“Regular sauna use lowers your sperm count.”

The research found this

The main study comes from Padova, Italy. Ten men with normal semen took two 15-minute sessions a week in a Finnish sauna at 80–90°C for three months. The researchers took measurements at four points between the first session and six months after the last.1Source 1 · Clinical trialSeminal and molecular evidence that sauna exposure affects human spermatogenesisGarolla A, Torino M, Sartini B, et al.. Human Reproduction, 2013.Read the paper ↗PubMed ↗ At the end of the course, sperm count and motility were strongly impaired (P < 0.001), and sex hormones had not changed significantly.

Tests on the sperm showed poorer DNA packaging, and the share with normal histone-to-protamine exchange fell from 78.7% to 69.0%.1Source 1 · Clinical trialSeminal and molecular evidence that sauna exposure affects human spermatogenesisGarolla A, Torino M, Sartini B, et al.. Human Reproduction, 2013.Read the paper ↗PubMed ↗ A score of mitochondrial function dropped from 76.8 to 54.0, and heat-stress genes such as HSP70 were switched up. Every change had reversed six months after the last session.

The paper's tables sit behind a paywall. A 2018 systematic review that extracted them reports sperm concentration falling from 89 to 31 million per milliliter and motile sperm from 58.0% to 36.1%.2Source 2 · Systematic reviewClinical Effects of Regular Dry Sauna Bathing: A Systematic ReviewHussain J, Cohen M. Evidence-Based Complementary and Alternative Medicine, 2018.Read the paper ↗PubMed ↗ Other summaries size it differently: one review calls it a 50% decline, and a Finnish health library says count fell to about a third.3Source 3 · ReviewEndocrine effects of sauna bathHuhtaniemi IT, Laukkanen JA. Current Opinion in Endocrine and Metabolic Research, 2020.Read the paper ↗4Source 4 · GuidanceSauna ja terveys [Sauna and health]Viinikka L. Lääkärikirja Duodecim, Terveyskirjasto, 2024.Read the paper ↗

One session can lower sperm numbers for a few weeks. After a single 20-minute sauna at 85°C, sperm numbers fell within a week and took about 5 weeks to return to normal.5Source 5 · Clinical trialThe Effect of a Single Sauna Exposure on SpermatozoaBrown-woodman PDC, Post EJ, Gass GC, et al.. Archives of Andrology, 1984.Read the paper ↗PubMed ↗ The scrotum normally runs cool. Its temperature sat about 2.5°C below body temperature in 258 young men measured at a room temperature of 21–25°C, and it rose from 32.7°C to 37.5°C in 10 men during a sauna.6Source 6 · Cross-sectional study[Scrotal temperature in 258 healthy men, randomly selected from a population of men aged 18 to 23 years old. Statistical analysis, epidemiologic observations, and measurement of the testicular diameters]Valeri A, Mianné D, Merouze F, et al.. Progres en urologie : journal de l'Association francaise d'urologie et de la Societe francaise d'urologie, 1993.Read the paper ↗PubMed ↗7Source 7 · Clinical trialA portable digital data recorder for long-term monitoring of scrotal temperaturesJockenhövel F, Gräwe A, Nieschlag E. Fertility and Sterility, 1990.Read the paper ↗PubMed ↗

Hot baths lower sperm output too. Twenty Chinese men who soaked their lower body in 43°C water for 30 minutes on 10 occasions saw sperm concentration fall to 15.5–28.8% of baseline by week 8, and output recovered by weeks 14–16.8Source 8 · Clinical trialEffect of transient scrotal hyperthermia on sperm parameters, seminal plasma biochemical markers, and oxidative stress in menXia W, Rao M, Zhao XL, et al.. Asian Journal of Andrology, 2015.Read the paper ↗PubMed ↗ In 3,041 North American couples trying to conceive, men's sauna use showed no link with time to pregnancy (fecundability ratio 1.04, 95% CI 0.73–1.48, for three or more saunas a month), though only 51 men went that often.9Source 9 · Cohort studyMale personal heat exposures and fecundability: A preconception cohort studyMcKinnon CJ, Joglekar DJ, Hatch EE, et al.. Andrology, 2022.Read the paper ↗PubMed ↗