What have studies of IHHT shown, and what have they not?

IHHT (intermittent hypoxia-hyperoxia therapy) is sold by clinics as sessions of breathing air with less oxygen, alternating with air with more oxygen, usually through a mask while resting. This page summarises published human studies: who took part, for how long and what was measured. It does not recommend IHHT or any protocol, and it says nothing about any clinic.

What is IHHT, and what is it not?

In IHHT you breathe low-oxygen air for a few minutes, then oxygen-enriched air, in repeated cycles. In the trials below, one session typically lasted 30–45 minutes. In a 2026 pilot trial a session was six cycles of 5 minutes at 10–14% oxygen and 3 minutes at 30–40% oxygen.

Several other methods also use low oxygen and are often confused with IHHT:

• Altitude training for athletes: living at altitude and training lower ("live high, train low"), or exercising in low oxygen. Most sports research is about these methods, not about resting IHHT.
• Intermittent hypoxia without the high-oxygen phase: used in spinal-cord-injury rehabilitation studies.
• Breathing-resistance "altitude" masks: a randomised study found that such a mask did not simulate altitude and worked more like a breathing-muscle trainer.
• Hyperbaric oxygen (HBOT), breath-holding and blood-flow-restriction cuffs: separate methods with their own evidence.

A result for one of these methods cannot be assumed to apply to IHHT.

What have clinical studies of IHHT found?

The studies are small, short and mostly in older people or patients with existing conditions:

• Older people in rehabilitation. In a randomised trial of 34 patients aged 64–92, adding IHHT to a 5–7-week training programme gave larger improvements on two cognitive screening tests and a 6-minute walk than training with ordinary air. A sham-controlled trial of 25 patients aged 77–94 found a benefit on one cognitive test and on mobility tests, but not on the other cognitive test.
• Prediabetes. In a placebo-controlled study of 55 patients, three weeks of IHHT or of hypoxia alternating with ordinary air both lowered blood glucose and some blood lipids, by similar amounts.
• Metabolic syndrome. In a randomised trial of 65 patients already on medication, three weeks of IHHT lowered blood pressure compared with sham breathing.
• Coronary artery disease. A non-randomised study of 46 patients reported better exercise tolerance after IHHT. The comparison group had completed standard cardiac rehabilitation, so the groups were not directly comparable.
• Blood pressure and heart rhythm in older adults. A 6-week trial of 22 people and a single-session trial of 16 people reported changes in blood pressure and heart-rate variability. Both were very small.

Two reviews pool this work. A 2022 systematic review found eight controlled studies of IHHT at rest; they were too different to combine statistically. A 2021 meta-analysis of hypoxia conditioning, including IHHT, in older patients with heart disease found lower resting blood pressure and heart rate, and no change in blood lipids or blood counts.

Which results were negative or mixed?

• In the trial of 25 older patients, adding IHHT did not raise BDNF, a protein linked to brain health, and did not lower the inflammation markers IL-6 and CRP.
• In a sham-controlled trial of 100 healthy adults, intermittent hypoxia changed some measures of blood flow in the brain but not cognitive performance.
• For exercising in low oxygen, a meta-analysis of 12 studies found no overall advantage for cognition over the same exercise in ordinary air, with very low certainty of evidence.
• For people with overweight or obesity, a meta-analysis of 9 trials found that exercise in low oxygen did not improve glucose, lipids or blood pressure more than the same exercise in ordinary air. Other reviews found small extra reductions in body fat.

What has not been shown?

No study found here shows that IHHT extends life, prevents dementia or reverses ageing. The trials measured short-term markers such as blood pressure, blood glucose, test scores and walking distance, usually for 3–7 weeks. A change in a marker is not the same as fewer heart attacks, prevented dementia or a longer life.

How strong is the evidence?

• Small samples: most IHHT trials had 16–65 participants.
• Short follow-up: weeks, not years.
• Few research groups: many of the trials share authors, so they are not fully independent confirmations.
• Commercial links: one prediabetes study lists an author from a company that makes IHHT equipment. The ongoing trial below includes authors from medical-device companies.
• Sports results do not transfer: large meta-analyses on oxygen uptake and athletic performance concern altitude and exercise protocols, not resting IHHT.

A finding that something "improved within the IHHT group" is weaker than a difference between the IHHT group and a sham group.

What do the studies say about safety?

The trials were run on selected participants under supervision. The 2021 meta-analysis reported no serious adverse events, and the single-session pilot reported none. Small, supervised trials cannot show how safe IHHT is for everyone. Whether it suits you is a question for your own doctor.

Which studies are still under way?

• REenergizeME (Denmark) is testing IHHT against placebo in 104 women with ME/CFS over 8 weeks. Only the protocol is published; there are no results yet.
• DRKS00038438 (University Hospital Tübingen) compares interval training, yoga and IHHT with sham in people with mild cognitive impairment. It was still recruiting at its 10 September 2026 update.

Sources checked in PubMed and the German Clinical Trials Register on 24 October 2026.