Three studies.
One conclusion.
Every claim KRYOS makes is grounded in peer-reviewed research. We do not run our own studies — we engineer around the ones that already exist. Three published papers form the foundation of the design: each is summarised below in plain language, with the original citation, methodology, and primary finding intact.
Underwear type and semen quality in 656 men
Researchers at the Harvard School of Public Health analysed semen samples from 656 men attending a fertility centre. They categorised participants by the type of underwear typically worn — natural-fibre boxers vs synthetic-fibre briefs — and controlled for age, BMI, smoking, and time of sample.
The men who reported wearing breathable natural-fibre underwear showed significantly higher sperm concentration, total sperm count, and motile sperm count than men in tight synthetic briefs. The differences held after statistical adjustment for confounders.
The mechanism is straightforward thermoregulation. Spermatogenesis is temperature-sensitive: scrotal temperatures above 35°C suppress sperm production. Synthetic, close-fitting underwear traps body heat against the testes; natural-fibre, looser-cut underwear allows the body's normal cooling to function.
Urinary phthalate metabolites and serum testosterone
Phthalates are plasticisers used to make polyester soft and pliable. They are not chemically bonded to the polymer — they leach. They are absorbed through skin, particularly the highly permeable skin of the inguinal region, and have been classified as endocrine disruptors by the European Chemicals Agency.
Researchers using NHANES data (a CDC-administered nationally representative health survey) measured urinary phthalate metabolite concentrations alongside serum testosterone in 2,208 men. They modelled the relationship statistically.
The finding: every doubling of urinary phthalate metabolites was associated with a measurable drop in free testosterone. The relationship held across age groups. The implication is that chronic, low-level phthalate exposure — exactly the exposure profile of someone wearing polyester underwear daily — is a measurable contributor to lower testosterone.
Scrotal hyperthermia and spermatogenic function
Mieusset and Bujan published the most-cited body of work on scrotal thermoregulation. Across multiple studies and replications spanning more than 1,200 men, they established the temperature thresholds at which sperm production and Leydig cell function begin to decline.
The scrotum is anatomically external for a reason: testes function optimally at 2–4°C below core body temperature. Sustained heating above 35°C — easily induced by synthetic close-fitting underwear within an hour of wear — produces measurable reductions in sperm concentration and motility. Recovery, when subjects return to natural-fibre underwear, takes 60–90 days, matching the spermatogenic cycle.
Natural-fibre cooling, by contrast, maintains the temperature differential the body evolved to require.
What this evidence is, and isn't
The Harvard cohort is observational, not a randomised controlled trial. Mechanistic studies on thermoregulation and phthalate endocrine disruption are causal. Together, the evidence is strong enough to engineer around — and weak enough that we will not promise personal outcomes. We will fund an RCT specifically on KRYOS when revenue allows. Until then: switching to natural-fibre, anatomically relieved, plasticiser-free underwear is a low-cost, evidence-supported choice. Nothing more, nothing less.