Mild Hyperbaric Oxygen Therapy vs Hard-Shell HBOT

The short answer: pressure and oxygen are the difference

The two chamber types differ in how much pressure they reach and what you breathe inside.

• Mild (soft-shell) chambers are fabric bags filled with air by a small pump. The UHMS says they operate at less than 1.4 ATA and are designed to be compressed only with air (UHMS, 2018). One FDA-cleared model reaches 1.2 to 1.3 ATA, about 3 to 4 psi above normal air pressure (FDA 510(k) summary K072757, 2007).
• Hard-shell chambers used for medical HBOT deliver pressure two to three times higher than normal air pressure (Cleveland Clinic, 2023). You breathe close to 100% oxygen.

ATA stands for "atmospheres absolute." Normal air pressure at sea level is 1.0 ATA. So 1.3 ATA adds about a third of an atmosphere, while 2.0 ATA doubles it.

The UHMS defines hyperbaric oxygen therapy as breathing near 100% oxygen while fully inside a chamber at 1.4 ATA or higher. By that definition, a soft-chamber session on air is not HBOT. The UHMS also says mild exposures with air "deliver no more oxygen to the body than breathing oxygen by mask at sea-level pressure" (UHMS, 2018).

Soft vs hard hyperbaric chambers compared

Sources for the table: UHMS (2018), FDA 510(k) summary K072757 (2007), FDA letter to health care providers (2025) and Cleveland Clinic (2023).

What is mild HBOT (mHBOT)?

Mild HBOT is the common name for sessions at low pressure, usually in a soft chamber. The UHMS says the use of low-pressure fabric chambers "is commonly called mild hyperbaric therapy (MHT or mHBOT)" (UHMS, 2018).

Two points trip people up:

• "Mild" is about pressure, not only the chamber. The UHMS notes that mild sessions are also offered in clinical hard chambers run at low pressure. A hard chamber alone does not tell you what treatment you get.
• There is no single standard for "mild." Some clinics and studies add extra oxygen or run at 1.4 ATA. The study in the research section below used 1.4 ATA with 35% to 40% oxygen. Ask the clinic for its exact settings.

FDA clearance and mild chambers

The FDA treats hyperbaric chambers as Class II medical devices. It clears them through the 510(k) process, and you can look up cleared devices under product code CBF in the FDA's 510(k) database (FDA, 2025).

For soft chambers, clearance covers one use. A 2007 FDA 510(k) summary for a portable mild chamber lists its indication as "Acute Altitude Sickness. As prescribed by or under the direction of a physician." The device it was cleared against is an older portable chamber built for altitude illness (FDA 510(k) summary K072757, 2007).

The UHMS position statement, revised in 2018, says the same:

• Low-pressure fabric chambers have FDA 510(k) clearance for acute mountain sickness only.
• Marketing them for any other condition is off-label.
• The UHMS does not recommend mild hyperbaric therapy for any medical purpose other than acute mountain sickness.
• The UHMS does not endorse "in home" therapy at any time.

Source: UHMS, 2018.

If a clinic says its mild chamber is "FDA-cleared," that is likely true for altitude sickness. It does not mean the FDA has cleared it for recovery, wellness or any other condition. For the conditions medical HBOT is cleared and covered for, see what HBOT treats.

What the research says about mild HBOT

The evidence for mild HBOT is thin. The UHMS says it is "not aware of any reliable clinical evidence" that pressure below 1.4 ATA has any therapeutic effect (UHMS, 2018).

Two studies show what the research does and does not cover.

A small study in healthy women (2023). Researchers in Japan put 16 healthy young women through two 70-minute sessions in random order. One session was at normal pressure and normal air. The other was at 1.4 ATA with 35% to 40% oxygen. After the mild session, the women had higher counts of natural killer cells (a type of immune cell) and higher parasympathetic ("rest") nerve activity. Oxidative stress markers did not rise (Nisa et al., Life, 2023).

What this study does not show: it had no patients, measured blood markers after one session and did not test whether anyone's health improved.

A trial in children with cerebral palsy (2001). This randomized trial gave 111 children 40 sessions over two months. About half breathed 100% oxygen at 1.75 ATA. The other half breathed air at 1.3 ATA, the pressure a mild chamber uses, as a comparison. Both groups improved by about the same amount, with no difference between them. Ear problems were reported in 27 children in the oxygen group and 15 in the 1.3 ATA air group (Collet et al., Lancet, 2001).

What this trial does not show: it was not designed to prove that 1.3 ATA air works. The authors said the improvement in both groups "deserves further consideration," but they did not conclude that mild pressure caused it.

Hard-shell chambers: monoplace and multiplace

Medical HBOT uses one of two kinds of hard chamber (Cleveland Clinic, 2023):

• Monoplace (one person). You lie on a table that slides into a clear tube.
• Multiplace (several people). A larger room-like chamber. You breathe oxygen through a mask or a light, see-through hood.

Sessions usually last one to two hours. These chambers must be built to the ASME PVHO-1 pressure-vessel standard, and the rooms that house them must meet fire codes for hyperbaric facilities (UHMS, 2018). The UHMS runs an accreditation program and encourages every hyperbaric facility to join it.

Medicare covers HBOT for a set list of conditions under its national coverage rule, NCD 20.29, when the treatment is given in a chamber (CMS, NCD 20.29). Private coverage varies by plan. Our insurance guide explains what is and isn't covered.

Home and portable hyperbaric chambers: safety first

Some people search for a home or portable chamber. Here is what the medical sources say before you think about buying or renting one:

• The UHMS does not endorse in-home therapy at any time (UHMS, 2018).
• Do not add oxygen. Soft chambers are designed for air only, and the UHMS says the FDA prohibits using them with supplemental oxygen. That includes running an oxygen concentrator into the bag.
• Fire is the main hazard with oxygen. The FDA warns of "a heightened risk of fire with use of oxygen at a high concentration." It has received reports of fires with HBOT devices that caused serious injuries and deaths (FDA, 2025).
• Supervision matters. The FDA tells providers to make sure the patient is "properly monitored and supervised for the duration of the treatment" (FDA, 2025). At home, there is usually no trained person watching.
• Hyperbaric chambers need a prescription. The UHMS says hyperbaric therapy requires a physician's prescription to buy and use.

We don't recommend any chamber or brand. For price ranges, see home chamber costs in our cost guide. For pressure-related risks such as ear pain, see HBOT side effects.

How to tell what a clinic offers

Clinic websites often say "hyperbaric" without saying which kind. Call and ask:

1. What pressure do you use, in ATA? Under 1.4 ATA is mild. Medical HBOT runs at 2.0 ATA or higher.
2. Is the chamber hard or soft? And if it is hard, do you run mild sessions in it?
3. What will I breathe? Air, extra oxygen, or close to 100% oxygen by mask, hood or a fully oxygen-filled chamber?
4. Who supervises the session? Is a trained physician or hyperbaric technician on site the whole time?
5. Is the facility accredited by the UHMS?
6. Is HBOT FDA-cleared, or covered by Medicare, for my condition? Your doctor can help you answer this.

If you have a wound, radiation injury or another serious condition, talk to your doctor first. A mild session is not a stand-in for medical HBOT.