[dharmaturtle]

This is a good point, and was brought up in a response to a study from Laukkanen: https://sci-hub.se/10.1001/jamainternmed.2015.3432

> heart rate increases up to 100 beats/min during sauna sessions at moderate temperatures and up to 150 beats/min during hotter saunas. Although not an issue for healthy individuals, such a cardiac challenge may feel uncomfortable for participants with poor cardiorespiratory fitness and pre-existing disease. Simple adjustment for disease vs no disease may not entirely solve this problem since reverse causation bias (ie, health status affects the likelihood of a sauna session) operates within disease groups; the more severe the disease, the greater the fear of cardiac challenge... A more robust finding at reduced risk of reverse causation bias would be a graded association between number of sauna sessions and mortality in an initially healthy, cardiorespiratory fit population, but this was not observed.

Laukkanen responded with

> ...we did observe graded inverse associations with sudden cardiac death (SCD), fatal coronary heart disease (CHD), and cardiovascular disease (CVD) events, which are the characteristics of a true inverse association between sauna bathing and outcomes. Furthermore, results were carefully adjusted for socioeconomic status (SES), physical activity, and cardiorespiratory fitness...

Notably Laukkanen didn't say that the graded association was in an "initially healthy" population, as the first reply specified.