upper bound from the four garments tested under simulated activity in a controlled chamber; a real run produces more sweat, more abrasion, and different airflow than the test setup, so the field value will differ. The order of magnitude — hundreds of thousands of particles per hour of vigorous wear — is what the underlying measurement supports.

The lead researcher, Professor Richard Thompson OBE of the University of Plymouth’s International Marine Litter Research Unit, summarised the finding this way: the emission of fibres while wearing clothes is of a similar order of magnitude to that from washing them, and constitutes a substantial direct release of microplastics that previous estimates had not accounted for. Earlier work had focused almost entirely on laundry as the source. This study showed wear is the other half of the picture.

The particles released during wear go primarily into the air immediately around the wearer. From there they are available to be inhaled, to settle on skin, and to be deposited in indoor environments. Research published in Scientific Reports using a breathing thermal manikin in indoor air has confirmed that synthetic fibres released from textiles are detectable in inhaled air, with polyester the dominant polymer identified (around 81% of synthetic particles in the samples tested).

We don’t claim that wearing a pair of polyester running shorts will, on its own, harm you, or give you PCOS. The research doesn’t support that, and neither do we.

What we do claim, and what the research does support, is that PFAS exposure is associated with elevated PCOS risk; that PFAS are commonly present in activewear; that sweat dramatically increases how much PFAS your skin can absorb from a treated textile; that polyester clothing releases microplastic particles directly into the air during normal wear, on a similar order of magnitude to laundry; that these particles are present in the air we breathe indoors; and that microplastics have now been identified in human blood, lungs, the placenta, and breast milk. The combination of those facts is enough for us to start with merino and organic cotton rather than with plastic.

We’ll add to this page as we go. If a claim isn’t here, we haven’t made it.

A working note on two natural-fibre options we keep coming back to.

When we set out to build a running short from natural fibres, two materials kept showing up on the shortlist: merino wool and lyocell (most often Lenzing’s Tencel branded version). Both are pitched as the sustainable answer to polyester. Both have legitimate claims. Neither is the obvious winner, and most of what’s written about the comparison is marketing copy from brands that sell one or the other. This is our attempt to lay out what we actually know, what’s well-evidenced, and where the claims get shakier.

what they are

Merino is a protein fibre — wool from Merino sheep, mostly grown in Australia and New Zealand. The fibre is crimped and has a scaly cuticle. The fineness is measured in microns; “fine” merino for next-to-skin garments typically sits between 17 and 19 microns. Coarser wool is itchy; finer wool isn’t.

Lyocell is a cellulosic fibre — regenerated from wood pulp, usually eucalyptus, using a solvent (NMMO) in a closed-loop process that recovers around 99% of the solvent for reuse. “Tencel” is Lenzing’s trademark for their lyocell. The fibre is smooth, fine, and structurally consistent in a way that natural protein fibres aren’t.

The headline trade-off: merino is a natural fibre that an animal grows; lyocell is a manufactured fibre derived from a plant. Both biodegrade. Neither is petrochemical.

moisture management

This is where the marketing copy gets loudest, and the evidence gets murkier.

The most-repeated claim is that lyocell wicks and dries roughly three times faster than merino. This number traces back to Lenzing’s own technical literature and shows up in copy from Icebreaker’s Cool-Lite range, Western Rise, and most lyocell-blend brands. We haven’t been able to find an independent peer-reviewed study that confirms the “3×” figure cleanly — it appears to come from Lenzing’s internal lab testing of specific yarn and fabric constructions. Treat it as directionally true (lyocell does release liquid moisture faster than wool) but specific to the fabrics tested.

What’s better established in the published literature:

• —Lyocell is highly absorbent for liquid moisture and releases it quickly. Studies on lyocell-blend fabrics consistently find higher water vapour permeability, absorbency, and wickability for lyocell-rich blends compared to other natural fibres (Aniş et al., Journal of the Textile Institute, 2020).
• —Merino’s moisture management story is different. It manages vapour very well — the fibre absorbs up to about a third of its own weight in moisture without feeling wet, and releases it slowly into the surrounding air. This is the “thermoregulation” claim (The Woolmark Company research on thermoregulation).

In running terms: lyocell can pull liquid sweat off the skin and move it through the fabric faster. Merino is better at buffering humidity at the skin and keeping the wearer comfortable as conditions shift — colder before a run, warmer mid-run, cool again at the end.

A short for a 5km tempo run on a humid day will probably feel drier in a lyocell-rich fabric. A short worn from a cool morning warm-up through a long run isn’t an obvious win for either.

odour

This is one of merino’s biggest marketing claims, and it’s also where the picture is more nuanced than the brand websites suggest.

The story most people know — that wool resists odour better than synthetics — is well supported. The Woolmark Company cites volunteer wear-trial research from AgResearch (NZ) and SIFO (Norway) finding that polyester base layers needed washing far more often than wool, with cotton in between. IWTO summarises the same study and the broader case for wool’s odour resistance.

But the picture gets more complicated when you look at the wider peer-reviewed literature on fibre and odour:

• —McQueen et al. (2007), in Textile Research Journal, found that polyester rated highest in odour intensity, with cotton and wool rating mid-to-low — broadly consistent with the wool-industry story.
• —But more recent work from McQueen’s group, summarised by the University of Alberta in 2024, found that cellulosic fibres (cotton and viscose) absorb and re-release smaller amounts of odour-causing compounds than polyester, nylon, or wool. The fibre-chemistry mechanism — how readily the fibre takes up oily odorants from sweat — sits separately from the bacterial-growth mechanism.
• —Šala et al. (2022), in Polymers, found that wool doesn’t actually kill or inhibit bacterial growth; bacteria attach firmly to wool fibres and form a biofilm, which can give the appearance of “antibacterial” behaviour in standard tests but is a different mechanism than what people imagine.

So: wool legitimately stays fresher than polyester in a multi-day-wear test. The often-repeated “merino is naturally antimicrobial” framing oversimplifies what’s actually happening — the fibre isn’t sterilising bacteria, it’s managing moisture and binding odorants in a way that delays the build-up most people notice. Lyocell’s odour performance, where it’s been tested directly, appears to sit closer to other cellulosics — competitive on odour through good moisture management rather than any antimicrobial mechanism.

For a running short specifically, the odour question is less critical than it is for a base layer or a hiking shirt — most runners wash their shorts after every wear, which means the gap between fibres in real-world use is narrower than the multi-day wear studies suggest. Both merino and lyocell will be fresher than polyester between washes; the difference between the two for a single-wear garment is small.

durability

This is where the marketing copy flips. The brands selling lyocell or lyocell-blend activewear (Tripulse, Albert Kreuz) lean hard on lyocell being more robust than wool — better tensile strength, less prone to thin spots and pinholes after repeated wear and washing. That’s largely true for the raw fibre: lyocell has higher tensile strength than wool, and lyocell is actually stronger when wet, which is unusual.

In practice, 100% merino garments — particularly fine-gauge knits at low GSM — are known to develop holes and thin spots faster than people expect, especially at high-abrasion zones. This is one of the reasons most performance merino brands (Icebreaker, Smartwool, Ortovox) blend their fabrics with nylon or polyester, accepting a small synthetic compromise in exchange for a garment that lasts twice as long.

A 100% merino running short with no synthetic blend will be more vulnerable to abrasion and washing fatigue than a lyocell or lyocell-blend equivalent at the same weight. This is a real consideration we’re weighing in our own product development.

sustainability

Both fibres have legitimate sustainability stories. Both also have weak points that don’t make it into the marketing.

Lyocell’s case: The closed-loop NMMO process recovers around 99% of the solvent. Eucalyptus and bamboo (the typical feedstocks) grow on marginal land, with minimal irrigation, and far faster than conventional cotton crops. A comparative LCA published in The International Journal of Life Cycle Assessment (2021) found lyocell has lower environmental impact than viscose across most categories. Lyocell is biodegradable.

Lyocell’s weak points: It’s still an industrial process — energy-intensive, requiring chemical solvents (even if mostly recovered), and dependent on the energy mix where it’s manufactured. Most Tencel comes from Lenzing’s plants in Austria and other locations with relatively clean grids; lyocell from regions with coal-heavy grids has a meaningfully larger footprint. End-of-fibre traceability is limited.

Merino’s case: Wool is a renewable annual crop on the sheep’s back. Certified under the Responsible Wool Standard (RWS), wool farms are audited for sheep welfare and land management. Wool biodegrades in soil within months. Garments tend to be worn longer and washed less, which dramatically reduces lifetime impact.

Merino’s weak points: Sheep produce methane. Wool’s per-kg carbon footprint at the farm gate is higher than most plant-based fibres before any of wool’s in-use advantages (less washing, longer wear) are counted. Land use per kilogram of fibre is high. Mulesing remains a contested practice; RWS-certified and NZ ZQ-certified wool prohibits it, but un-certified Australian wool can still come from mulesed flocks.

The honest answer is that the two fibres are roughly comparable at the cradle-to-gate stage, with merino’s footprint dominated by farming and lyocell’s dominated by manufacturing energy. Over the full life of a garment — including wear, washing, and end-of-life — merino’s lower washing frequency and slower wear-out (when blended) tip the balance back. This is why almost every credible sustainability comparison ends with “it depends on the specific product, the specific supply chain, and how the garment is used.”

where we’ve landed

For our current short, we’re building with merino, not lyocell.

We want to be honest about how this trades off. Running shorts aren’t a multi-day-wear garment. Most runners peel them off sweaty and they go straight in the wash, which means merino’s strongest argument — staying fresh across days of wear without laundering — barely applies here. The case for merino in a running short rests on different ground: feel against skin during the run, vapour buffering as body temperature shifts from warm-up to mid-run to cool-down, and the per-garment sustainability profile (no microplastic shedding into waterways with every wash, biodegradable at end of life).

Lyocell would win on pure liquid-sweat throughput. We’ve chosen merino because we think the rest of the picture matters more for the kind of running this short is built for — anything from an easy 5k to a long Sunday run, in mixed conditions, where comfort across changing body temperatures beats peak cooling under continuous heavy sweat.

That said, we’re actively interested in a merino/lyocell blend for a future style — most likely a hot-weather race short where the priority shifts from comfort across temperature changes to maximum cooling under continuous high sweat output. If we go that direction, it’ll be because the fabric is genuinely better for that use, not because the blend is fashionable.

We’ll update this page as we test more fabrics and as the research evolves.

Further reading

• —The Woolmark Company — Wool research and thermoregulation
• —IWTO — The link between wool and odour resistance
• —McQueen Textile Research Group, University of Alberta — Odour in Textiles publication list
• —Comparison of life cycle assessment between lyocell fiber and viscose fiber in China — Int. J. LCA, 2021
• —Antibacterial properties of non-modified wool — Polymers, 2022
• —Aniş et al. — Thermophysiological comfort of silk/lyocell blended fabrics