The findings of this long-term prospective study of over 14 years follow-up suggest that the cardiovascular benefits of sauna bathing may exist in both men and women. Our new results show that addition of information on the frequency of sauna bathing improved the prediction and reclassification of the long-term risk for CVD mortality. A higher frequency of sauna bathing sessions per week was related to a decreased risk of fatal CVD events independent of conventional cardiovascular risk factors as well as several other potential confounders. The risk of fatal CVD events decreased with increasing sauna sessions in a dose-response manner with no threshold effect. For the duration of sauna bathing per week, we observed a decrease in risk of CVD mortality with increasing duration of sauna bathing, though further work may be required to ascertain whether a curvilinear or linear shape best describes the relationship. The association was strong and also independent of several established and emerging risk factors. Except for the evidence of effect modification by diabetes status for the association between sauna frequency and CVD mortality, the associations were not modified significantly by levels or categories of several clinically relevant characteristics including gender. However, findings from the subgroup analyses should be interpreted with caution given the multiple statistical tests of interaction and the low event rates in these subgroups.

Several mechanisms can be postulated to underpin the protective effects of sauna bathing on cardiovascular mortality. Dry and hot sauna baths have been shown to increase the demands of cardiovascular function [5, 22, 38]. Sauna bathing causes an increase in heart rate which is a reaction to the body heat load. Heart rate may be elevated up to 120–150 beats per minute during sauna bathing, corresponding to low- to moderate-intensity physical exercise training for the circulatory system without active muscle work [30, 39,40,41]. Acute sauna exposure has been shown to produce blood pressure lowering effects [42], decrease peripheral vascular resistance [42, 43] and arterial stiffness [17, 44], and improve arterial compliance [18]. Short-term sauna exposure also activates the sympathetic nervous and the renin-angiotensin-aldosterone systems and the hypothalamus-pituitary-adrenal hormonal axis, and short-term increases in levels of their associated hormones have been reported [45]. Repeated sauna exposure improves endothelial function, suggesting a beneficial role of thermal therapy on vascular function [16,17,18, 46]. Long-term sauna bathing habit may be beneficial in the reduction of high systemic blood pressure [42], which is in line with previous evidence showing that blood pressure may be lower among those who are living in warm conditions with higher ambient temperature [47, 48]. We have demonstrated that regular sauna bathing is associated with a lowered risk of future hypertension [6]. Typical hot and dry Finnish sauna increases body temperature which causes more efficient skin blood flow, leading to a higher cardiac output, whereas blood flow to internal organs decreases [22]. Sweat is typically secreted at a rate which corresponds to an average total secretion of 0.5 kg during a sauna bathing session [5, 39]. Increased sweating is accompanied by a reduction in blood pressure and higher heart rate, while cardiac stroke volume is largely maintained, although a part of blood volume is diverted from the internal organs to body peripheral parts with decreasing venous return which is not facilitated by active skeletal muscle work [49]. However, it has been proposed that muscle blood flow may increase to at least some extent in response to heat stress, although sauna therapy-induced myocardial metabolic adaptations are largely unexplored [30, 50]. There is also evidence that regular long-term sauna bathing (average of two sessions per week) increases left ventricular ejection fraction [46]. Heat therapy may improve left ventricular function with decreased cardiac pre- and afterload, thereby maintaining appropriate stroke volume despite large reductions in ventricular filling pressures [16, 38, 51,52,53]. Additionally, previous studies have demonstrated a positive alteration of the autonomic nervous system and reduced levels of natriuretic peptides, oxidative stress, inflammation, and norepinephrine due to regular sauna therapy [15, 30, 43, 53, 54].

Our current results highlight a substantial risk reduction of fatal CVD events in men and women, with frequent sauna use of over four times per week and duration of sauna bathing of more than 45 min/week. The data suggests that a history of more frequent sauna use is associated with a decrease in the risk of fatal CVD in a linear dose-response manner. Our data was based on the total weekly duration of sauna sessions, and therefore, we are unable to make any comments regarding the minimum duration of a single session that may confer benefits. However, based on historical data, a typical sauna session usually ranges from 5 to 20 min [30], although longer sauna bathing sessions may be used depending on the individual [22]. The findings also show that frequency of sauna bathing has incremental predictive value to CVD mortality beyond conventional risk factors and has the ability to reclassify subjects across clinically relevant risk thresholds. There was no statistically significant evidence of effect modification by gender. Regular Finnish sauna bathing is safe and may have several additional health benefits. Patients with a previous myocardial infarction, stable angina pectoris, or heart failure can usually enjoy sauna bathing without any significant adverse cardiovascular effects [5, 22, 55]. In this long-term follow-up study, CVD mortality rate among most active sauna users (i.e., those participants with sauna of four to seven times per week) was 2.7 cases per 1000 person-years, indicating low risk. However, in a specific group of older individuals who are prone to orthostatic hypotension, sauna baths should be taken cautiously due to possible sudden drop in blood pressures which may occur just after a hot and dry bath [22, 30, 56]. Hypotension during and immediately after sauna can be easily prevented by appropriate fluid intake to avoid dehydration [16, 30]. Further investigation into the value of regular sauna bathing in CVD risk reduction and prevention in general populations is warranted.

Several strengths of the current study deserve consideration. This is the first prospective evaluation of the associations of both frequency and duration of sauna bathing with the risk of cardiovascular mortality in a general population including both genders. Our cohort was well characterized with a long-term follow-up period, and there were no losses to follow-up. This representative sample of middle-aged to elderly men and women who use saunas makes it possible to generalize the observed results in Northern European populations; however, prospective studies should be conducted in populations who are not accustomed to regular sauna bathing. We adjusted for a comprehensive panel of lifestyle and biological markers and included subgroup as well as risk prediction analyses using sensitive measures such as the − 2 log likelihood. Our findings were robust to the exclusion of the first 5 years of follow-up, minimizing the possibility of reverse causation bias as the explanation for our findings. Several limitations of the current study also merit consideration. As with all observational epidemiological studies, exposure assessments based on self-administered questionnaires are prone to misclassification and recall bias. Our findings from hot Finnish sauna bathing with an average temperature of approximately 80 °C cannot be directly applied to other type of steam rooms and warm water therapy which may operate at lower temperatures than a relatively dry traditional sauna and do not allow humidity changes achieved by pouring water on the heated rocks [30]. Good ventilation is a feature of a typical sauna which makes it comfortable to stay for longer periods while sauna bathing. The relatively low event rate for cardiovascular deaths (N = 181) precluded detailed assessment of (i) effect modification by relevant clinical characteristics on the associations and (ii) dose-response relationships of the associations. Though we accounted for many potential confounders to ensure the validity of our associations, there is a potential for residual confounding. It is possible that underlying diagnosed or undiagnosed diseases may have an effect on sauna bathing habits, suggesting reverse causality; however, our subgroup analyses according to various clinical characteristics were consistent and the associations remained robust in several sensitivity analyses, independent of many underlying clinical conditions and exclusion of the first 5 years of follow-up. Sauna bathing habits may have changed during follow-up due to probable changes in health habits or other incident diseases of participants occurring over the long period of time; however, any changes may be minimal as sauna habits are fairly stable in the Finnish population [30]. We could not account for the longer-term duration and regularity of sauna use prior to the study entry because of the lack of data. However, it is a common way to assess usual lifestyle activities using baseline questionnaires in long-term epidemiological studies. Secondly, we were unable to assess the associations between sauna bathing and CVD mortality risk when comparing people who used sauna with people who did not use sauna at all (control group). Indeed, the majority of Finnish people are accustomed to having a sauna bath regularly at least once per week, as it is traditionally part of the Finnish culture [30, 40]. The associations were unchanged in a subsidiary analysis which employed a combination of people who did not use sauna baths and those who had a single sauna session per week as a reference comparison.

In Finland, sauna is easily accessible to the majority of the population independently of socioeconomic and educational backgrounds. Sauna bathing is an activity that has been a tradition in Finland for thousands of years, and our data shows minor differences in annual salary levels according to the sauna frequency groups (in years 1998–2001; see Table 1), suggesting that sauna ownership does not correlate with financial status in Finland. It is therefore highly unlikely that these factors may explain the observed findings on sauna and fatal CVD events in this population. Indeed, SES did not differ when comparing one vs. four to seven times per week frequency groups; SES level was the highest among those using sauna two to three times per week. Based on our cross-sectional baseline data, the most frequent sauna use was directly related to the level of physical activity, BMI, energy intake, and alcohol consumption. Though there is a possibility that factors such as physical activity could potentially explain these finding, it is unlikely as our analysis accounted for the role of physical activity. Furthermore, our recent research evidence suggests that a combination of regular physical fitness and sauna baths is associated with a substantial reduction in the risk of fatal cardiovascular and all-cause mortality events compared with each modality alone [57, 58]. We have shown that even participants with low fitness levels have a reduced risk of mortality when combined with frequent (3–7 sessions per week) or infrequent (≤ 2 sessions per week) sauna use. However, mortality risk is substantially reduced in those with very high fitness levels combined with frequent use of sauna. Other studies have also reported similar findings. Iwase and colleagues demonstrated enhance metabolism in participants when isotonic exercise was performed during sauna exposure [59]. On the effects of sauna bathing on athletes, Ridge and Pyke demonstrated an augmentation in acute physiological responses when sauna exposure followed exercise [60]. In another study in which six male distance runners completed 3 weeks of post-training sauna bathing, study participants experienced an enhancement in endurance running performance [61]. The overall findings show that physical activity or fitness and sauna bathing each have independent effects on vascular disease [57, 58], which suggests that the beneficial effects of sauna bathing on CVD mortality is not due to physical activity or exercise.