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Short answer: In human studies, breathing hydrogen gas has usually been well tolerated, and none of the 15 studies we reviewed reported a serious adverse event that investigators attributed to hydrogen. Side effects did show up, though: a small drop in blood oxygen, more low-oxygen episodes after surgery, dizziness and a mild nasal-lining injury from a breathing mask, and, in one infant trial, more abnormal heart-enzyme, digestive and liver-test results than with air. The studies were small and short, and several didn't describe how they checked for side effects. The most serious documented harms came from the machines themselves: hydrogen is flammable, and inhalers have exploded.
This page covers breathing hydrogen gas, often called hydrogen inhalation therapy. For drinking it, see hydrogen water side effects and hydrogen tablets: side effects and evidence.
What side effects have hydrogen inhalation studies reported?
Mostly none or mild ones, with a few exceptions worth knowing. Here are 15 human studies of inhaled hydrogen, with doses as reported. Studies without a comparison group can't show whether an effect came from hydrogen.
| Study | Who took part | Gas and dose (as reported) | Length | Adverse events reported |
|---|---|---|---|---|
| Cole 2021 [1], no comparison group | 8 healthy adults | 2.4% hydrogen in medical air, 15 L/min by high-flow nasal cannula | 24–72 hours nonstop | No clinically significant adverse events; small, clinically insignificant rises in hematocrit and platelet counts |
| Grepl 2026 [2], placebo-controlled crossover | 20 healthy young women | Hydrogen gas vs room air (concentration not given in the abstract) | 60 minutes | Blood oxygen saturation 95.9% vs 96.7% on air; judged not clinically significant; heart-rate variability unchanged |
| Chen 2021 [3], no comparison group | 20 adults aged 31–60 | 66.7% hydrogen / 33.3% oxygen at 3 L/min by nasal tube | 2 or 4 hours a day for 2 weeks | Main T-cell counts unchanged; the share of some immune-cell subtypes fell; clinical side effects not described |
| HYBRID II, Tamura 2023 [4], placebo-controlled | 73 adults in a coma after cardiac arrest | 2% hydrogen with oxygen through a ventilator | 18 hours | Adverse events in 95% vs 88% within 72 hours; serious ones in 18% vs 21%; an independent committee attributed none to hydrogen |
| Katsumata 2017 [5], open-label | 20 adults with a STEMI heart attack | 1.3% hydrogen with 26% oxygen during the artery-opening procedure | During the procedure | No hydrogen-related severe adverse events |
| Ono 2017 [6], randomized (control group got standard medication) | 50 adults with a recent stroke | 3% hydrogen | 1 hour twice a day for 7 days | No significant adverse effects; oxygen saturation improved |
| Yoritaka 2021 [7], placebo-controlled | 20 adults with Parkinson's disease (15 analyzed) | 6.5% hydrogen in air at 2 L/min | 1 hour twice a day for 16 weeks | No adverse events; no benefit on symptoms |
| Guan 2020 [8], open-label, not randomized | 90 adults in hospital with COVID-19 (44 on hydrogen–oxygen) | 66% hydrogen / 33% oxygen at 6 L/min by nasal cannula | About 7.7 hours a day until discharge | Worse cough 6.8% vs 8.7%; chest tightness 2.3% vs 21.7%; abnormal lab results 2.3% vs 13.0%; no serious adverse events |
| Zheng 2021 [9], compared with oxygen | 108 adults with a COPD flare-up | Hydrogen–oxygen at 3 L/min (2 L/min hydrogen) by nasal mask | 6–8 hours a day for 7 days | Adverse events 63% vs 78% on oxygen; severe ones 3.7% vs 13.0%; device-related dizziness, nasal lining injury and worse wheezing in 2 people vs 3 on oxygen |
| Botek 2022 [10], placebo-controlled | 50 adults recovering from COVID-19 | 300 mL/min of pure hydrogen by nasal cannula, at home | 1 hour twice a day for 14 days | No side effects reported |
| Lin 2022 [11], sham-controlled | 153 adults aged 65+ after surgery (170 randomized) | 66.7% hydrogen / 33.3% oxygen by nasal cannula | 1 hour after surgery | Low blood oxygen (below 90%) in 7% vs 3%, especially after chest surgery; pain slightly higher on day 1; other side effects similar |
| Ji 2026 [12], compared with a nitrogen–oxygen mix | 150 adults having day surgery | Hydrogen–oxygen mix (66.7% hydrogen) by nasal cannula | Before and after surgery | Adverse events 14.5% vs 24.3%, a difference that wasn't statistically significant |
| Liu 2022 [13], placebo-controlled | 60 adults aged 50–70 with high blood pressure (56 finished) | 66% hydrogen / 33% oxygen at about 30–60 mL/min (about 0.2–0.4% inhaled hydrogen) | 4 hours a day for 2 weeks | No side effects reported in either group |
| Gao 2025 [14], compared with air | 66 adults with sleep problems (41 on hydrogen–oxygen) | 66.7% hydrogen / 33.3% oxygen at 60 mL/min by nasal tube | 1 hour twice a day for 7 days | Not reported (no adverse-event section) |
| Chen 2026 [15], placebo-controlled | 53 infants with epileptic spasms (27 on hydrogen–oxygen) | 66.6% hydrogen / 33.3% oxygen at 3 L/min by face mask, added to standard treatment | 1 hour four times a day for 14 days | Overall similar (25 vs 21 children); raised heart-enzyme results 13 vs 5 children, digestive problems 20 vs 11, abnormal liver tests 4 vs 0; no serious events; no benefit on spasms |
A few details the table can't show:
- The infant trial is the clearest warning sign. Infants breathed hydrogen–oxygen gas by face mask for four hours a day, on top of their usual treatment. Compared with babies who breathed air, more had raised heart-enzyme results, digestive problems and abnormal liver tests. All four liver-test problems resolved, two by the end of the trial and two within a week. The authors called the treatment "generally safe and feasible" but said heart, liver and gut function should be closely monitored [15]. The paper's percentages don't match its group sizes, so we report the counts.
- Masks and cannulas cause their own problems. In the COPD trial, device-related side effects in the hydrogen group included dizziness, an injury to the nasal lining and worse wheezing [9].
- One uncontrolled study raised an immune question. After two weeks of high-flow hydrogen–oxygen, the share of several immune-cell subtypes fell in 20 middle-aged adults, and the authors suggested it might suppress immune function in healthy people [3]. With no comparison group, it's unclear whether hydrogen caused the change or what it means for health.
- "No side effects" often wasn't backed by a method. Some studies reported no side effects without describing how they checked [10, 13], and the insomnia trial had no adverse-event section at all [14]. The studies with the most careful safety checks, such as predefined adverse events and independent review, include the two that used premixed gas at 2% to 2.4% [1, 4].
Is hydrogen inhalation safe?
For short, supervised use at low concentrations, the evidence is mostly reassuring. Longer and home use are much less studied, and home machines add a fire risk.
- Hydrogen isn't poisonous. It's non-toxic, but in large amounts in an enclosed space it can push out the oxygen you need to breathe [16, 17].
- Low, premixed concentrations have the best safety data. Eight healthy adults breathed 2.4% hydrogen for up to 72 hours without clinically significant adverse events [1], and patients after cardiac arrest breathed 2% hydrogen for 18 hours without adverse events attributed to it [4].
- Doses in other studies were very different. By our rough estimate, the hydrogen people actually inhaled ranged from about 0.1% to more than 10% across studies, so a safe result at one dose doesn't carry over to another. See how much hydrogen human studies used.
- Long-term use hasn't been tested. The longest study in our table lasted 16 weeks [7]; most lasted days to two weeks.
Can hydrogen inhalation lower blood oxygen?
Yes, slightly, in some studies. Any gas you breathe that isn't oxygen dilutes the oxygen in each breath. In a crossover study, an hour of hydrogen inhalation lowered healthy young women's average oxygen saturation from 96.7% to 95.9%, which the authors judged not clinically significant [2]. After surgery, low blood oxygen (below 90%) was more common during hydrogen–oxygen inhalation, 7% vs 3%, especially after chest surgery; the authors called it manageable by adjusting oxygen [11].
The effect depends on the dose and the gas. A 2026 modeling paper estimates that breathing 30% hydrogen would cut inhaled oxygen to about 14.7%, and it advises keeping inhaled hydrogen below about 30% unless extra oxygen is given, with oxygen monitoring at high doses [18]. Reaching 30% would take roughly 2 to 10 L/min of hydrogen for most adults, more than most home machines make [18]. Gas that contains oxygen can have the opposite effect: in the COVID-19 study, oxygen saturation improved more with hydrogen–oxygen gas than with standard care [8], and the stroke study also reported better oxygen saturation [6].
Can hydrogen inhalation machines explode?
Yes. Hydrogen is flammable in air at concentrations from about 4% to 75%, and it ignites far more easily than gasoline vapor [16, 17]. Premixed gas below 4% can't burn, but pure hydrogen and the hydrogen–oxygen gas that many machines make can ignite or detonate near a flame or spark [18].
- Japan's Consumer Affairs Agency has recorded incidents including a hydrogen–oxygen inhaler whose lid flew off with a bang that left the user's ears ringing, and a leaking home hydrogen generator that exploded and injured the person [19].
- A 2026 modeling paper notes documented inhaler explosions that caused facial fractures, a torn airway and internal bleeding [18], and a 2026 review describes a published case of an explosion inside a patient's airway [20].
For the full explanation, including why 1–4% mixtures can't burn and why hydrogen–oxygen gas is riskier, see Is hydrogen flammable? in our hydrogen machine guide.
Who should be cautious with hydrogen inhalation?
Check with a clinician first if any of these apply. In most cases that's because these groups haven't been studied, not because harm has been proven.
- Lung or heart disease, or low blood oxygen. Oxygen dilution matters more if your oxygen is already low. The modeling paper advises keeping inhaled oxygen above about 15%, especially for people with breathing or heart problems or at high altitude [18]. Low-oxygen episodes were more common after surgery in the hydrogen group [11].
- Babies and children. The one placebo-controlled trial in infants we found reported more heart-enzyme, digestive and liver-test abnormalities than air [15]. A flow that gives an adult a modest dose can give a small child a much larger one [18].
- Pregnancy. None of the studies in our table was designed to test safety in pregnancy, so it's unknown.
- High altitude. Air already has less oxygen at altitude, which raises the risk of low oxygen from any added gas [18].
- Smokers, and anyone using a machine near flames, heaters or sparks. Hydrogen ignites very easily [16].
- Medicines. None of the studies in our table tested drug interactions. Tell your clinician if you use hydrogen alongside treatment.
How does inhaling hydrogen compare with drinking hydrogen water?
The risks are different. Reported side effects of hydrogen water are mostly mild gut and urinary symptoms, and magnesium tablets add a magnesium limit to watch; see hydrogen water side effects and hydrogen tablets. Inhalation adds two risks that drinking doesn't: diluted oxygen and flammable gas. For the wider research on hydrogen and sleep, see the sleep section of our evidence-graded guide to hydrogen benefits.
What the evidence doesn't show
- Long-term safety. The longest study lasted 16 weeks [7]; we found no data on daily use over months to years.
- Rare side effects. The studies enrolled 8 to 170 people, too few to catch problems that affect a small share of users.
- Careful tracking in every study. Only some studies defined how they would record adverse events or used independent reviewers [1, 4, 9]; others gave a one-line summary or nothing [10, 13, 14].
- Whether results hold across doses. Studies used very different machines and doses, from a trickle of hydrogen–oxygen to premixed gas through a ventilator, so safety at one dose doesn't prove safety at another.
- Independence. Two of the four authors of the insomnia trial worked for the company that made the device [14]. The cardiac arrest trial was funded by a gas supplier, which had no role in the analysis [4]. The explosion reports come from employees of a company that sells low-concentration inhalers and argues against high-concentration designs [19, 20]. We're funded by a hydrogen company too; our methodology explains how we weigh studies.
A correction. An earlier version of this page said its conclusions came from "100+ studies" and cited animal studies and a hydrogen-water study in children as evidence that inhalation is safe. We couldn't support the "100+" figure for human inhalation safety data, so we replaced it with the 15 human studies above.
Frequently asked questions
What are the side effects of hydrogen inhalation?
In human studies, most people had no side effects or mild ones. Reported effects include a slight drop in blood oxygen, more low-oxygen episodes after surgery, dizziness and a mild nasal-lining injury from a breathing mask, and, in one infant trial, more abnormal heart-enzyme, digestive and liver-test results than with air. No study we reviewed attributed a serious adverse event to hydrogen.
Is hydrogen inhalation safe for daily use?
Studies have tested daily use for up to 16 weeks without serious problems attributed to hydrogen, but they were small, and nobody has studied daily use for years. The biggest practical risks at home are fire, because many machines make flammable gas, and lower oxygen at high flows.
Can hydrogen inhalation lower your oxygen level?
It can, a little. In a study of healthy young women, an hour of hydrogen lowered oxygen saturation from 96.7% to 95.9% on average. After surgery, low blood oxygen was more common during hydrogen–oxygen inhalation (7% vs 3%). People with lung or heart disease should ask a clinician first.
Is hydrogen inhalation safe for children?
It hasn't been shown to be. The one placebo-controlled trial in infants we found reported more raised heart-enzyme results, digestive problems and abnormal liver tests with hydrogen–oxygen gas than with air, and no benefit for their seizures. Don't give hydrogen gas to a child without a doctor's supervision.
Can a hydrogen inhalation machine explode?
Yes. Hydrogen burns in air from about 4% to 75% and ignites very easily. Pure hydrogen and hydrogen–oxygen gas from electrolysis machines can ignite or detonate near a flame or spark, and explosions have caused injuries. Premixed gas below 4% can't burn.
Sources
- Cole AR, et al. Safety of prolonged inhalation of hydrogen gas in air in healthy adults. Critical Care Explorations. 2021. PubMed · Our summary
- Grepl P, et al. Sixty-minute inhalation of molecular hydrogen decreases blood oxygen saturation but does not alter autonomic cardiac regulation at rest in healthy females: a randomized, double-blind, placebo-controlled crossover study. Canadian Journal of Physiology and Pharmacology. 2026. PubMed · Our summary
- Chen JB, et al. High-flow hydrogen inhalation might suppresses the immune function of middle-aged participants: a self-controlled study. Medical Gas Research. 2021. PubMed · Our summary
- Tamura T, et al. Efficacy of inhaled hydrogen on neurological outcome following brain ischaemia during post-cardiac arrest care (HYBRID II): a multi-centre, randomised, double-blind, placebo-controlled trial. eClinicalMedicine. 2023. PubMed · Our summary
- Katsumata Y, et al. The effects of hydrogen gas inhalation on adverse left ventricular remodeling after percutaneous coronary intervention for ST-elevated myocardial infarction — first pilot study in humans. Circulation Journal. 2017. PubMed · Our summary
- Ono H, et al. Hydrogen gas inhalation treatment in acute cerebral infarction: a randomized controlled clinical study on safety and neuroprotection. Journal of Stroke and Cerebrovascular Diseases. 2017. PubMed · Our summary
- Yoritaka A, et al. Randomized double-blind placebo-controlled trial of hydrogen inhalation for Parkinson's disease: a pilot study. Neurological Sciences. 2021. PubMed · Our summary
- Guan WJ, et al. Hydrogen/oxygen mixed gas inhalation improves disease severity and dyspnea in patients with coronavirus disease 2019 in a recent multicenter, open-label clinical trial. Journal of Thoracic Disease. 2020. PubMed · Our summary
- Zheng ZG, et al. Hydrogen/oxygen therapy for the treatment of an acute exacerbation of chronic obstructive pulmonary disease: results of a multicenter, randomized, double-blind, parallel-group controlled trial. Respiratory Research. 2021. PubMed · Our summary
- Botek M, et al. Molecular hydrogen positively affects physical and respiratory function in acute post-COVID-19 patients: a new perspective in rehabilitation. International Journal of Environmental Research and Public Health. 2022. PubMed · Our summary
- Lin H, et al. Hydrogen gas treatment improves postoperative delirium and cognitive dysfunction in elderly noncardiac patients. Journal of Personalized Medicine. 2022. PubMed · Our summary
- Ji S, et al. The impact of perioperative hydrogen gas inhalation on postoperative recovery in day surgery patients: a research study. Scientific Reports. 2026. PubMed · Our summary
- Liu B, et al. The effect of a low dose hydrogen-oxygen mixture inhalation in midlife/older adults with hypertension: a randomized, placebo-controlled trial. Frontiers in Pharmacology. 2022. PubMed · Our summary
- Gao YH, et al. Effect of hydrogen-oxygen inhalation on sleep disorders and abnormal mood: a single-blind, randomized controlled trial. Medical Gas Research. 2025. PubMed · Our summary
- Chen C, et al. A randomized controlled double-blind study on the brain protection of infantile patients with epileptic spasm syndrome through atomized inhalation of hydrogen-oxygen gas. Frontiers in Neurology. 2026. PubMed · Our summary
- US Department of Energy (College of the Desert). Hydrogen Fuel Cell Engines and Related Technologies, Module 1: Hydrogen Properties (Rev 0, December 2001). energy.gov
- NOAA Office of Response and Restoration. Hydrogen — chemical datasheet. CAMEO Chemicals. cameochemicals.noaa.gov
- LeBaron TW, et al. Respiratory-physiology modeling of therapeutic hydrogen inhalation: defining the fraction of inspired hydrogen (FiH₂) and flow-rate requirements. Respiratory Research. 2026. PubMed · Our summary
- Ichikawa Y, et al. Guidelines for the selection of hydrogen gas inhalers based on hydrogen explosion accidents. Medical Gas Research. 2023. PubMed · Our summary
- Ichikawa Y, et al. Preventable in-body hydrogen explosions from high-concentration H₂ inhalers in Japan — switch to safe, low-concentration hydrogen therapy. International Journal of Risk & Safety in Medicine. 2026. PubMed · Our summary
How we wrote this: drafted with AI assistance from the sources above and checked against each source by the Hydrogen Studies editorial team. This is general information, not medical advice — talk to your clinician before changing any treatment.