The book's Chapter 13 covers the famous 2014 endotoxin study (cited in the Notes) and its limits: healthy young men, no active control, and no way to tell whether the breathing, the cold, or the meditation did the work. A decade on, the evidence has been reviewed as a whole.
Nine papers covering eight trials met the review's criteria. Across them, the method raised adrenaline and interleukin-10 and lowered pro-inflammatory cytokines, consistent with the original finding. Effects on exercise performance were mixed, and effects on breathing measures after exercise were mixed too. The reviewers call the inflammatory finding promising and ask for trials in people who actually have inflammatory conditions, which is the test that has not been run. A 2025 semi-randomized trial comparing 29 days of the method against meditation found the method's effect on daily state stress grew over the month while meditation's shrank, with little difference in trait measures at the end.
The book's position has not moved: the fast breathing drops CO2, the cold is a stressor, and the two together are not a beginner's practice. Never do the breathing in or near water; people have drowned doing exactly that. The book's bellows breath page carries the same rule for the gentler version.
The studies
Almahayni O, Hammond L. Does the Wim Hof Method have a beneficial impact on physiological and psychological outcomes in healthy and non-healthy participants? A systematic review. PLOS ONE 19(3), 2024, e0286933. Read the abstract
- Sample
- 9 papers, 8 trials
- Design
- PRISMA systematic review, PROSPERO-registered.
- Found
- Reduced inflammation via raised epinephrine and IL-10 and lower pro-inflammatory cytokines; mixed effects on exercise performance and respiratory measures.
- Limits
- Few, small trials, mostly healthy participants; no trials in inflammatory disease.
Fox, Biddell, King. A semi-randomised control trial assessing psychophysiological effects of breathwork and cold immersion. Scientific Reports 15, 2025, 43879. Read the abstract
- Sample
- Not stated in abstract
- Design
- 29-day semi-randomized comparison of the Wim Hof Method against meditation as an active control.
- Found
- The method's daily effect on state stress, energy and clarity grew across days while meditation's declined; minimal between-condition trait changes at 29 days.
- Limits
- Semi-randomized; self-report state measures drive the headline; nuanced physiological and sleep findings.
Where this lands in the book
Techniques
Rooms
Two rules, no exceptions
Never practice any technique on this site while you are driving, operating machinery, or doing anything that needs your full attention. The slow ones can make you drowsy, faster than you’d expect. And never practice a breath-hold, bellows breath, or any vigorous breathing in or near water. Read Before you begin for the doctor’s list and the rest.
Verdict scale: Science means trials or meta-analyses with consistent findings; Promising means real trials that are small, few, or unreplicated; Mixed means the studies point both ways; Mechanism means it shows how something works in the body, not that it helps in life. Checked against the published abstracts in October 2026. Not medical advice.