What to Expect During Your First HBOT Session

The most eventful part of a first hyperbaric session is usually the first three minutes, when the chamber pressurizes and your ears have to catch up.

After that it gets pretty boring, which is the point. You sit in a clear-walled chamber, breathe through a mask or hood, and the pressure holds steady until the session ends. Most people read, listen to something, or fall asleep.

Still, "you'll be fine" isn't a useful answer if you've never done it. So here's the actual walkthrough: what happens minute by minute, what your ears are going to do about it, what to wear, and who genuinely shouldn't get in the chamber.

What Actually Happens, Start to Finish

You'll get changed first. Cotton clothing, nothing synthetic, and everything metal or electronic stays outside the chamber. Pure oxygen environments make ordinary things flammable, so this part is not negotiable anywhere that runs a chamber properly.³

Then you get in and the door seals. You'll hear a hiss as the chamber pressurizes, and over the next few minutes the pressure climbs toward 2.0 ATA. This is the descent phase, and it's where your ears do their work.

Once you're at pressure, that's basically it. You breathe through a mask or a hood, the sound settles into a steady hum, and the session runs its clock. Staff are with you the whole time and you can talk to them.

At the end, pressure comes back down gradually so your body can adjust.³ Your ears may pop again on the way up, usually less dramatically than on the way down. Then you're out, you change, and you go about your day. There's no recovery time and no reason you can't drive yourself home, though some people feel a little sleepy afterward.³

Your Ears Are the Part Worth Preparing For

Middle ear barotrauma is the most common side effect of hyperbaric therapy, and it's worth understanding rather than worrying about.

The reported incidence swings wildly across the literature, from about 2% up to 84% in patients who can breathe on their own.¹ That range looks alarming until you read why it's so wide. Studies define the condition differently, treat very different patient populations, and, critically, vary in how well they teach people to equalize pressure in the first place.¹ A five-year French analysis of 2,610 hyperbaric patients found ear barotrauma in 10.04% of them, and the cases that did occur skewed toward older patients, women, and people with a history of ear, nose, and throat problems.²

Two details from that research matter for a first-timer. Risk is highest during your initial sessions and doesn't climb as your treatment course goes on.¹ And when barotrauma does happen, it's usually mild: in one large series, 84% of cases were minor eardrum irritation or slight redness, with no ruptures at all.¹

The fix is unglamorous. Yawn, swallow, wiggle your jaw, or hold your nose and swallow a sip of water.³ Do it early and often as the pressure starts climbing rather than waiting until your ears already ache. If you can't clear them, say so immediately. Pressurization can be slowed or paused, and the research is clear that going down too fast makes barotrauma more likely.¹

If you're congested from a cold or allergies on the day of your session, reschedule. A blocked eustachian tube is the single most predictable way to have a bad first experience.

What It Feels Like at Pressure

Warmer than you'd expect, for one. Compressing air heats it, so the chamber gets noticeably warm during the descent and then evens out. It's also loud enough during pressurization that conversation gets awkward, then quieter once you're holding steady.

Beyond that, honestly, most people report very little. There's no sensation of the oxygen itself. Some people feel a mild fullness in their sinuses. Many fall asleep.

The side effects that show up in the clinical literature are worth knowing about, with one important caveat: nearly all of those numbers come from full medical treatment courses, meaning 20 to 40 daily sessions at pressures of 2.0 to 3.0 ATA. That's a very different exposure than a single 30-minute recovery session.

• Oxygen toxicity seizures are the most feared complication and the rarest. Recent estimates put the rate at roughly 1 in 2,000 to 3,000 treatments across the 2.0 to 3.0 ATA range, and there's no evidence of long-term effects from an isolated episode.¹
• Temporary nearsightedness shows up in 20% to 40% of people who complete at least 20 daily treatments, and vision typically returns to baseline afterward.³
• Sinus pressure and congestion are common and usually mirror whatever your sinuses were already doing.³

Is It Claustrophobic?

This is the question people actually want answered, and the honest response depends on the chamber.

Hospital chambers are often monoplace units, meaning a single clear tube you lie down inside. Those are the ones that trigger confinement anxiety, though even there the reported rate is about 8 events per 10,000 treatments.¹ Claustrophobia is common in the general population, so a handful of people in any chamber will feel some version of it.¹

Defiant runs an OxyHealth 64D, which is a hard-shell multiplace chamber that seats two people sitting upright. You can see out, you can talk to staff, and you can bring someone in with you, which resolves the anxiety question for most people who were worried about it. If you're on the fence, book a 30-minute session rather than a 90 and see how you feel.

For a deeper look at how chamber construction affects pressure and experience, read our comparison of hard-shell and soft-shell hyperbaric chambers.

What to Wear and What to Leave in the Locker

Cotton, and not much else. Here's the short list:

• Wear: 100% cotton clothing. Comfortable, loose, nothing you'd mind sitting in for an hour.
• Leave outside: phones, watches, earbuds, jewelry, hearing aids, lighters, anything battery-powered, anything metal.
• Skip that morning: hair products, oils, lotions, perfume, and most cosmetics. Petroleum-based products in particular can ignite at lower temperatures in a high-oxygen environment.³
• Mention beforehand: any implanted device, including pacemakers, and any medication you're taking. Some older implanted devices don't tolerate a pressurized environment.³
• Use the restroom first. Obvious, but a 90-minute session is a long time.

You can typically bring water in. Ask when you book, since it varies by facility.

How Long Should Your First Session Be?

Thirty minutes, in almost every case.

Defiant offers 30, 60, and 90-minute sessions at $99, $185, and $225 respectively, with a $49 first session for new clients. Starting at 30 gives you a real read on how your ears handle pressurization and whether the chamber bothers you, without committing an hour and a half to finding out.

If the first one goes smoothly, moving up to 60 minutes is a reasonable next step. How many sessions make sense after that depends entirely on what you're using it for, which we covered in detail in how many HBOT sessions you actually need.

Who Shouldn't Get in the Chamber

Some people should not do hyperbaric therapy at all, and some should wait. Cleveland Clinic lists the following as reasons hyperbaric oxygen may be unsafe:³

• A collapsed lung (pneumothorax)
• Lung conditions that raise the risk of one, including COPD, emphysema, and cystic fibrosis
• A current fever or cold
• A recent ear injury
• Recent ear surgery

Pregnancy is also a contraindication outside of emergency carbon monoxide poisoning, because the pressure change may constrict placental blood vessels.³

None of that is a screening you do yourself. Tell the provider your full medical history and current medications at your consult, and let them make the call.

Being Straight About What HBOT Is Cleared For

The FDA has cleared hyperbaric chambers for a specific list of conditions, including carbon monoxide poisoning, decompression sickness, gas embolism, gas gangrene, crush injuries, certain non-healing wounds, radiation injury, severe anemia, compromised skin grafts, and sudden hearing or vision loss.⁴

Recovery, soreness, training load, and general wellness are not on that list. The FDA has also stated plainly that hyperbaric chambers are not proven to treat cancer, Lyme disease, autism, or Alzheimer's disease, and warns consumers away from clinics that claim otherwise.⁴

We'd rather say that up front than let you find it out later. Research suggests hyperbaric oxygen may support recovery, inflammatory response, and tissue healing, and the mechanism for why it might is well described. But the evidence base for wellness and athletic use is thinner than the marketing around it, and anyone telling you otherwise is selling. For an honest read on where that research actually stands, see our evidence review on HBOT and recovery.

Booking a First Session in Lisle

Defiant's chamber is a hard-shell OxyHealth 64D that seats two and runs up to 2.0 ATA, at 5100 Lincoln Ave in Lisle, serving Naperville, Downers Grove, Wheaton, and Chicago's western suburbs. Sessions are 30, 60, or 90 minutes, and new clients can book a first 30-minute session for $49.

Before your first session, the team walks you through ear equalization so you're not learning it while the pressure is already climbing. Bring your medication list and mention any ear or sinus history. If you're planning to pair hyperbaric with other recovery work, the contrast therapy protocol is a common combination.

Frequently Asked Questions

Last updated September 4, 2026.