Sauna use, especially dry Finnish sauna, triggers intense sweating and genuine fluid loss. We explain why this raises measured hematocrit without any change in red blood cell count, and how long you need to wait for the result to return to normal.
Yes — especially after a dry sauna
A sauna session, particularly in a dry Finnish sauna at high temperature and low humidity, can genuinely raise measured hematocrit. This is the effect of plasma volume loss through sweat — a concentration of the blood, not an actual increase in red blood cell count. The effect is temporary and resolves with rehydration.
Hematocrit is the percentage of blood volume occupied by red blood cells — the ratio of red cell volume to total blood volume. In a sauna, the body sweats intensely, losing fluid mainly from the intravascular space (plasma) to cool itself through evaporation. A smaller plasma volume with the same number of red blood cells automatically raises measured hematocrit — a phenomenon called hemoconcentration. This is exactly the same mechanism we describe with ordinary dehydration, just triggered by heat rather than, say, exercise or restricted fluid intake.
It's worth highlighting the difference from genuine erythrocytosis — an actual increase in red blood cell mass produced by the bone marrow (as happens, for example, with testosterone therapy or chronic hypoxia). Hemoconcentration after a sauna doesn't increase the number of cells in the body — it simply, temporarily concentrates the same number of cells into a smaller volume of fluid. The difference matters practically: hemoconcentration resolves within hours of rehydration, while genuine erythrocytosis takes weeks.
Effects of dry and mist saunas on circulatory and thermoregulatory functions in humans
Moderate evidenceIwase S, Kawahara Y, Nishimura N, Takada H, Nagata M, Niimi Y, Miwa C · Health · 2013
A study comparing a 10-minute session in a dry infrared sauna (70°C, 15% humidity) with a session in a mist sauna (40°C, 100% humidity) in healthy men. A statistically significant rise in hematocrit was observed only after the dry sauna — accompanied by a clearly larger loss of plasma volume than in the mist sauna, which the authors link to more intense sweating at high temperature and low humidity.
View studyThe scale of fluid loss in a single sauna session depends heavily on temperature, duration, and individual sweating intensity — typically, losses range from several hundred milliliters to over a liter of fluid during a single, longer session, which is enough to noticeably concentrate the blood. The effect on hematocrit appears already during the session itself and persists until the lost fluid is replaced — for most people this is a matter of hours, not days, provided the fluid loss wasn't very large.
It's also worth remembering the broader context: sauna is just one of several factors that can temporarily inflate hematocrit (alongside, for example, exercise-induced dehydration or body position during the draw) — the mechanism is always the same, dilution or concentration of plasma, regardless of the specific cause.
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How to avoid an inflated result after sauna use
Elevated hematocrit after a sauna means the body started producing more red blood cells.
This is usually a purely dilutional effect — the same number of red blood cells concentrated into a smaller plasma volume after fluid loss through sweat, not a real increase in erythropoiesis. Genuine erythrocytosis develops over weeks, not hours, and doesn't resolve with simple rehydration.
No — data suggest the effect is more pronounced after a dry sauna at high temperature and low humidity than after a mist sauna, likely due to more intense sweating and a greater loss of plasma volume in the dry sauna.
It usually returns to baseline within a few hours of full rehydration, as long as the fluid loss wasn't very large. That's a much shorter timeframe than genuine erythrocytosis, which persists for weeks.
Yes, which is why when monitoring hematocrit in the context of testosterone therapy, it's worth scheduling tests on days without a recent intense sauna session and while well hydrated, to avoid misinterpreting the result as genuine erythrocytosis.
By itself, as a temporary hemoconcentration effect, it isn't dangerous and resolves with rehydration. It mainly matters when it affects the interpretation of a test done for another reason, such as monitoring hormone therapy.
dr Katarzyna Lewandowska
Specialist physician in cardiology, cardiovascular-prevention consultant
Katarzyna works as a cardiologist at a Warsaw teaching hospital and has spent years focused on cardiovascular prevention — trying, as she puts it, to convince people to change their habits before they end up on her ward, not after. She joined VitMode after a series of conversations with Anna at a lifestyle-medicine conference, where the two discovered they shared the same frustration: an internet full of contradictory claims about cholesterol, aspirin and heart supplements, with no clear signal of what's actually backed by research. She reviews content on cardiovascular health, lipid panels and pharmacological prevention, consistently distinguishing what helps a statistical population from what makes sense for a specific person. Off duty, she road-cycles — not for performance, but because, in her words, it's hard to write credibly about prevention without practicing it yourself.
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Kasia W. 2 weeks ago
Very clearly explained, especially the interactions section — I hadn't seen it laid out this well anywhere else.
Marek T. a month ago
Are you planning to update this with the newest study from this year? I saw an interesting meta-analysis.
