The Wim Hof Method

The Wim Hof Method combines cold exposure with a forceful breathing exercise, and it is famous for one striking experiment in which trained practitioners resisted an injected bacterial toxin better than untrained volunteers. That result is real, but a follow-up pilot study suggested the breathing does most of the work and the cold mainly amplifies it, and every trial of the method has been small, mostly in men and rated very low quality.

Don't take up the method expecting a stronger immune system; no trial has tested it against real infection. Never do the breathing in or near water, because it can make you black out, and never hold your breath under cold water.

What it is

The method bundles regular cold exposure with a breathing exercise: cycles of deliberate hyperventilation — fast, forceful breathing — practised with or without long breath-holds. The cold half is covered on Cold exposure. This page is about the claim that the package changes how the immune system responds.

The famous experiment

In a small randomised trial of 24 people, those trained in the method were given an intravenous dose of endotoxin — a fragment of bacteria that triggers a short, flu-like inflammatory reaction. Compared with untrained volunteers, they showed higher adrenaline (epinephrine), more of the anti-inflammatory signal interleukin-10, a blunted rise in pro-inflammatory signals (tumour necrosis factor-α, interleukin-6 and interleukin-8) and fewer symptoms.^[1]

Two limits matter. The trial could not say which part of the method did it. And an endotoxin injection is a brief laboratory challenge, not a real infection or an autoimmune disease.

Taking the method apart: the breathing does most of the work

A follow-up separated the pieces. Forty-eight healthy men were randomised to cold-exposure training, the breathing exercise, both, or no training, and then given the same endotoxin challenge.^[2]

Cold training alone left both the inflammatory response and the adrenaline surge untouched, though this group was the only one with significantly milder peak flu-like symptoms than the untrained men.
The breathing exercise produced both.
Cold was not simply inert. It significantly amplified the breathing exercise's anti-inflammatory effect, and the combination was the most potent arm — the authors' own conclusion.

So the popular "cold boosts immunity" story, built on the first experiment, is mostly a story about breathing.

How good is the evidence overall?

A systematic review of the Wim Hof literature found eight trials, each with 15–48 participants. It rated every one very low quality and at high risk of bias, and 86% of the participants were men.^[3]

Overall the evidence is weak and preliminary: a real and interesting laboratory effect, tested in small groups of mostly men, against an experimental challenge rather than a real infection.

Safety: keep the breathing and the breath-holds out of the water

Breath-holding is what turns cold water dangerous for the heart. Breathing freely with your head out of cold water, about 2% of immersions in healthy young people produce a heart-rhythm disturbance; submerged with a held breath, 62–82% do.^[4]

The breathing exercise carries its own drowning risk. Hyperventilating before a breath-hold lets you stay under water longer, but it can make you lose consciousness from lack of oxygen with no warning. In 58 such blackouts during underwater swimming, all in good swimmers and most in guarded pools, 23 people died.^[5] US health officials traced 16 drownings and near-drownings in New York State to deliberate hyperventilation and other breath-holding games underwater.^[6] Do the breathing sitting or lying down on dry land, never in or under water.

The full list of who should avoid cold water — heart-rhythm conditions, certain medicines, pregnancy and more — is on Cold exposure.

What to do

Treat the immune claims as unproven. The evidence is a laboratory toxin challenge in small groups of mostly men, not protection against infection.
Never hold your breath with your face under cold water, and never do the hyperventilation rounds in or near water.
Build up the cold part gradually, with the same precautions as any cold-water practice.