Sauna and Blood Clots: Does Heat Raise DVT Risk?
Key insights
- The only long-term study on the question followed 2,242 Finnish men for a median of 24.9 years and found that those who used a sauna 2–3 times a week had a 33 per cent lower risk of venous thromboembolism (deep vein thrombosis or pulmonary embolism) than men who used it once a week or less 1.
- Men using the sauna 4 or more times a week showed no clear difference in risk either way; that group was small and the estimate was statistically uncertain, so it is not evidence of harm or of extra benefit.
- Frequent sauna use in the same Finnish cohort was associated with lower C-reactive protein, fibrinogen and white-cell counts 11 years later, all markers linked to clotting and inflammation 2 3.
- In a small laboratory study, raising core temperature by 2°C markedly reduced circulating platelet- and endothelial-derived microparticles, which take part in clot formation 4.
- The realistic acute concern is fluid loss, not heat itself: rehydrate after every session, and if you have a history of DVT, a clotting disorder or take anticoagulants, agree a plan with your clinician first.
Search for “sauna and blood clots” and you will find two opposite worries. One says heat thins the blood and gets it moving, so a sauna must be good for circulation. The other says sweating dehydrates you and thickens the blood, so a sauna must raise the risk of a clot. Both sound plausible, and both are usually offered without a single study behind them.
The question matters. Venous thromboembolism (VTE), the umbrella term for deep vein thrombosis (DVT) in the legs and pulmonary embolism (PE) in the lungs, is one of the most common cardiovascular problems in adults. People who have had a clot, long-haul travellers, and anyone on hormone therapy reasonably want to know whether a regular sauna habit is helping or hurting.
The encouraging news is that there is real long-term data. The honest caveat is that it comes from one cohort of middle-aged Finnish men, so it answers the question for that group better than it answers it for everyone.
Why heat could plausibly push clot risk either way
Clots in the veins form when three conditions line up: slow or pooled blood flow, damage or activation of the vessel lining, and blood that is primed to coagulate. Heat touches all three. A sauna dilates blood vessels and roughly doubles cardiac output, which speeds flow through the skin and limbs. Repeated heat exposure also appears to calm systemic inflammation and the vessel lining. On the other side of the ledger, a session can cost several hundred millilitres of sweat, and losing plasma water temporarily concentrates the blood. Which effect dominates over years is an empirical question, and that is where the Finnish data comes in.
What the Finnish cohort actually found
The evidence comes from the Kuopio Ischaemic Heart Disease (KIHD) study, the same eastern Finnish cohort behind the well-known sauna and mortality findings. Researchers recorded the sauna habits of 2,242 men aged 42 to 61 who had no history of VTE, then followed them for a median of 24.9 years. Over that time, 146 men (6.5 per cent) had a first venous thromboembolism 1.
Compared with men who used the sauna once a week or less, those who used it 2–3 times a week had a hazard ratio of 0.67 (95% CI 0.46–0.96) after adjustment for established risk factors including lifestyle, a 33 per cent lower risk. For men using it 4 or more times a week, the hazard ratio was 0.92 (95% CI 0.51–1.68): a wide interval that includes both benefit and harm, most likely because relatively few men sat in that group 1.
The headline, then, is not that sauna prevents clots. It is that in a large group of regular sauna users followed for a quarter of a century, there was no sign that habitual sauna bathing raised VTE risk, and moderate use was associated with less of it.
The likely mechanisms: inflammation, fibrinogen and platelets
Inflammation and coagulation are tightly linked, and this is where the plausible explanation sits. In 2,269 KIHD men with blood drawn at baseline and again 11 years later, those who used the sauna 4–7 times a week had, compared with once-weekly users, 1.66 mg/L lower C-reactive protein, 0.16 g/L lower fibrinogen and 0.49 × 10⁹/L fewer white blood cells at follow-up, after adjusting for age, weight, smoking, physical activity and other factors 2. Fibrinogen matters here directly: it is the protein that becomes the fibrin mesh of a clot, and it drives blood viscosity.
The same research group separately examined C-reactive protein and clots in 2,420 KIHD men with 119 VTE events. The within-cohort association was modest and borderline (hazard ratio 1.17 per standard deviation of CRP, 95% CI 0.98–1.40), but their pooled analysis of published prospective studies concluded that higher CRP is associated with higher VTE risk 3.
There is also a more direct laboratory signal. When ten healthy young men were heated until core temperature rose by about 2°C, circulating microparticles shed by platelets and by the vessel lining fell markedly 4. These particles are pro-coagulant, so a reduction fits the idea that heat nudges the blood towards a less clot-prone state. The caveats are real: it was a water-perfused suit rather than a sauna, the sample was small, and it measured a single session.
The acute concern: fluid loss and thicker blood
The dehydration worry is not baseless. During and just after a session, sweat loss shrinks plasma volume and the blood is temporarily more concentrated. For most people this reverses once fluids are replaced; it becomes relevant when sessions are long, alcohol is involved, or someone already has other risk factors. We cover the numbers in our guide to sauna hydration and electrolytes.
Interestingly, the body adapts in the opposite direction over time. In seven trained cyclists who added 30 minutes of sauna at 87°C after training on consecutive days, plasma volume expanded during the sauna block 5. Repeated heat exposure, in other words, tends to increase the fluid portion of the blood rather than deplete it, provided you drink enough to support the adaptation.
Realistic expectations
This is observational evidence from one cohort of white, middle-aged Finnish men, published as a short research letter, with sauna habits measured once at baseline. Men who sauna regularly may also be healthier in ways the statistics could not fully capture. There are no randomised trials, no data on women, and nothing on people who have already had a clot or who take anticoagulants. The fair reading is that regular, moderate sauna use does not appear to increase clot risk in healthy middle-aged men and may be associated with a lower one. Sauna is not a substitute for medical clot prevention after surgery, injury or long-haul travel.
A practical, clot-conscious sauna routine
Frequency: the lowest-risk pattern in the data was 2–3 sessions a week. Duration: 15–20 minutes per round is typical of the Finnish habits studied. Hydration: drink water before and after, and add electrolytes after long or repeated rounds. Alcohol: skip it before and during sessions, as it compounds fluid loss. Movement: stand up slowly and walk around during cool-down rather than lying still for long periods.
Check with your clinician first if you have had a DVT or pulmonary embolism, have a known clotting disorder, take anticoagulants, are pregnant or recently post-partum, use oestrogen-containing medication, or are recovering from surgery or a long period of immobility. And if you notice one-sided calf swelling, pain, warmth or redness, or sudden breathlessness or chest pain, seek medical assessment promptly rather than trying to sweat it out.
The Contrast Market Perspective
The Finnish data describes steady, moderate, repeatable sauna use: the same temperature, the same routine, a few times a week, for decades. That kind of consistency depends on equipment that holds its set temperature accurately, heats evenly, and is comfortable enough to use without pushing sessions longer or hotter than you intended. If you are planning a home sauna around a long-term health routine, we can help you match heater output, room size and controls to the way you actually use it. Schedule a consultation.
References
Footnotes
- Kunutsor SK, Mäkikallio TH, Khan H, Laukkanen T, Kauhanen J, Laukkanen JA (2019). Sauna bathing reduces the risk of venous thromboembolism: a prospective cohort study. European Journal of Epidemiology. PubMed ↩︎
- Kunutsor SK, Laukkanen T, Laukkanen JA (2018). Longitudinal associations of sauna bathing with inflammation and oxidative stress: the KIHD prospective cohort study. Annals of Medicine. PubMed ↩︎
- Kunutsor SK, Seidu S, Blom AW, Khunti K, Laukkanen JA (2017). Serum C-reactive protein increases the risk of venous thromboembolism: a prospective study and meta-analysis of published prospective evidence. European Journal of Epidemiology. PubMed ↩︎
- Bain AR, Ainslie PN, Bammert TD, Hijmans JG, Sekhon M, Hoiland RL, Flück D, Donnelly J, DeSouza CA (2017). Passive heat stress reduces circulating endothelial and platelet microparticles. Experimental Physiology. PubMed ↩︎
- Stanley J, Halliday A, D'Auria S, Buchheit M, Leicht AS (2015). Effect of sauna-based heat acclimation on plasma volume and heart rate variability. European Journal of Applied Physiology. PubMed ↩︎