Podcast episode 104 (in German)

Long-term alkaline bathing – thirty hours in the water, and what really happens

October 12, 2026

Around 30 hours in the bathtub over 3 days, and afterwards the claim is: that went deep into the body and the organs. The sentence is even true — only the route runs through the kidneys and circulation instead of the skin, and the skin pays for the duration.

What it is about

An alkaline bath brings the water to a pH of usually 8.5; the scene itself names 90 to 120 minutes as standard and 5 hours as the upper limit. The episode examines a practice far beyond that: 10 to 12 hours a day, 2 or 3 days in a row. The usual justification, that acid leaves the body through the skin, does not hold; that job is done by the kidneys. What does hold is immersion itself. Water pressure shifts blood from the legs into the chest and heart, the body interprets this as too much volume and excretes water and sodium — the same diuresis divers know. Added to this are measured relaxation and studies on baths for back pain. The bottom line: the effect is real and well measured, and with duration the price paid by the skin’s outer horny layer rises.

Key points

The kidneys excrete around 71 millimoles of acid per day, measured in a metabolic study in 18 postmenopausal women — the skin plays no significant role in this balance.
13 healthy men passed 758 milliliters of urine and 56 millimoles of sodium in 3 hours of immersion, a good 3 grams of table salt.
After 5 hours in water, urine flow and sodium excretion at 32 and 34.5 degrees were six times higher, at 36 degrees only three times — warmer is not more.
The only measurement over many hours comes from 16 divers in water at 5 degrees: plasma volume minus 17 percent, plus clearly elevated values for potassium, calcium, magnesium and zinc.
Just 3 hours under a water patch increased the water content of the entire horny layer and its permeability in 10 people; small pools of water formed between the horny cells.
A 2006 meta-analysis found, in 5 randomized studies on spa therapy for low back pain, an advantage of around 27 millimeters on a 100-millimeter pain scale compared with waiting lists.
In 12 patients with chronic heart failure, the normal circulatory response to immersion was completely absent — known heart failure should be discussed with a physician beforehand.

Where the data stop

There are no studies on baths of more than 10 hours. The longest immersion trials in thermoneutral water end at 5 hours; what happens between 5 and 12 hours appears in no paper. The diver measurement comes closest but fits only to a limited extent, because cold produces its own diuresis and the dry suit changes the water pressure. The temperature finding, too, is derived from 5 hours, not 10. And the back studies test spa protocols with 20-minute baths over 2 or 3 weeks; they do not say that more hours help more.

For the skin, the direction is clear, the threshold is not. The barrier recovers after a bath, but with 10 hours in the water and a 14-hour break the time window becomes tight, and the alkaline water raises the pH of the acid mantle more than plain water. Anyone who bathes over days and replaces fluid only with plain water also dilutes their sodium further — in extreme cases to the point of hyponatremia.

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Published on October 12, 2026.

The topic in the database

Hours in warm water. The justification on the package does not hold – the effect exists anyway, just via a different route.Long-term alkaline bathing

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Information only, not medical advice and not a usage or dosage recommendation. Prescription-only and unapproved substances belong in the hands of a physician. Last updated: 2026-09-22.