The sauna, also known as the Finnish bath, is a traditional practice that has been carried out mainly in Finland for nearly 2000 years. It is characterized by dry air and high temperatures. It consists of repeated short, and occasionally long, exposure to a hot environment with temperatures ranging from 70˚C to 100˚C. Sauna baths are no longer limited to Nordic countries but have become popular worldwide [1]. In Burundi, sauna practice is especially common in the city of Bujumbura.

There are different types of saunas, such as smoke or underground saunas. The modern sauna consists of a room made mainly of untreated wood, with structures for lying down or sitting, and a heater containing stones. The room measures at least 3 m² to ensure a proper balance of heat, humidity, and ventilation. The temperature ranges between 70˚C and 100˚C, with the optimal range being around 80˚C to 90˚C at the user’s face level and about 30˚C at floor level. Air humidity is usually between 10% and 20%, but it can be temporarily increased by bursts of steam generated when water is poured onto hot stones. Drainage is ensured through the floor. Ventilation must be effective, with frequent air changes (3 to 8 times per hour). A sauna bath consists of a series of short stays inside the sauna lasting from 5 to 20 minutes, depending on the user’s tolerance and comfort, interspersed with cooling periods (shower, swimming in cold water, outdoor exposure, or resting in a room at ambient temperature) [2].

The sauna induces acute physiological changes in renal and cardiac function [3]. Profuse sweating appears rapidly, with an average water loss of about 0.5 to 1 liter. Cutaneous blood flow increases, while visceral and muscular blood flow decreases. Renal blood flow decreases by 0.4 L/min. Cardiac output rises along with heart rate. The effects of the sauna on blood pressure are variable. Conversely, cold exposure after leaving the sauna stimulates the sympathetic nervous system, causing cutaneous vasoconstriction, a decrease in heart rate, and an increase in blood pressure. Skin temperature drops to around 33˚C, but core temperature remains stable. Within hours after the sauna session, cardio-circulatory function returns to baseline. Fluid loss is compensated by oral hydration after the session [4].

The study aimed to assess the physiological effects of sauna exposure on cardiovascular and renal parameters in apparently healthy adults.

This study aimed to explore the physiological effects of a sauna session on cardio-circulatory and renal functions among users of six sauna facilities in Bujumbura. The results provide interesting insights both into the immediate physiological responses to sauna use and the structural characteristics of local facilities.

The characteristics of the sauna rooms showed partial compliance with standard norms, with an average surface area of less than 10 m², ventilation often missing, and heating exclusively by wood. These factors may influence air quality and heat distribution, and therefore the physiological response of users [4]. The absence of thermometers in all observed facilities is particularly concerning, as it prevents precise control of temperature, an essential factor for both safety and efficacy of sauna sessions [2].

From an anthropometric and sociodemographic perspective, men represented two-thirds of the sample and were older and more corpulent than women. Most participants were unaware of the risks associated with sauna practice, reflecting poor health education, as also reported in other low-income countries [1].

The present study found a statistically significant decrease in mean arterial pressure after sauna exposure, whereas systolic and diastolic blood pressure remained unchanged. This pattern suggests that sauna exposure may induce a modest acute reduction in vascular resistance without producing clinically meaningful changes in conventional blood pressure measurements. Similar transient hemodynamic responses have been reported in previous studies and are generally attributed to heat-induced vasodilation and redistribution of peripheral blood flow [5][6]. Nevertheless, the magnitude of the observed MAP reduction was small, and therefore the findings should not be interpreted as evidence of a substantial hypotensive effect of sauna use. However, in a literature review on both healthy individuals and patients with heart disease, Ptak et al. [6] concluded that sauna use, particularly Waon therapy, helps reduce blood pressure, improve vascular function and myocardial perfusion, decrease symptoms of heart failure, and enhance exercise tolerance among patients. Heart failure patients therefore represent a target population that may derive significant clinical benefits from such approaches. Moreover, some studies suggest that regular sauna use may also have a preventive effect in healthy individuals, protecting them against future development of hypertension [7]. Finally, this finding suggests a mild acute vasodilatory effect of heat exposure, consistent with previous studies reporting transient reductions in vascular resistance following sauna use [6][8].

From a renal standpoint, no significant changes were observed in urea or creatinine levels, indicating short-term stability of renal function. These findings are consistent with the work of Kunutsor SK et al. [9], who showed that well-monitored sauna sessions do not impair renal function nor increase the risk of future chronic kidney disease. However, it is important to note that the studied population was mostly young and without major medical history. In individuals with chronic kidney disease or susceptibility to dehydration, the effects may differ considerably. Indeed, fluid loss from intense sweating may cause hemoconcentration, reduced renal blood flow, or even transient increases in creatinine among sensitive subjects [4]. The absence of significant change in this study could also be explained by rapid rehydration or by the short exposure time. Nevertheless, it should be emphasized that hyperthermia puts the kidneys in a state of vulnerability. As body temperature rises, blood flow is redirected to the skin to promote cooling, thereby diverting blood from internal organs such as the kidneys in order to maintain blood pressure regulation. At the same time, hyperthermia and dehydration increase metabolic demands to conserve fluids and electrolytes. Together, these mechanisms can create a local imbalance between supply and demand, leading to tissue hypoxia potentially damaging to renal tissues. These findings highlight that heat-related hyperthermia may cause subclinical kidney injury, with possible long-term consequences on renal function [10]. Studies with prolonged exposure, longitudinal follow-up, or targeting specific populations are needed to better assess the nephrological implications in the medium and long term [9]. It is well known that sauna users usually ensure adequate hydration during their sessions. Furthermore, although hematological results (hemoglobin, leukocytes, platelets) did not change significantly, this may reflect either a rapid adaptive response or insufficient sensitivity of the tests to detect subtle variations after a single session.

The absence of significant variation in hemoglobin, platelets, leukocytes, and red blood cells indicates that immediate hematological changes remain minimal after a single exposure. This contrasts with some reports of moderate hemoconcentration following heavy sweating, often observed after longer or repeated exposures [11][12]. Moreover, although the mean weight loss was small and not significant, it can be mainly attributed to acute dehydration due to sweating, as observed in other heat exposure contexts. Post-session rehydration is therefore essential to avoid hydro-electrolytic imbalances.

The modest effect observed on cardiovascular parameters may be related to the moderate duration of sessions and their intermittent nature for most participants. Even a single sauna session provides immediate cardiovascular benefits, reducing arterial stiffness and blood pressure without disturbing electrolytes. This may help explain why regular sauna practice is associated with a reduced risk of cardiovascular events [13]. In more intensive studies, involving several weekly sessions or prolonged exposure, more pronounced effects have been observed, including improved blood pressure profile, heart rate variability, and endothelial function [14]-[16].

Note also that the psychological or subjective impact, not evaluated here, remains an important area for further exploration. Several studies have demonstrated improved well-being, reduced stress, and significant anxiolytic effects after sauna use [17]-[19].

This study has several limitations. The relatively small sample size may have limited the statistical power to detect subtle changes in some physiological and biochemical parameters. The study assessed only the immediate effects of a single sauna session and therefore cannot provide information on long-term cardiovascular or renal outcomes. The sauna exposure was not completely standardized across facilities, as session duration, heat cycles, cooling periods, and environmental conditions may have varied between establishments, potentially introducing exposure heterogeneity. Direct markers of hydration status and electrolyte balance, such as serum sodium, potassium, osmolality, or urine-specific gravity, were not measured. Consequently, the absence of significant changes in serum creatinine and urea should be interpreted cautiously, as subclinical fluid and electrolyte shifts may have occurred without being captured by the available biomarkers. Finally, the study was conducted among apparently healthy sauna users in a single city, which may limit the generalizability of the findings to other populations.

This study evaluated the acute cardiovascular, renal, and hematological effects of sauna use among adults attending six sauna facilities in Bujumbura. Most sauna facilities showed suboptimal conditions, including inadequate ventilation and the absence of thermometers. Men constituted the majority of participants, and many users were unaware of potential sauna-related health risks. Sauna exposure produced a modest but significant reduction in mean arterial pressure, suggesting transient heat-induced vasodilation. However, systolic and diastolic blood pressure remained unchanged. No significant changes were observed in serum creatinine, urea, hemoglobin, leukocytes, platelets, or red blood cells, indicating short-term physiological stability in healthy individuals. Weight loss after the session was minimal and likely related to mild dehydration. These findings support the short-term safety of sauna use in healthy adults. Nevertheless, individuals with renal disease or a risk of dehydration may require caution. Further large-scale and longitudinal studies are needed to evaluate the long-term cardiovascular, renal, and psychological effects of sauna exposure.

This study was funded by the University of Burundi.