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Kunutsor, Laukkanen and Laukkanen recorded baseline sauna habits in 1,935 Caucasian men aged 42 to 61 from the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD), a prospective cohort in eastern Finland. The men were followed for a median of 25.6 years. The endpoint combined three conditions: chronic obstructive pulmonary disease, asthma and pneumonia. Over follow-up, 379 hospital-diagnosed cases were recorded.
Compared with men who used the sauna at most once a week, men with two to three sessions had a hazard ratio of 0.73 (95 % CI 0.58 to 0.92), a 27 % lower relative risk. Against the same reference group, four or more sessions a week came with a hazard ratio of 0.59 (0.37 to 0.94), a 41 % lower relative risk. The models were adjusted for several major risk factors for respiratory disease.
The data are observational, from men only and from eastern Finland only. The endpoint groups three very different diseases, so the result says little about any one of them. The upper confidence limit for the group with four or more sessions is 0.94, close to the threshold of statistical significance.
The main message is the step between the two groups rather than a single figure: the relative risk reduction rose from 27 to 41 percent as frequency increased. The authors discuss improved lung function and mucociliary clearance, reduced airway inflammation and a heat shock protein response as possible mechanisms. The title of the original paper speaks of reduced risk, while the data describe an association. They do not show that sauna use prevents respiratory disease in an individual.