The science

The physiology (A). The testicles need to sit at roughly 34 degrees - a few degrees below the rest of the body. That is why they hang outside. The body forms about 100 million sperm a day, and a single sperm takes roughly three months to form and mature. The Swedish health service 1177 states plainly that tight underwear or daily hot baths can stall production - and that it starts again when you stop.

The central study (B). Ten healthy Italian men used a sauna twice a week, 15 minutes at 80–90 degrees, for three months. Sperm count and motility dropped sharply - more than halved according to several summaries. Six months after they stopped, everything was normal again (Garolla et al. 2013, Human Reproduction). Hormones were unaffected.

What that means in practice:

• The effect is real, dose-dependent and temporary. If you go once a week and are not planning children, there is nothing to worry about.
• The effect lags. What a semen sample shows today reflects how you lived two to three months ago.
• Count on three months for recovery - a full sperm cycle. The Garolla study saw normal values after six months.

Other heat sources that have been studied:

• Tight underwear. In the largest study - 656 men at a fertility clinic - boxer wearers had 25% higher sperm concentration and 17% higher count (Mínguez-Alarcón et al. 2018). A 2026 review notes that no study has shown an effect on actual pregnancies: loose fit is a supportive low-risk measure, not a treatment.
• Heatwaves and prolonged sitting raise temperature. A study of 818 men in Singapore found 46% higher odds of low sperm count after 90 days with outdoor temperatures above 29.8 degrees.
• Hot tubs and hot baths. Clinics such as Mayo Clinic, Nordic IVF and Kry generally advise against them while you are trying to conceive.

What Swedish experts say. Endocrinology professor Mikael Lehtihet argues that heat and cooling matter “only for men who are borderline and actively trying to conceive”. Andrologists Elenkov, Lundberg Giwercman and Giwercman write in Läkartidningen that there is “still no evidence that lifestyle changes lead to improved fertility”. We think both deserve quoting.

Sources: 1177 · Garolla 2013 · Mínguez-Alarcón 2018 (ESHRE) · Transl Androl Urol 2026 · Euronews 2024 · Läkartidningen 2022 · Metro Mode 2026 · Mayo Clinic · Nordic IVF · Kry.

• Sweat is more than 99 percent water, with salts, lactate and urea. Its job is cooling (Baker 2019; 1177).
• Researchers who did the maths found that even heavy sweating removes roughly 0.02 percent of the environmental pollutants we take in daily - 0.04 percent at maximum effort (Imbeault et al. 2018).
• PFAS, so-called forever chemicals, are not excreted in sweat - shown even by the group most positive about sweat as an excretion route (Genuis et al. 2013).
• Microplastics are particles and cannot pass through sweat glands. The measurement methods behind the headlines have also been questioned by chemists.
• The largest systematic review of sauna bathing calls the evidence here “scant and incomplete” (Hussain & Cohen 2018). Fact-checker Science Feedback: Unsupported.
• Bryan Johnson’s own percentages are one person’s spot samples of substances that leave the body within hours, while he also changed his water filter and cutting board.

Why we still say “shower after the sauna”: hygiene, cooling down, and rinsing off salt and sweat that dry out the skin. Not because anything is reabsorbed - no study supports that.

And the scrub? Heat softens the outermost skin layer so dead cells come away more easily - that is common to Finnish birch whisks, Turkish kese and Korean seshin. The dermatology association AAD says: gently, about once a week, never on irritated skin, moisturise afterwards. That is all it does - and it is enough.

Sources: Baker 2019 · Imbeault 2018 · Genuis 2013 · Hussain & Cohen 2018 · Science Feedback 2024 · McGill OSS · AAD.