“Frequent sauna use cuts your stroke risk by 61%.”

The research found this

The figure comes from a 2018 paper in Neurology. At an examination between 1998 and 2001, researchers in Kuopio asked 1,628 men and women aged 53 to 74 about their sauna habits. None of them had had a stroke. Over a median of 14.9 years, 155 of them had one.1Source 1 · Cohort studySauna bathing reduces the risk of stroke in Finnish men and womenKunutsor SK, Khan H, Zaccardi F, et al.. Neurology, 2018.Read the paper ↗PubMed ↗

Against people who went once a week, those who went four to seven times had a hazard ratio of 0.39 once age and sex were taken into account. A hazard ratio compares how often something happens in two groups, so 0.39 means a 61% lower rate of stroke. Adding established heart risk factors barely moved the number, and with physical activity and socioeconomic status included it was 0.38. The link didn't differ between men and women or by age.1Source 1 · Cohort studySauna bathing reduces the risk of stroke in Finnish men and womenKunutsor SK, Khan H, Zaccardi F, et al.. Neurology, 2018.Read the paper ↗PubMed ↗

The full paper is behind Neurology's paywall. Its abstract leaves out the size of each group, the number of strokes in each, the estimate for two or three sessions a week and the full list of adjustments. The authors' later review gives a hazard ratio of 0.42 for ischemic stroke (from a blocked artery) and 0.33 for hemorrhagic stroke (from bleeding). For hemorrhagic stroke, the confidence interval (the range the true value plausibly falls in) runs from 0.07 to 1.51, and that range includes no effect.2Source 2 · ReviewCardiovascular and Other Health Benefits of Sauna Bathing: A Review of the EvidenceLaukkanen JA, Laukkanen T, Kunutsor SK. Mayo Clinic Proceedings, 2018.Read the paper ↗PubMed ↗ The review's counts for the two types are 129 and 34. Those add up to 163 strokes, though the paper reported 155.

Japan offers a larger comparison with a different kind of heat. Researchers there followed 30,076 adults from 1990 to 2009. People who soaked in a hot tub almost every day had a 26% lower hazard of stroke than people who bathed twice a week or less.3Source 3 · Cohort studyHabitual tub bathing and risks of incident coronary heart disease and strokeUkai T, Iso H, Yamagishi K, et al.. Heart, 2020.Read the paper ↗PubMed ↗ For bleeding inside the brain the hazard ratio was 0.54, and bleeding around the brain showed no link. Once strokes in the first ten years were excluded, the trend was no longer statistically significant, so chance could explain it.

Two letters to Neurology questioned the result. Dippel and colleagues argued that healthier people keep bathing often, and that similar links with Alzheimer's disease and respiratory illness point to that bias.4Source 4 · ReviewReader response: Sauna bathing reduces the risk of stroke in Finnish men and women: A prospective cohort studyDippel DW, Fransen PS, Ikram MK. Neurology, 2019.Read the paper ↗PubMed ↗ A second letter raised a worry about people in their sixties. Many of them have undetected narrowing in the arteries to the brain, and heat and fluid loss could cut blood flow there.5Source 5 · ReviewReader response: Sauna bathing reduces the risk of stroke in Finnish men and women: A prospective cohort studyYu Y, Guo WT, Yan XL, et al.. Neurology, 2019.Read the paper ↗PubMed ↗ In reply, the authors said the result still stood when they excluded the first two years of follow-up and people with existing disease. They also agreed that observational studies can't prove cause.6Source 6 · ReviewAuthor response: Sauna bathing reduces the risk of stroke in Finnish men and women: A prospective cohort studyKunutsor SK, Laukkanen T, Laukkanen JA. Neurology, 2019.Read the paper ↗PubMed ↗