Short read
Zaccardi and colleagues analysed data from the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD) in a prospective cohort study published in the American Journal of Hypertension in 2017. The participants were middle-aged Finnish men whose blood pressure was normal at the start. They were followed for about two decades. The endpoint was new-onset hypertension, not the short-term drop in blood pressure after a session.
Measured against men who used the sauna once a week, more frequent sauna use was associated with a markedly lower risk of developing hypertension over the follow-up period. The size of that difference is quoted in secondary sources but has not yet been checked against the original paper, so no figure is given here.
The study includes men only, from the same Finnish cohort as the mortality findings. It is therefore not independent confirmation of those results but a further analysis of the same data. As an observational study, it shows an association and not a cause.
The authors discuss repeated widening of the blood vessels, improved endothelial function and fluid loss through sweating as plausible mechanisms. The value of the finding is its long time frame: it adds a view over decades to data that otherwise describe acute changes in blood pressure. It does not show that sauna use prevents high blood pressure in an individual.