A 29% reduction in sick days. That’s the number you’ll find on nearly every ice bath company’s website, every cold plunge influencer’s feed, every listicle about the immune benefits of cold water. It comes from a 2016 Dutch study published in PLOS ONE, and it is real. But what the study actually found is more interesting and more modest than the way it gets quoted.
Here’s what the researchers discovered when they tracked over 3,000 participants through 30 consecutive days of cold showers: the cold shower group’s illness days were statistically no different from the control group’s. What changed was their behaviour when they did get sick. They took 29% fewer days off work. And the duration of the cold shower made no difference at all. Thirty seconds produced the same result as ninety.
That gap is the difference between a biological claim and a behavioural one, between “cold water strengthens your immune system” and “cold water may make you tougher when illness hits.” The honest version is more useful than the marketing version. So here’s the full picture, claim by claim.
What the Buijze study really tells us
Geert Buijze, at the academic medical centre in Amsterdam, led what remains the largest randomised controlled trial on cold showers and health outcomes. Participants finished their morning showers with 30, 60, or 90 seconds of cold water, or showered normally. Follow-up ran for 90 days.
That 29% sickness-absence reduction survived adjustment for age, sex, lifestyle factors, and regular physical activity. But the mechanism wasn’t immunological in any obvious way. Participants reported the same number of illness days as the control group; they simply showed up to work more when they felt unwell. Whether that reflects improved perceived wellbeing, greater tolerance for discomfort, a placebo-driven sense of hardiness, or some combination, the study can’t say. And regular physical activity independently reduced sickness absence by 35%, slightly outperforming the cold showers. Combined, the two produced a 54% reduction. Exercise wasn’t a footnote in this study. It was the co-lead.
The biomarker question: does any of it translate?
Dozens of studies over the past three decades have documented immune marker changes following cold water exposure. White blood cell counts rise. Lymphocyte numbers increase. Natural killer cell activity ticks up. Inflammatory cytokine profiles shift. Researchers have observed these effects since Janský and colleagues first reported them in 1996, and the pattern is consistent enough to be real. Acute cold exposure activates the sympathetic nervous system, triggers norepinephrine release, and produces a measurable immune cell mobilisation.
But a white blood cell count rising for an hour after a cold plunge is an acute stress response. Your white blood cell count also rises after a hard run, a frightening experience, or a strong cup of coffee. What actually matters is whether repeated cold exposure produces lasting, functional improvements in immune competence: fewer infections, faster recovery, better surveillance. On that question, the evidence is far thinner than the market suggests.
François Haman, a thermal physiologist at the University of Ottawa who has studied human cold-stress responses for over two decades, has been characteristically direct. In his assessment, the immune evidence is “totally unclear.” The biomarker changes exist. Their clinical significance remains unestablished.
What the most recent meta-analysis found
In early 2025, Tara Cain, a health scientist at Coventry University, and colleagues published the most comprehensive systematic review and meta-analysis of cold water immersion’s effects on immune function to date, pooling data from 11 randomised controlled trials involving 3,177 participants.
Their verdict was clear: no significant effects on immune function either immediately after or one hour after cold water immersion. The only real-world illness outcome in the entire evidence base came from a single study, Buijze’s cold shower trial, which found reduced absence rather than reduced illness.
“While there have been claims that cold-water immersion experiences can boost your immunity and mood, we found very little evidence to support these claims,” Cain said when the review was published. Anyone who says the evidence clearly shows cold water “boosts your immune system” is overstating what we actually know.
What the cold might not be doing
In 2022, Esperland and colleagues published a review of 104 studies on the health effects of cold water exposure and raised a point that should make every cold plunge company uncomfortable. Many of the health benefits attributed to cold exposure, they argued, may not be causal. Regular cold water swimmers tend to be more physically active, handle stress differently, maintain social connections through swimming groups, and exhibit higher psychological resilience. Any or all of these factors could explain their better health outcomes.
A study by Collier and colleagues makes the point concrete. Researchers tracked upper respiratory infections in regular cold water swimmers compared with their non-swimming partners. Cold swimmers reported significantly fewer infections: 3.0 episodes per year versus 5.5. But when compared with regular pool swimmers exercising in warm water, the difference disappeared. Cold wasn’t the active ingredient. Exercise was.
Esperland’s review also flags social connection — regular cold water swimmers tend to swim in groups, and communal activity from Finnish sisu traditions is itself a well-documented health variable. Disentangling the effect of cold itself from the effect of being the kind of person who voluntarily gets into cold water remains the central methodological challenge this field hasn’t solved.
The Wim Hof complication
Readers familiar with cold exposure research will have encountered the 2014 study by Kox and colleagues, published in PNAS, which showed that Wim Hof Method practitioners could voluntarily influence their immune response when injected with bacterial endotoxin. Trained participants produced more anti-inflammatory cytokines and fewer pro-inflammatory ones. Their flu-like symptoms were milder.
Here’s the problem: the Wim Hof Method combines cold exposure with specific breathing techniques and meditation. Kox’s team did not isolate cold from the other components, and later research has suggested the breathing exercises may be the primary driver of the observed immune effects. This study is evidence for a combined intervention. It cannot be cited as evidence that ice baths reduce inflammation alone.
The dose-response sweet spot
If the evidence for cold-exposure immune benefits is modest, is there a frequency at which whatever benefit exists is maximised? A 2025 cross-sectional study of 1,233 regular cold water immersion practitioners in Poland offers the most useful answer currently available.
Researchers found an inverted U-shaped dose-response curve for both mental health outcomes and upper respiratory tract infection rates. Benefits increased with frequency up to roughly twice per week. Beyond that, they plateaued, and at very high frequencies appeared to decline. In hormetic terms, this makes sense: a moderate stressor triggers adaptive responses, while too much of the same stressor overwhelms them. The same principle governs exercise training, and this finding may be the most practically useful one in the cold-immune literature — one almost nobody discusses.
The implication: more is not better. Twice a week appears to be a reasonable frequency for someone interested in immune-supportive cold exposure, and a far more sustainable commitment than daily plunging, which is where most people’s practice falls apart.
Practical guidance from the evidence
Most of the research protocols used water between 10°C and 15°C, with immersion times ranging from one to several minutes. Buijze’s cold shower study suggests that even 30 seconds of cold may be sufficient for the behavioural resilience effect. Most commercial cold plunges operate colder, typically 6–8°C, which is worth knowing: you’re likely getting a more intense stimulus than the study participants did, and the dose-response calculation may shift accordingly.
Geography also matters in ways few people consider. Buijze’s study was conducted during winter in the Netherlands, where cold tap water runs at roughly 10°C. A cold shower in Bali uses water closer to 27°C. From Icebaths.com’s 500+ installations across Southeast Asia, this is a consistent operational reality: if you live somewhere warm and rely on cold showers rather than a temperature-controlled plunge, your cold stimulus is significantly milder than what was tested. A dedicated cold plunge closes that temperature gap regardless of climate.
Frequency, then, is the variable most within your control. The Polish study points to twice a week as a reasonable target. Three times is probably fine. Daily plunging isn’t supported by the dose-response data for immune outcomes, and Buijze’s finding that exercise independently outperformed cold showers should shape how you think about priorities. Cold exposure works best as one element in a broader practice, not as a standalone immune intervention.
Heather Massey, a researcher at the University of Portsmouth and a cold water swimmer herself, has captured the field’s position with useful clarity: the evidence for inflammatory benefits is building, but it’s early. The honest response to that isn’t disappointment. It’s calibration.
The honest position
Cold exposure probably contributes something to immune health: the biological plausibility is there, the dose-response data points to a real signal at moderate frequencies, and the association between regular cold water practice and fewer respiratory infections is consistent enough to take seriously. But the most recent meta-analysis found no significant immune effects across pooled trial data. The largest trial found people took less time off work, not that they got sick less. Cold exposure is one thread in a larger fabric alongside consistent movement, adequate sleep, managed stress, and the kind of social connection that often comes bundled with any committed physical practice.
Understanding this is not a reason to stop plunging. What’s defensible isn’t the dramatic case, where a three-minute ice bath supercharges your white blood cells into illness-fighting readiness. It’s the quieter case: that building a consistent practice of voluntary discomfort, held alongside other good habits, cultivates a resilience the research hasn’t fully quantified. Science will catch up. It tends to, once the early practitioners have already moved on to the next question.
