What to Expect at Your First Hyperbaric Oxygen Therapy Session

Your First HBOT Session in Brief

A first hyperbaric oxygen therapy (HBOT) session usually takes one to two hours. You are screened by a clinician, change into cotton clothing, and lie or sit in the chamber while it is slowly pressurised. Your ears will feel full and pop, much like on a descending plane. At treatment pressure you breathe oxygen and rest, and at the end the pressure is brought back down. Most people can return to normal activities straight afterwards, according to Cleveland Clinic.

This guide walks through each step, using hospital guidance and published research, so you know what is normal and what to tell staff.

Emergency treatment works differently. If you suspect carbon monoxide poisoning or decompression illness, call your local emergency number first. Emergency HBOT happens in hospital chambers. Divers can also call the DAN Emergency Hotline on +1-919-684-9111.

Before You Arrive: Screening

The Undersea and Hyperbaric Medical Society (UHMS) says treatment should be based on an evaluation and prescription by a licensed physician trained in hyperbaric medicine. Expect questions about your medical history, medications, lungs, ears and sinuses. A 2017 review of HBOT side effects explains why some of these matter:

• Collapsed lung. An untreated (unvented) pneumothorax is an absolute contraindication to HBOT, because it can become a life-threatening tension pneumothorax during decompression. A history of lung disease, a previous pneumothorax or chest surgery should prompt at least a chest X-ray.
• Colds and congestion. An acute upper respiratory infection is a relative contraindication for planned treatment, because blocked sinuses and ears are harder to equalise.
• Diabetes. HBOT can lower blood sugar in some people with diabetes, so facilities set a minimum blood glucose level before treatment. Expect a blood sugar check before each session.
• Heart failure. Patients with a history of heart failure may need a baseline echocardiogram.
• Eyes. If you are likely to have a long course, you may be offered an eye examination first.
• Claustrophobia. Tell staff early. Coaching can help, and some patients do better in a multiplace chamber.

What to Wear and What to Leave Behind

Oxygen makes materials easier to ignite, so clinics control what goes into the chamber.

Clothing. UHMS guidance says clothing must be issued by the facility or approved by its safety coordinator. 100% cotton garments are allowed, and some cotton-polyester blends are too. Silk, wool and synthetic materials should stay out unless approved. The FDA notes that wool and synthetic fabrics may produce more static electricity.

Skin and hair products. Skip lotions, oils, hairspray, perfume, aftershave and makeup on treatment days unless the clinic approves them. The European Code of Good Practice for HBOT lists aerosols, aftershave, alcohol, and petroleum-based lubricants and grease among prohibited materials. Cleveland Clinic advises discussing any skin care products you use, because pure oxygen can make some products ignite at lower temperatures.

Leave outside:

• Phones, smartwatches, earbuds and other electronics (see phones and batteries in hyperbaric chambers)
• Metal objects, such as jewellery
• Lighters, matches and anything that produces heat or sparks
• Non-diving watches and pens, which the European code also lists as prohibited

Cleveland Clinic notes that you may only be allowed to bring a small bottle of water into the chamber.

Arriving at the Clinic

On your first visit, allow extra time for paperwork, consent and a briefing. The UHMS says a safely run service does a pre-treatment "time-out" before each session to confirm your identity, your treatment plan and safety readiness. In a monoplace chamber using 100% oxygen, you should also be fitted with a grounding strap to prevent static discharge.

You will be treated in one of two chamber types:

• Monoplace chamber: a clear, tube-like chamber for one person, typically filled with oxygen.
• Multiplace chamber: a room-sized chamber for several people, pressurised with air. You breathe oxygen through a mask or hood, and a staff member can be inside with you.

During the Session

Compression: the "descent"

As pressure rises, the air in your middle ear is squeezed and your ears feel blocked. Middle ear barotrauma is one of the most common side effects of HBOT, and the review found the risk is higher during the first treatments. Patient education and active coaching during compression help reduce it.

Ways to equalise your ears include:

• Swallowing or yawning. Cleveland Clinic suggests yawning, or holding your nose and swallowing a sip of water.
• Toynbee manoeuvre: pinch your nostrils and swallow.
• Valsalva manoeuvre: pinch your nostrils and blow gently through your nose. Divers Alert Network (DAN) warns that doing this too forcefully can cause damage.
• Frenzel manoeuvre: close your nostrils and the back of your throat, then make a "k" sound.

Equalise early and often, and never force it. If you feel pain, tell the operator straight away. If you cannot equalise at all, a doctor may recommend ear treatment before further sessions.

At treatment pressure

For most accepted medical indications, treatment pressure is between 2.0 and 3.0 ATA (atmospheres absolute). The UHMS says all its approved indications need at least 2.0 ATA with near-100% oxygen.

Most of the session is spent at this pressure, breathing normally. How long depends on your protocol. In one randomised trial of diabetic foot ulcers, patients breathed oxygen for 85 minutes in a 95-minute session. Some protocols include short periods of breathing air, called air breaks, to lower the risk of oxygen toxicity.

The UHMS says trained staff should watch and hear you continuously throughout the session, so you can speak to them at any time. Many people rest or sleep.

Decompression: the "ascent"

At the end, the pressure is reduced gradually. Your ears may pop again as air expands. Keep breathing normally and do not hold your breath: lung overinflation during decompression is a rare risk, mainly for people with trapped air in the lungs or who hold their breath.

After the Session

Cleveland Clinic says you can return to your usual activities immediately after a session. Common and less common effects include:

• Ear fullness or pain. Most middle ear barotrauma resolves when the ears are not repeatedly injured. Tell staff before the next session if your ears hurt.
• Sinus pain, especially if you have a cold or allergies.
• Blurred distance vision. With repeated sessions at 2.0 ATA or more, short-sightedness (myopia) has been reported in 25% to 100% of patients after several weeks. It usually reverses within 3 to 6 weeks after treatment ends, but can take 6 to 12 months. The review advises against getting a new permanent glasses prescription until at least 8 weeks after your course, and says clinics should monitor whether your vision stays safe for driving.
• Low blood sugar in people with diabetes. This is a legitimate concern, but clinically relevant low blood sugar is not common.
• Oxygen toxicity seizure. This is rare, estimated at about 1 in 2,000 to 3,000 treatments in more recent studies. Seizures stop when oxygen is reduced and have no known lasting effects.

For the full list and how clinics reduce each risk, read HBOT side effects.

How Many Sessions Will You Need?

That depends on why you are being treated. Cleveland Clinic says sessions may be scheduled from once a week to five days a week, for up to 30 or more sessions. Published protocols for accepted uses look like this:

In the diabetic foot ulcer trial, 52% of patients in the oxygen group had a healed ulcer at one year, compared with 29% in the placebo group, who breathed air in the chamber. Among patients who completed more than 35 sessions, the figures were 61% and 27%.

There is no established session count for wellness uses, because evidence for those uses is limited. If a clinic sells a package, ask what the number is based on.

Tips for a Smooth First Session

• Arrive early so paperwork and the safety check are not rushed.
• Mention any cold, congestion or new medication on the day, since blocked ears and sinuses are harder to equalise.
• Practise gentle equalising at home, such as swallowing with your nose pinched.
• Dress in plain cotton and skip skin and hair products.
• Speak up about ear pain, anxiety or feeling unwell as soon as you notice it.

Finding Your First HBOT Clinic

Look for a clinic with physician oversight, trained chamber staff and clear safety procedures. Our guide on how to choose an HBOT clinic lists what to check. When you are ready, find HBOT clinics near you.

Sources

• Cleveland Clinic, Hyperbaric Oxygen Therapy: What It Is and Benefits, Side Effects (last updated January 2023).
• Heyboer M 3rd, Sharma D, Santiago W, McCulloch N. Hyperbaric Oxygen Therapy: Side Effects Defined and Quantified. Adv Wound Care. 2017;6(6):210-224.
• Undersea and Hyperbaric Medical Society, Ten Guidelines for Patients and Referring Physicians in Selecting Safe Hyperbaric Oxygen Treatments (April 2025).
• U.S. Food and Drug Administration, Follow Instructions for Safe Use of Hyperbaric Oxygen Therapy Devices: Letter to Health Care Providers (25 August 2025).
• Kot J, Desola J, Lind F, et al. A European code of good practice for hyperbaric oxygen therapy: Review 2022. Diving Hyperb Med. 2023;53(4 Suppl):1-17.
• Divers Alert Network, 6 Methods to Equalize Your Ears and Emergency Assistance.
• Weaver LK, Hopkins RO, Chan KJ, et al. Hyperbaric oxygen for acute carbon monoxide poisoning. N Engl J Med. 2002;347(14):1057-1067.
• Löndahl M, Katzman P, Nilsson A, Hammarlund C. Hyperbaric oxygen therapy facilitates healing of chronic foot ulcers in patients with diabetes. Diabetes Care. 2010;33(5):998-1003.
• Centers for Medicare and Medicaid Services, National Coverage Determination 20.29: Hyperbaric Oxygen Therapy.
• NHS England, Hyperbaric oxygen therapy services (all ages): service specification (January 2025).

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Medical Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting hyperbaric oxygen therapy, particularly if you have existing medical conditions. Individual experiences vary.