What's in Your Kid's Sports Underwear?

This report started because of a question neither of us could answer.

We've spent our working lives in baseball and softball training. Mike has run a training facility for over 18 years. Mark has coached thousands of youth players through Applied Vision Baseball and Art of Baseball. We've watched kids grow up from eight-year-olds to college commits. We've also watched ourselves and our peers grow older.

A few years ago, something started bothering us. A lot of our former teammates, a lot of adults we came up playing ball with, were dealing with things that didn't seem to affect our fathers' generation at the same ages or in the same numbers. Fertility issues. Low testosterone diagnoses in their 30s and 40s. Hormone imbalances. Cancers nobody saw coming.

We're not doctors. We weren't looking to diagnose anything. But we started reading.

What we found made us uncomfortable. Peer-reviewed research, published in respected journals, linking long-term exposure to a specific family of chemicals to the exact kinds of health issues showing up in our generation. And those chemicals were turning up in a place we hadn't thought to check: athletic apparel. Including the underwear worn every day by the kids we coach.

So we did what any reasonable person would do once armed with that information. We set out to build an underwear product that didn't contain any of it. Two years later, we did. And we sent it to SGS, one of the largest independent testing organizations in the world, for verification.

This report is the research case behind that work. It's written in plain English for parents, but it cites real peer-reviewed studies, real regulatory findings, real certifications, and real independent lab test results. Where we make a claim, we cite the source. Where we cannot claim something with confidence, we say so.

A note on what this report is and isn't.

It is a plain-English synthesis of peer-reviewed research, independent lab testing, regulatory findings, and publicly available industry data. It cites real sources. It shows real results.

It is not medical advice. It is not a diagnosis. It is not a claim that any specific product has caused any specific harm to any specific person. The relationship between chemical exposure and human health is complex, and responsible researchers are careful about that complexity. We are too.

What we can say with confidence is this: the research is real, the exposure pathway is real, the chemicals are real, the industry-wide compositions are real, and parents deserve to know enough to make their own decisions.

Read this at your own pace. Share it freely. Forward it to coaches, to other parents, to your pediatrician, to your group chat. The only thing we ask is that you don't edit or excerpt it in ways that distort the findings.

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— Mark & Mike

Executive Summary.

For parents who want the 60-second version of what's in their kid's sports underwear:

01The category default is plastic.

Almost all conventional and performance kids' sports underwear is made from synthetic materials, primarily polyester, nylon, elastane, and spandex, which are plastic-derived fabrics manufactured using a family of chemicals linked by peer-reviewed research to hormone disruption, reduced testosterone, fertility issues, and elevated cancer risk. This is not a niche concern. It is the default composition of the youth athletic apparel category.

02"Natural" marketing usually has a synthetic catch.

Even briefs marketed as "organic" or "non-toxic" typically contain synthetic content. The most common failure point is the waistband or leg openings, where 5% to 20% synthetic fiber (typically nylon, elastane, or spandex) is placed at the exact zone of maximum skin contact and friction.

03Sports multiply chemical exposure. They don't just add it.

Heat from physical activity unlocks chemical bonds in plastic fabrics. Sweat opens the skin's pores, creating absorption channels. Friction creates microscopic fabric tears that release microplastics and skin abrasion that increases absorption. For a youth athlete wearing synthetic briefs 6 to 10 hours per week, the result is chronic, stacking exposure during the developmental years when hormones are forming.

04The skin inside underwear is among the most absorptive on the body.

This is not a metaphor. A foundational 1967 dermatological study by Feldmann and Maibach demonstrated that chemicals applied to the scrotum absorbed at approximately 42 times the rate of the forearm, a finding refined and consistently confirmed in subsequent research.

05Children are physiologically more vulnerable than adults to chemicals in clothing.

They have thinner skin, higher skin-surface-area per body-weight ratio, developing hormone systems, immature detoxification pathways, and longer lifespans over which effects can manifest. The Endocrine Society and the National Academies of Sciences, Engineering, and Medicine have both formally acknowledged these amplified pediatric vulnerabilities.

06The hormone-decline trends are already visible at the population level.

Testosterone in young American males has dropped approximately 40% in 40 years. Sperm counts in Western men dropped more than 50% in five decades. These declines are documented across multiple peer-reviewed longitudinal studies and are consistent with what endocrinologists would predict from sustained population-wide exposure to endocrine-disrupting chemicals like those used in synthetic apparel.

07"Forever chemicals" (PFAS) are pervasive in apparel testing.

A 2023 study by Arnika, IPEN, and 13 partner organizations tested 72 samples from 13 countries and found PFAS in 64% of them. Separate investigations by Toxic-Free Future, Silent Spring Institute, and Mamavation have repeatedly found PFAS in popular activewear brands, including items marketed as "eco" or "non-toxic."

08The Attack Briefs lab results are documented.

The Attack Sports Performance Briefs have been independently tested by SGS under Report SL72507344844001TX dated October 28, 2025. The results follow.

09Parents can act without buying any specific brand.

Any underwear that is 100% natural fiber (certified organic merino wool, certified organic cotton, or hemp) is a meaningful upgrade over synthetic alternatives. Checking the waistband composition is the single most effective additional screening step.

The remainder of this document provides the research, regulatory findings, independent test results, and action steps behind each of these statements.

1.1The quiet takeover of synthetic apparel.

The materials kids wear to practice every day did not always look the way they do now.

Until roughly the 1960s, athletic wear was dominated by natural fibers. Cotton dominated base layers and undergarments. Wool was standard for colder-weather athletics. These materials had limitations, but they did not shed microplastics, did not contain endocrine-disrupting plasticizers, and did not require chemical treatments to function.

Beginning in the late 1960s and accelerating through the 1970s and 1980s, the apparel industry shifted rapidly toward synthetic fibers. Polyester. Nylon. Elastane (spandex). These materials are petroleum-derived plastics, manufactured through chemical processes that require additives, softeners, stabilizers, and dyes. The shift happened because synthetics are cheaper to produce at scale, easier to dye in bright colors, lighter than many natural fibers, and compatible with performance coatings.

Consumer athletic wear followed the cost curve. By the 2000s, the overwhelming majority of youth sports apparel was synthetic. By the 2020s, it's effectively universal. Walk into any sporting goods store and read the fabric composition labels. The rare natural-fiber option is usually a specialty product at three to five times the price of the default.

This is the context in which every child growing up today is dressed for their sport. It is also the context in which the chemical exposure described in this report is occurring, continuously, during the most biologically sensitive years of their development.

1.2The chemical family of concern.

The chemicals most consistently associated with health concerns in synthetic kids' sports underwear and youth athletic apparel fall into five main categories:

Each chemical family has its own peer-reviewed research literature. Each is present in varying amounts in conventional synthetic athletic apparel. Sections 4 through 6 address each in detail.

1.3Why this matters now.

Youth sports participation in America is at record levels. More children than ever are playing competitive sports. Travel ball programs, tournament schedules, and year-round training have extended the weekly hours a typical young athlete spends in sports-specific apparel.

At the same time, markers of male reproductive health have been in measurable decline:

• Testosterone in young American men is down approximately 40% over 40 years (Travison et al., 2007, and subsequent replications).
• Sperm counts in Western men have dropped more than 50% in the last five decades (Levine et al., 2017; Levine et al., 2022).
• Puberty timing has shifted earlier in girls and later in boys, consistent with endocrine disruption patterns (Lee et al., 2019; Fudvoye et al., 2019).
• Testicular cancer incidence has roughly doubled in young men since the 1970s in most industrialized countries.
• Anogenital distance in male infants (a biomarker of fetal androgenization) has decreased in populations with higher maternal phthalate exposure (Swan et al., 2005, and replicated in subsequent cohorts).

No single cause. But chronic exposure to endocrine-disrupting chemicals is repeatedly cited as a significant factor.

The parent reading this report has no control over many of the contributing factors. What you can control is what your child wears.

This section describes what the kids' sports underwear market actually looks like, based on publicly available fabric composition information from manufacturer product pages and published industry testing by third-party organizations.

We did not conduct chemical testing on competitor products. What follows is aggregate analysis of what the category actually uses, drawn from information the manufacturers publish themselves.

2.1Mass-market cotton-blend boxer briefs.

The default option in nearly every retail underwear aisle. Price points usually $2 to $4 per pair in multi-packs.

Typical body composition, based on publicly listed manufacturer data:

• Solid colors: 95% cotton / 5% spandex (or elastane)
• Heather variants: approximately 57% cotton / 38% polyester / 5% spandex (polyester added for the dye effect)
• Fly and fly facing: typically 91% cotton / 5% polyester / 4% spandex mesh

Typical waistband composition.

Rarely disclosed in detail on consumer-facing packaging. Industry-standard waistbands in this category are constructed of woven polyester and elastane, sometimes with a nylon component. Waistbands are where undeclared synthetic content is most commonly concentrated.

Even the "cotton" options in mass-market boxer briefs contain 5% or more synthetic fiber in the body fabric. The waistband composition is typically more synthetic than the body. This is not a criticism of any specific brand. It is a description of what the category is.

2.2Performance and athletic-branded boxer briefs.

Marketed as moisture-wicking, quick-drying, "micro-stretch," "performance," or similar language. Price points $8 to $20 per pair.

Typical body composition, based on publicly listed manufacturer data:

• "Micro-stretch" and moisture-wicking styles: typically 90% polyester / 10% spandex
• Compression or high-performance blends: 83% polyester / 17% spandex, or 92% recycled polyester / 8% spandex
• Hybrid designs: polyester / modal / spandex blends, or polyester / cotton / spandex

Performance coatings.

Moisture-wicking and quick-dry fabrics frequently use fluorinated chemistry (PFAS) to achieve their performance claims. Independent industry testing has repeatedly found PFAS in performance activewear:

• The 2023 Arnika/IPEN study tested 72 outdoor and clothing samples from 13 countries and found PFAS in 46 of them (63.8%), including items marketed for children. PFOA, a PFAS chemical globally banned under the Stockholm Convention, was among the most commonly detected compounds.
• Toxic-Free Future reported that 72% of outdoor apparel and furnishings labeled as stain- or water-resistant contained detectable PFAS.
• Silent Spring Institute research found PFAS in more than 60% of apparel labeled "eco-green," "non-toxic," "waterproof," or "stain-resistant."
• Mamavation investigations have found PFAS indicators in roughly 25% of tested activewear items from popular brands.

2.3Youth athletic underwear with integrated cup or compression.

Marketed specifically for baseball, football, lacrosse, hockey, and other contact sports. Often sold alongside protective cup systems. Price points $15 to $35 per pair.

Typical body composition.

• Compression polyester / spandex blends, often 90% polyester / 10% spandex or higher spandex content
• Some include nylon components for stretch or support elements
• Pouch and cup-holder designs frequently use synthetic composite materials

These products are worn specifically during athletic competition, which maximizes heat, sweat, and friction exposure (see Section 5). They're designed to sit tight against the skin for extended periods (full game or practice). The combination of composition, usage duration, and anatomical position makes them the highest chemical-exposure underwear product category in the youth market.

2.4"Clean" and "natural" marketed briefs.

Marketed with claims like "organic," "natural," "non-toxic," "plastic-free," or "chemical-free." Price points $15 to $40 per pair.

Typical body composition.

• Most common: 95% organic cotton / 5% elastane
• Premium merino-based: approximately 84% merino wool / 12% nylon / 4% spandex
• Hemp-based: 53% hemp / 44% organic cotton / 3% spandex
• Bamboo/viscose variants: 95% viscose / 5% spandex

The main body fabric is usually genuinely a natural fiber, often GOTS-certified. The marketing claims are usually defensible on that basis. But there's typically 3% to 20% synthetic content somewhere in the construction, and that synthetic content is disproportionately concentrated in the waistband and leg openings.

Parents paying the premium for "clean" options often get a partial upgrade over mass-market synthetics, the body fabric is better, but the waistband problem is almost universal. Section 7 addresses why nylon waistbands are the most concerning element of this pattern.

2.5Fully synthetic-free briefs.

A narrow and rare category. Defined as: body fabric is 100% certified natural fiber (no blended synthetics, not even small percentages), and waistband is made of a natural material such as natural rubber wrapped in organic cotton.

2.6The industry reality.

None of the analysis above requires specialized knowledge or testing to verify. A parent with a product label and five minutes can replicate the category analysis above for any brand they're considering. The reason it reads as surprising is that the category has not, historically, made it easy for parents to understand what they're actually buying.

The Lab-Tested Alternative

Attack Sports Performance Briefs →

100% GOTS-certified organic merino wool. Natural rubber waistbands. PFAS, BPA, and phthalates: not detected by SGS.

3.1Certification overview.

This section documents the independent third-party testing of the Attack Sports Performance Briefs fabric. The original report is available in full to any parent, researcher, journalist, or regulatory authority upon request.

SGS is one of the world's largest testing, inspection, and certification organizations. Their chemical testing methods reference international standards (ISO, AATCC, AFIRM, EN). We did not influence which tests were run or how they were interpreted.

3.2Fiber content verification.

Test method: AATCC TM20-2021 (standard test method for fiber analysis).

The fiber composition claimed on the product label was independently verified. This is not always the case in textile testing; undeclared synthetic blends are a common finding when labs verify consumer-facing fabric claims.

3.3Bisphenol testing: BPA and BPS.

Test method: AFIRM RSL 2025 protocol using solvent extraction with LC-DAD/MS and LC-MS/MS analysis.

"Not Detected" means no measurable BPA or BPS was present above the lab's 0.10 mg/kg reporting limit, which is 100 times below the client requirement for BPA and 2,000 times below the client requirement for BPS.

3.4Phthalate screening: 12 compounds.

Test method: ISO 14389:2022 using GC-MS analysis. Reporting limit: 0.005% for each compound.

3.5Total Fluorine: the PFAS indicator.

Test method: EN 14582:2016 using Ion Chromatography (IC).

Total Fluorine is the screening test used to detect the presence of PFAS. PFAS compounds are defined by their carbon-fluorine bonds (a bond type rarely found in nature), so detectable organic fluorine in a consumer textile is a strong indicator of PFAS. "Not Detected" at a 20 mg/kg reporting limit means no detectable fluorine was present, which indicates no PFAS.

3.6Fiber fragment release during home laundering.

Test method: AATCC TM212-2021 using AATCC accelerated laundering (Option B: water only) with oven-dried specimens and filters. Four specimens tested.

3.7Microplastics from textile sources: ISO method.

Test method: ISO 4484-1:2023, the international standard for determining material loss from fabrics during washing.

3.8A critical interpretation note.

Both methods measure fiber fragment mass during simulated washing. They were developed to quantify microplastic release from synthetic textiles. For Attack Briefs, what's released is biodegradable wool. For a synthetic garment, the same mass is plastic.

For Attack Briefs, the mass released during these tests represents natural merino wool fibers, not microplastic particles. Wool is a biodegradable protein fiber (keratin) that breaks down in the environment within months to a few years. A synthetic garment tested under these same methods would release the same mass as petroleum-derived plastic microfibers that persist in the environment for decades to centuries, enter water systems, and accumulate in the human food chain.

This is not a rhetorical point. The mass number is similar. What's released is fundamentally different. It is the distinction between shedding compostable hair and shedding plastic particles.

For reference: microplastic release from synthetic textiles during home laundering is the subject of growing regulatory concern in the European Union and California, with France having passed legislation (Law No. 2025-188) restricting PFAS and related synthetic textile releases beginning January 2026.

3.9Certifications on file.

Attackletics maintains the following documentation and will provide copies to any parent, researcher, journalist, or regulatory authority who requests them:

• GOTS (Global Organic Textile Standard) certification for the merino wool source
• OEKO-TEX Standard 100 certification for the dyes used in colored variants
• Full SGS Report SL72507344844001TX (the report documented in this section)
• Natural rubber sourcing documentation for the waistband material

4.1Phthalates in apparel.

What they are. A family of chemical compounds used as plasticizers (softeners) in synthetic materials. Common in vinyl products, soft plastics, fabric softeners, and the synthetic fabric and dye processes used in most athletic apparel. Common ones in apparel: DEHP, DBP, BBP, DINP, and related compounds.

What phthalates do to the body.

Phthalates are well-documented anti-androgens. They interfere directly with the body's ability to produce and use testosterone.

The mechanism is specific. Phthalates disrupt the StAR (Steroidogenic Acute Regulatory) protein and the CYP17 enzyme pathway. Both are essential components of how the Leydig cells in the testes synthesize testosterone from cholesterol. When these pathways are disrupted, testosterone production drops.

Peer-reviewed research consistently finds:

• Lower serum testosterone in adolescents and adults with higher measured phthalate metabolite levels in urine (Meeker and Ferguson, 2014, analyzing NHANES data).
• Reduced fetal testosterone development and shortened anogenital distance (a biomarker of androgenization) in male infants whose mothers had higher phthalate exposure during pregnancy (Swan et al., 2005).
• A Danish study that measured phthalate metabolites in maternal breast milk found significant decreases in serum measures of free testosterone and Leydig cell function in 3-month-old boys.
• A dose-response relationship across studies, which strengthens the causal inference (Radke et al., 2018, systematic review of human epidemiological evidence).

Regulatory context.

The U.S. has banned six specific phthalates from children's toys and childcare articles at a threshold of 0.1% by weight (Consumer Product Safety Improvement Act). No equivalent restriction applies to children's apparel.

4.2Bisphenol A (BPA) and related bisphenols.

What they are. A family of industrial chemicals used in the production of plastics, epoxy resins, and certain dye and fabric treatment processes. The most-studied is BPA. Industry has moved partially toward BPS and BPF as substitutes, but emerging research suggests similar concerns.

What BPA does to the body.

BPA is an endocrine disruptor that acts through two main mechanisms:

1. Estrogen mimicry. BPA's molecular shape allows it to bind to estrogen receptors in the body and produce estrogen-like effects. In males, this means more estrogen signaling than is biologically appropriate, with downstream effects on testosterone balance, sperm production, and secondary sex characteristics.

2. Androgen receptor blockage. BPA also binds to androgen receptors and blocks them. Even if testosterone is present at normal levels, it can't do its job if the receptors it needs to bind to are occupied.

Peer-reviewed research findings:

• Leydig cell studies consistently show decreased testosterone synthesis following BPA exposure through disruption of steroidogenic enzyme expression and cholesterol transport (Rochester, 2013, comprehensive literature review).
• Animal studies demonstrate impaired male fertility, reduced sperm count, and altered sexual development in offspring following maternal BPA exposure.
• A study by Meeker et al. (2010) found associations between urinary BPA levels and reduced semen quality and increased sperm DNA damage in men attending an infertility clinic.
• According to the CDC's National Report on Human Exposure to Environmental Chemicals, nearly all Americans age 6 and older have detectable levels of BPA, phthalate metabolites, and other common endocrine-disrupting chemicals in their urine.

The "BPA-free" catch. Many products now labeled "BPA-free" have simply substituted BPS or BPF, which emerging research suggests have similar or in some cases stronger endocrine-disrupting activity. "BPA-free" does not mean bisphenol-free unless the manufacturer specifies.

4.3PFAS: Per- and Polyfluoroalkyl Substances ("forever chemicals").

What they are. A family of more than 10,000 synthetic chemicals characterized by extremely strong carbon-fluorine bonds, which makes them resist water, oil, stains, and environmental degradation. They are called "forever chemicals" because they do not meaningfully break down in the environment or in the human body.

Why PFAS are in athletic apparel.

PFAS compounds are the industry-standard solution for water-resistant, stain-resistant, and certain moisture-wicking performance fabrics. They are used as coatings on many performance synthetic materials. In EU data, textile applications account for about 11% of total PFAS usage, but textiles are responsible for roughly 31% of PFAS environmental emissions.

The scale of PFAS contamination in apparel.

What PFAS do to the body.

PFAS compounds bioaccumulate. Once they enter the body, they do not leave quickly. Measured half-lives in the human bloodstream range from several years to decades for the more persistent compounds.

• The National Academies of Sciences, Engineering, and Medicine 2022 report identified PFAS as linked to elevated cholesterol, reduced birth weight, and weakened antibody response, with specific concerns for children.
• The C8 Health Study, one of the largest PFAS epidemiological studies ever conducted, established "probable links" between PFAS exposure and six specific disease categories, including testicular and kidney cancers, thyroid disease, ulcerative colitis, pregnancy-induced hypertension, and high cholesterol.
• Elevated PFAS levels are associated with reduced testosterone and altered reproductive hormone profiles (Grandjean and Clapp, 2015).
• Dermal absorption research (2022, Environmental Science & Technology) demonstrated significant skin absorption of PFAS in animal models, reinforcing that skin is a meaningful route of exposure, not just ingestion or inhalation.
• The CDC has acknowledged PFAS exposure can cause liver damage, asthma, thyroid disease, prostate, kidney, and testicular cancers, decreased fertility, and elevated blood pressure in pregnant women. In children: low birth weight, bone variations, and accelerated puberty.

Regulatory momentum.

• California AB 1817 restricts PFAS in apparel (effective 2025).
• New York S6291 prohibits PFAS in apparel (with limited outdoor-gear exception until 2028).
• Colorado HB 22-1345 phases out PFAS in outdoor clothing with disclosure requirements until full ban in 2028.
• France Law No. 2025-188 bans PFAS in textiles, footwear, and waterproofing agents for consumer use beginning January 2026.
• Denmark is prohibiting import and sale of PFAS-containing clothing and footwear to consumers from July 2026.
• European Chemicals Agency (ECHA) is advocating for a universal PFAS restriction under REACH.

4.4Microplastics in apparel and human tissue.

What they are. Plastic particles smaller than 5 millimeters. They enter the environment from the breakdown of larger plastic items, including synthetic textiles. Nanoplastics (smaller than 1 micrometer) can cross biological barriers that larger particles cannot.

How synthetic apparel produces microplastics.

Polyester, nylon, elastane, and other synthetic fabrics shed fibers continuously through normal wear and through washing. Friction accelerates shedding. A single load of laundry with synthetic clothing can release hundreds of thousands to millions of microplastic fibers into the wastewater stream.

Direct exposure concerns.

Beyond environmental distribution, microplastics from underwear can enter the body directly through dermal absorption through sweat-opened pores; inhalation of fiber fragments released during dressing and movement; and direct mucosal contact in the genital region.

Where microplastics have been detected in human tissue.

• A 2023 study in Environmental Science & Technology documented microplastics in human testicular tissue in every sample examined.
• Microplastics have been detected in human blood, placental tissue, breast milk, and meconium (the newborn's first stool).
• Animal studies exposing rodents to polystyrene microplastics at low doses demonstrate reduced sperm count, DNA fragmentation in sperm, impaired Leydig cell function, and decreased testosterone via the LH-mediated LHR/cAMP/PKA/StAR pathway (Jin et al., 2022).

4.4.1The polyamide (nylon) finding.

A 2023 peer-reviewed study (Zhou et al., Ecotoxicology and Environmental Safety) compared the testosterone adsorption capacity of different microplastic polymers. The question was: which plastics most efficiently absorb testosterone out of biological fluids, thereby reducing its bioavailability?

This matters because polyamide (nylon) is the most common synthetic fiber used in the waistbands of "clean" underwear brands and is the dominant fiber in almost all conventional athletic apparel waistbands.

This means the plastic most likely to interfere with testosterone bioavailability is being placed against the skin at the precise anatomical location with the highest absorption rate, during the activities (heat, sweat, friction) most likely to maximize fiber release and skin uptake.

From a risk-reduction standpoint, if a parent reduces only one aspect of their child's chemical exposure from apparel, eliminating nylon from the waistband is likely the single highest-impact change they can make.

4.5Toxic dyes (azo and disperse dyes).

What they are. Most synthetic athletic apparel is dyed with azo dyes or disperse dyes, both of which are petroleum-derived synthetic colorants. A subset of azo dyes can break down under normal conditions (heat, sweat, washing) to release aromatic amines, some of which are classified as carcinogenic or possibly carcinogenic by the International Agency for Research on Cancer (IARC Monograph Volume 99).

What toxic dyes do.

• Skin sensitization and contact dermatitis, particularly in children with existing sensitive skin conditions
• Absorption through sweat-opened pores during physical activity
• Potential long-term exposure concerns from dyes that release carcinogenic aromatic amines

Regulatory context. The European Union restricts azo dyes that break down into specific aromatic amines (REACH Regulation EC 1907/2006, Annex XVII). The United States does not have equivalent federal apparel-specific regulations. A product sold in a U.S. retailer may contain dyes that would be restricted under EU rules.

Certifications to look for. OEKO-TEX Standard 100 is the most widely recognized third-party certification for textile chemical safety and specifically addresses dye and finish safety. GOTS certification also sets rigorous dye requirements for organic textiles.

The chemicals described in Section 4 are present in conventional synthetic apparel whether the wearer is sedentary or athletic. But sports do not produce a linear increase in exposure. They produce a multiplicative one.

5.1Heat accelerates chemical migration.

Plastics and dyes are held in the fabric matrix through chemical bonds that are temperature-sensitive. When temperature rises, the matrix becomes less stable, and additives (plasticizers, stabilizers, dye fixatives, coatings) begin to migrate out of the fabric and onto whatever is in contact with it.

Peer-reviewed polymer and textile chemistry literature has documented:

• BPA and phthalate migration rates from synthetic materials increase substantially with temperature.
• Dye bleed and fixative migration are documented functions of temperature.
• PFAS-based performance coatings show increased release under thermal stress.

A youth athlete playing baseball in a July dugout, running a summer soccer practice, or grinding through an indoor basketball tournament is wearing fabric at elevated temperatures for hours at a time. The chemical migration happening during those sessions is meaningfully higher than what would occur in a sedentary context.

5.2Sweat opens the skin.

Sweating is not just cooling. It is a biological process that changes the permeability of the skin.

When sweat glands activate: pores open to release fluid; the stratum corneum (outer skin layer) becomes hydrated and more permeable; the aqueous environment on the skin surface can dissolve polar chemicals that might otherwise remain bound to the fabric; and the normal acidic skin pH shifts, affecting chemical absorption dynamics.

Pharmaceutical research has long exploited this phenomenon. Transdermal drug patches are intentionally designed to work on sweat-opened skin. The same mechanism applies, unintentionally, to chemical absorption from sweaty synthetic apparel.

5.3Friction amplifies both release and absorption.

Every athletic movement creates friction between fabric and skin. That friction has two effects:

On the fabric: Microscopic fibers break and release microplastic particles. Laboratory testing shows synthetic athletic apparel shedding orders of magnitude more microplastic material during simulated athletic use than during static wear.

On the skin: Microscopic abrasion temporarily increases permeability. Even below the threshold of visible chafing, the surface layer of skin is being mechanically disturbed in ways that increase chemical and particle uptake.

5.4The stacking effect.

When heat, sweat, and friction occur together, which they always do during athletic activity, the effects multiply rather than add. Hot fabric releases more chemicals. Sweating skin absorbs more readily. Friction releases more particles and creates more entry points. Each factor amplifies the others.

For a youth athlete in 6 to 10 hours of practice and game time per week, wearing synthetic briefs during every session, the chronic exposure profile is not comparable to the exposure profile of the same garment worn sedentarily. It's substantially worse.

This occurs during the exact developmental years when hormone systems are forming and when reproductive organs are most susceptible to endocrine disruption.

5.5The absorption-site problem.

The chemical exposure from underwear specifically is compounded by the region of skin involved.

A foundational 1967 study by Feldmann and Maibach (Journal of Investigative Dermatology) established that percutaneous (through-skin) absorption varies dramatically by body region.

Underwear sits against that exact region, every day, for hours. Add heat, sweat, and friction, and the exposure math gets worse in the very place it matters most for male hormonal and reproductive health.

A 2022 study published in Environmental Science & Technology on dermal absorption of poly- and perfluoroalkyl substances in rats demonstrated significant skin absorption of PFAS compounds, reinforcing that skin is a meaningful route of exposure for these chemicals, not just ingestion or inhalation.

Adult exposure to endocrine-disrupting chemicals is a concern. Child exposure is a larger concern. Not because children are fragile, but because of specific, well-documented physiological differences.

6.1Skin anatomy and absorption.

Children have thinner skin than adults, with less developed stratum corneum (the outermost protective skin layer). Thinner skin is more permeable. More chemicals get through.

Children also have a larger skin surface area relative to their body weight. A small child has roughly three times the surface-area-to-body-weight ratio of an average adult. Any chemical exposure through the skin therefore represents a proportionally larger body burden in a child than in an adult.

The Endocrine Society's position paper on endocrine-disrupting chemicals specifically identifies children's greater skin absorption surface relative to body weight as one of four primary reasons children face amplified exposure risks compared to adults. The Pew Charitable Trusts and the American Academy of Pediatrics have issued consistent guidance to the same effect.

6.2Developing hormone systems.

The endocrine system develops progressively from conception through late adolescence. Specific developmental windows are especially sensitive to chemical interference:

• Prenatal: The fetal endocrine system begins organizing in utero. Maternal exposure to EDCs during pregnancy has been associated with measurable effects on male reproductive development (Swan et al., 2005; replicated in subsequent cohorts).
• Early childhood: Brain and hormone system development continues rapidly. Multiple peer-reviewed reviews have identified this window as high-vulnerability for EDC exposure (Braun, 2017, Nature Reviews Endocrinology).
• Puberty: The hormonal cascade that drives puberty is itself a chemical communication system. Exogenous chemicals that mimic or block those signals have been linked to altered pubertal timing and trajectory (Lee et al., 2019).

"In utero, early postnatal life, and/or pubertal development are highly susceptible periods to exposure."

6.3Immature detoxification pathways.

The liver enzyme systems that metabolize and clear foreign chemicals from the body develop gradually. Cytochrome P450 enzyme activity, responsible for much of the body's detoxification capacity, is substantially lower in young children than in adults.

What that means in practice: a chemical dose that an adult can clear in hours may persist in a child's body for days. Body burden accumulates differently in children than in adults.

6.4Longer latency for effects to manifest.

Many EDC-associated health effects, especially those related to reproductive function and cancer, emerge years or decades after exposure. A child exposed during developmental years has a longer horizon over which exposure-related effects can develop.

Peer-reviewed research on testicular dysgenesis syndrome (TDS), a collection of male reproductive conditions hypothesized to stem from fetal and early-childhood EDC exposure, suggests that effects including reduced semen quality, testicular cancer risk, and fertility issues may not become evident until adulthood, long after the exposure window.

6.5Behavioral and physiological amplifiers.

Children also exhibit behaviors that amplify exposure. They put hands and objects in their mouths. They play close to the floor where dust accumulates (floor-level PFAS, phthalates, and flame-retardant dust are well documented). They inhale at higher rates relative to body weight. The totality of these factors means children's per-body-weight exposure to environmental chemicals typically exceeds that of adults.

6.6The youth athlete compounds every factor.

A child who plays sports combines:

Each of these factors is documented in peer-reviewed literature. Stacked together, they describe the exposure profile of the population the sports apparel industry is selling synthetic clothing to every day.

7.1The pattern.

A parent reads a product page. It says "organic cotton" or "made with merino wool." The marketing claims are clean. The branding is reassuring. The price is higher than conventional, and the parent pays the premium expecting what's advertised.

Then, deep in the fine print of the product details, comes the composition line:

• "95% organic cotton, 5% elastane"
• "84% merino wool, 12% nylon, 4% spandex"
• "Made with certified organic materials" (with waistband composition undisclosed)

The 5% to 20% synthetic content is not a rounding error, and it is usually concentrated in two places: the waistband and the leg openings. Those are exactly the high-friction, high-skin-contact zones.

7.2Why brands do this.

Stretch and fit. Elastane and spandex give underwear its shape memory. Achieving comparable fit without them requires different engineering (natural rubber, fabric tension design, or specific cut patterns), which is harder and more expensive to manufacture at scale.

Cost. Synthetic blends are cheaper to produce than 100% natural alternatives, especially in waistband form. Natural rubber is more expensive than elastane by a significant margin.

Inertia. Manufacturing supply chains are built around synthetic waistbands. Breaking out of that default requires either paying a premium to specialty mills or building entirely new supplier relationships.

7.3The specific problem with nylon waistbands.

As discussed in Section 4.4.1, polyamide (nylon) microplastics demonstrated the highest testosterone adsorption capacity of any plastic tested in peer-reviewed comparative analysis (Zhou et al., 2023).

When a brief is 95% organic cotton with a nylon waistband, the product is placing the single worst plastic (from a hormone-interference standpoint) in direct contact with the skin at the perineal region (the highest-absorption region on the body), in the exact zone where movement friction is greatest.

7.4A 30-second audit every parent can do.

If a brand won't disclose the waistband composition, treat that as meaningful information.

7.5What truly clean kids' sports underwear looks like.

A fully clean underwear product, by the definitions used in this report, has:

• Body fabric: 100% natural fiber (organic merino wool, organic cotton, or hemp), certified by a recognized standard (GOTS for natural fiber certification; OEKO-TEX for chemical safety including dyes).
• Waistband: 100% natural material (natural rubber from trees, cotton-wrapped natural elastic, or entirely fabric-based construction).
• Dyes: OEKO-TEX certified or undyed.
• No PFAS coatings for water or stain resistance.
• No synthetic "performance" treatments.

This is a narrow category. Very few brands in the market meet every one of these criteria.

8.1Immediate actions — free, this week.

1. Audit your kid's underwear drawer. Pull out every pair. Check the label on the inside waistband. Identify any that are fully synthetic (100% polyester, nylon, or blends without any natural fiber). These are the highest-exposure items. At minimum, reduce the frequency of their use for long athletic sessions.

2. Read the fabric composition on your kid's gear before your next purchase. Not the marketing copy. The actual fiber percentage line. Apply the 30-second audit from Section 7.4.

3. Stop drying synthetic athletic wear in the dryer on high. Heat accelerates chemical migration. Low or no-heat dry settings reduce the rate of additive breakdown in synthetic materials. Not a full solution, but a free one.

4. Wash new synthetic athletic wear before first use. A significant amount of surface residue (dye excess, fabric finishes, softener residues) washes out in the first cycle. Use a non-fragranced, mild detergent. Avoid fabric softeners.

5. Air out gear between uses, especially in a gear bag. Stale, damp, enclosed gear bags increase the contact time between skin, sweat residue, and synthetic chemicals. Hang-drying and airing reduces that contact window.

8.2Gradual swaps — moderate cost.

Replace the highest-contact items first. Priority order:

• Underwear (direct skin contact, most absorptive region)
• Base layers worn under uniforms (extended contact, temperature-amplified exposure)
• Socks (blood-rich skin, frequent friction)
• Practice shirts (broad contact, frequent heat and sweat)

Look for certified natural fiber alternatives. GOTS-certified organic cotton and merino wool are the most widely available options. Any fully natural-fiber option is a meaningful improvement over a fully synthetic one.

Don't let perfect be the enemy of good. If you can't afford to replace everything at once, focus on the most-worn items and the highest-contact items. A partial swap is better than waiting for a total overhaul.

8.3Questions to ask brands.

Brands that answer clearly and specifically are generally trustworthy. Brands that deflect, refuse, or answer only in marketing copy are generally not.

8.4Longer-term awareness.

The chemical story in athletic apparel is one piece of a larger picture. Other meaningful exposure pathways for the developing athlete include water bottles and food storage containers (BPA, phthalates), non-stick cookware and stain-resistant furniture (PFAS), synthetic fragrances in laundry detergent and body care (phthalates), food packaging especially plastic-wrapped or canned items (BPA, phthalates), and sports equipment materials (synthetic grips, certain protective gear components).

Parents already attentive to some of these (the water bottle swap, the sunscreen audit, the seed oil conversation) may find that athletic apparel is simply the next logical item on the list.

We spent two years of our lives on this because we couldn't unsee what we saw once we started looking.

If you've read this far, you're the rare parent who actually engages with source material instead of headlines. That kind of reader is the reason we wrote this.

The research is real. The exposure is real. The chemicals are real. The SGS test results on our own product are real and fully documented. The category of parents who are willing to act on real information is growing faster than most brands realize.

Whether you buy Attack Briefs or not is not the most important thing that happens as a result of this report. The most important thing is that you now know.

If you have questions, email us. If you find errors in our research or citations, tell us and we'll correct them. If you want to share this report, share it freely. Forward it to coaches, other parents, your pediatrician, your group chat. The only thing we ask is that you don't edit or excerpt it in ways that distort the findings.

We're not going to solve the youth apparel industry's chemistry problem alone. But parents can. One purchasing decision at a time, one conversation at a time.

Thank you for caring enough to read to the end.

— · —

— Mark & Mike

Co-founders, Attackletics

Full Research Citations.

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