“Regular sauna use lowers your sperm count.”

The research found this

In the main study, researchers in Padova, Italy, had 10 men with normal semen take two 15-minute sessions a week in a Finnish sauna at 80–90°C for three months. They took measurements at four points, from the first session to 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. The share with a normal histone-to-protamine exchange (a step in packing DNA tightly) 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 for how well the sperm's energy-producing mitochondria worked dropped from 76.8 to 54.0, and heat-stress genes such as HSP70 became more active. Every change had reversed six months after the last session.

The paper's tables are behind a paywall. A 2018 systematic review that extracted them reports sperm concentration falling from 89 to 31 million per milliliter, and the share of 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 the drop 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 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. In 258 young men measured in a room at 21–25°C, its temperature was about 2.5°C below body temperature. During a sauna, it rose from 32.7°C to 37.5°C in 10 men.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 soaked their lower body in 43°C water for 30 minutes on 10 occasions, and by week 8 their sperm concentration had fallen to 15.5–28.8% of its starting level. 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 ↗ Among 3,041 North American couples trying for a baby, men's sauna use showed no link with how long pregnancy took. A fecundability ratio compares the monthly chance of conceiving, and 1 means no difference. For men who took three or more saunas a month it was 1.04, with a plausible range (95% CI) of 0.73–1.48, and 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 ↗