“Regular sauna use means you catch fewer colds.”

The research found this

The prevention claim goes back to a trial published in 1990. Edzard Ernst and colleagues had 25 volunteers take regular saunas and 25 controls avoid them, and both groups recorded their colds for six months.1Source 1 · Clinical trialRegular Sauna Bathing and the Incidence of Common ColdsErnst E, Pecho E, Wirz P, et al.. Annals of Medicine, 1990.Read the paper ↗PubMed ↗ The sauna group had significantly fewer colds. Most of the difference came in the last three months, when their rate was roughly half that of the controls. Colds lasted as long and felt as bad in both groups.

The abstract doesn't say the groups were randomized, and people logged their own colds with no lab test to confirm them. A 2018 review looked for later trials of sauna or hot baths for preventing colds and found none.2Source 2 · ReviewViral Upper Respiratory InfectionBarrett B. Integrative Medicine, 2018.Read the paper ↗

One session shifts immune cells for a short time. After 30 minutes at 73°C, white blood cell counts rose in 51 adults, and neutrophil and lymphocyte counts were back to their starting levels 30 minutes later.3Source 3 · Clinical trialAcute Finnish sauna heat exposure induces stronger immune cell than cytokine responsesHeinonen IH, Koivula T, Hollmén M, et al.. Temperature, 2026.Read the paper ↗PubMed ↗ In a head-to-head test with traditional and infrared saunas, only 45 minutes in 40.5°C water changed cytokines (the immune system's chemical messengers) or immune cells.4Source 4 · Crossover trialComparison of thermoregulatory, cardiovascular, and immune responses to different passive heat therapy modalitiesAtencio JK, Reed EL, Wiedenfeld Needham K, et al.. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 2025.Read the paper ↗PubMed ↗

The pneumonia link comes from a Finnish study called KIHD. Researchers followed 2,210 men for a median of 25.6 years. Those who took four or more saunas a week had a 37% lower risk of hospital-diagnosed pneumonia than men who went once a week or less (a hazard ratio of 0.63). The plausible range for that estimate ran up to 1.00 (no difference).5Source 5 · Cohort studyFrequent sauna bathing may reduce the risk of pneumonia in middle-aged Caucasian men: The KIHD prospective cohort studyKunutsor SK, Laukkanen T, Laukkanen JA. Respiratory Medicine, 2017.Read the paper ↗PubMed ↗ In a later analysis, frequent sauna use without high fitness showed no significant link.6Source 6 · Cohort studyHigh fitness levels, frequent sauna bathing and risk of pneumonia in a cohort study: Are there potential implications for COVID‐19?Kunutsor SK, Laukkanen JA. European Journal of Clinical Investigation, 2021.Read the paper ↗PubMed ↗

Using heat to treat a cold has also failed in trials. In a randomized trial of 157 adults with fresh colds, breathing hot sauna air for about three minutes on three days didn't ease overall symptoms.7Source 7 · Randomized trialVisiting a sauna: does inhaling hot dry air reduce common cold symptoms? A randomised controlled trialPach D, Knöchel B, Lüdtke R, et al.. Medical Journal of Australia, 2010.Read the paper ↗PubMed ↗ A Cochrane review of trials found no clear benefit from heated, humidified air.8Source 8 · Systematic reviewHeated, humidified air for the common coldSingh M, Singh M, Jaiswal N, et al.. Cochrane Database of Systematic Reviews, 2017.Read the paper ↗PubMed ↗ The Finnish sauna researchers wrote in 2021 that sauna body temperatures are too low to kill or inactivate the virus that causes COVID-19.9Source 9 · ReviewFinnish sauna and COVID-19Kunutsor SK, Lavie CJ, Laukkanen J. Le infezioni in medicina, 2021.Read the paper ↗PubMed ↗