Hyperbaric oxygen therapy (HBOT) is considered safe when performed in a properly equipped clinical facility, with serious complications being rare. The most common side effect, middle ear pressure discomfort, occurs in roughly 2% of patients. Seizures, the most feared complication, happen in about 3 out of every 10,000 treatments at standard pressures. That said, safety depends heavily on the type of chamber, the pressure used, and whether you have certain pre-existing conditions.
The Most Common Side Effect: Ear Pressure
Middle ear barotrauma is the side effect you’re most likely to experience. It happens because the air-filled space behind your eardrum needs to equalize pressure with the chamber as pressure rises. Unlike ascending in an airplane, where your ears pop passively, the pressurization phase of HBOT requires you to actively equalize, using techniques like swallowing, yawning, or gently blowing against pinched nostrils. If you can’t equalize fast enough, the pressure difference stretches or irritates the eardrum, causing pain and sometimes fluid buildup.
Clinical centers reduce this risk in a few ways. Slowing the rate of pressurization gives you more time to equalize. Some facilities use oral decongestants like pseudoephedrine before treatment, which can reduce the severity and frequency of ear problems. Nasal steroid sprays and antihistamines are also used, though practices vary between centers. If you have a sinus infection or upper respiratory infection at the time of treatment, your provider will likely postpone your session because congestion makes equalization much harder.
Temporary Vision Changes
An estimated 60% of patients undergoing a course of HBOT will notice at least a small shift in their vision, typically a mild increase in nearsightedness. This happens because prolonged exposure to high-concentration oxygen temporarily changes the shape of the lens inside the eye. The shift is subtle for most people, about one line on a standard eye chart, and it reverses on its own within 3 to 6 weeks after finishing treatment. In uncommon cases, the change can linger for 6 to 12 months before fully resolving. This is why providers generally recommend holding off on updating your eyeglass prescription until well after your treatment course is complete.
Seizure Risk by Pressure Level
Oxygen toxicity affecting the brain can trigger a seizure during treatment. This is the complication patients worry about most, but the actual numbers are reassuring at standard clinical pressures. At 2.0 atmospheres absolute (the lower end of therapeutic pressure), no seizures were recorded in over 16,000 treatments. At the more commonly used range of 2.4 to 2.5 atmospheres, the incidence is about 0.03%, or roughly 3 in 10,000 sessions, based on data from nearly 139,000 treatments across three facilities.
The risk climbs with higher pressures. Patients treated for decompression sickness at 2.6 to 2.9 atmospheres had a seizure rate of 0.6%. Among carbon monoxide poisoning patients treated at 2.8 to 3.0 atmospheres, the rate reached 2.5%. These higher pressures are used only for specific emergency conditions, not for routine wound healing or other elective indications. If a seizure does occur during treatment, it typically stops once oxygen pressure is reduced, and it does not cause lasting brain injury.
Claustrophobia and Anxiety
About 2% of patients experience significant confinement anxiety during HBOT. Monoplace chambers, which enclose a single person in a clear tube, are the most likely to trigger this. Multiplace chambers, which resemble small rooms where several patients sit together, are generally better tolerated. If you know you’re claustrophobic, let your provider know before your first session. Many facilities can offer a brief orientation visit so you can see the chamber and practice being inside before committing to a full treatment.
Who Should Not Receive HBOT
The only condition that absolutely rules out HBOT is an untreated pneumothorax, a collapsed lung with trapped air. Under increased pressure, that trapped air can expand dangerously. Several other conditions don’t disqualify you entirely but require extra caution:
- Obstructive lung disease or pulmonary blebs (small air pockets in the lung tissue), which raise the risk of air trapping during depressurization
- Recent ear or chest surgery, where healing tissues may be vulnerable to pressure changes
- Active upper respiratory or sinus infections, which make ear equalization difficult
- Uncontrolled fever, which may lower the threshold for oxygen-related seizures
Critically ill patients, particularly children, face a substantially different risk profile. In pediatric ICU populations, complications including drops in blood pressure (63%), airway spasms (34%), and ear bleeding (13%) have been documented. These numbers reflect the fragility of that patient population, not the typical outpatient experience.
Clinical Chambers vs. Soft-Sided Home Chambers
The safety profile described above applies to treatments in regulated clinical facilities using hard-shell chambers built to engineering standards for pressure vessels designed for human occupancy. Soft-sided “bag” chambers sold for home use are a different situation entirely, and the Undersea and Hyperbaric Medical Society has issued a direct consumer warning about them.
The concerns are serious. Many soft-sided chambers are not registered with the FDA. Some sellers claim their product is equivalent to the Gamow bag, a portable device originally designed only to treat acute mountain sickness at altitude with compressed air, not to deliver oxygen therapy. The Gamow bag was never certified for use with supplemental oxygen. Despite this, many soft-sided chambers are sold with oxygen concentrators, creating fire risk when concentrated oxygen is used under pressure inside a fabric enclosure.
Additional problems include the quality and purity of the breathing gas, which often doesn’t meet medical-grade standards required by fire protection codes. Users are frequently allowed to bring cell phones, tablets, and other electronics into these chambers, and static grounding mechanisms required for clinical chambers are almost never present. Canada’s health authority has banned soft-sided chambers entirely. Many of these devices are manufactured outside the U.S. and shipped in parts to avoid border inspection, then assembled after arrival.
How Clinical Facilities Manage Fire Risk
Because HBOT involves high-concentration oxygen, fire safety is tightly regulated. Clinical chambers must be housed in rooms dedicated exclusively to hyperbaric operations. Only 100% cotton or cotton-polyester blends (no more than 50% polyester) are permitted inside the chamber. Silk, wool, and synthetic fabrics are prohibited. Electrical circuits inside the chamber can be de-energized immediately if a fire is detected, and every treatment room must have a fire extinguisher rated for multiple fire types.
Patients are typically asked to change into facility-provided cotton scrubs, remove jewelry, and leave all personal electronics outside the chamber. Grounding the chamber and the patient to prevent static discharge is a standard part of the pre-treatment checklist, particularly in monoplace systems. These precautions are part of what separates a regulated clinical facility from an unregulated home setup.