Reference
Hyperbaric Oxygen Therapy Glossary: ATA, Monoplace, Barotrauma and 40 More Terms Explained
HBOT stands for hyperbaric oxygen therapy: breathing near-100% oxygen in a chamber pressurized to at least 1.4 atmospheres absolute (ATA). Sea level is 1 ATA. This glossary explains ATA and more than 40 other terms you will meet on this site, in insurance rules and in research, in plain English.
Start Here
Pressure, Explained First: What ATA Means
Almost every hyperbaric term connects back to pressure, so it is worth getting the units straight first. ATA stands for atmospheres absolute: the total pressure around you, including the air pressure you already live in. At sea level you are at 1 ATA. A chamber at 2.0 ATA is at twice that pressure.
Some gauges show pressure above the surrounding air instead, written as psig (pounds per square inch, gauge). Divers and emergency protocols often use depth: every 33 feet of seawater (fsw), or about 10 meters (msw), adds one more atmosphere.
Why it matters: the Undersea and Hyperbaric Medical Society (UHMS) defines HBOT as breathing near-100% oxygen at 1.4 ATA or more, and every indication it recognizes calls for at least 2.0 ATA. Hard-shell chambers commonly treat at 2.0 to 2.4 ATA, while soft "mild" chambers typically stop at about 1.3 ATA. Under pressure, extra oxygen dissolves directly into your blood plasma, which is the core idea behind the therapy.
| ATA | psig (above surface) | Equivalent seawater depth | kPa (absolute) | Where you'll see it |
|---|---|---|---|---|
| 1.0 | 0 | 0 fsw / 0 msw | 101 | Sea level, normal air pressure |
| 1.3 | 4.4 | 10 fsw / 3 msw | 132 | Typical soft "mild" chamber |
| 1.4 | 5.9 | 13 fsw / 4 msw | 142 | UHMS minimum for the definition of HBOT |
| 2.0 | 14.7 | 33 fsw / 10 msw | 203 | Minimum for UHMS-recognized indications |
| 2.4 | 20.6 | 46 fsw / 14 msw | 243 | Classic protocol for wound and radiation indications |
| 2.5 | 22.0 | 50 fsw / 15 msw | 253 | Upper end of common wound and radiation protocols |
At Generations Wellness, your physician prescribes the pressure for your sessions. To see how pressure and oxygen work together, read how HBOT works.
Glossary
Hyperbaric Terms A–C
Air break
A short, scheduled period during a hyperbaric session when the patient breathes air instead of oxygen. Many treatment protocols include air breaks to lower the total oxygen dose and the risk of oxygen toxicity.
Why it matters: If your plan includes air breaks, they are a routine safety step, not a sign that something went wrong.
Angiogenesis
The growth of new small blood vessels from existing ones. Researchers study angiogenesis as one way repeated HBOT sessions may support healing in tissue with a poor blood supply, such as tissue damaged by radiation.
Why it matters: It helps explain why HBOT is given as a course over several weeks rather than as a single treatment. How pressure and oxygen work.
Arterial gas embolism (AGE)
A medical emergency in which gas bubbles enter the arteries and block blood flow, for example after a diving accident. Air or gas embolism is on the FDA's list of cleared uses for hyperbaric chambers, and it is treated urgently in hospital-based chambers.
Why it matters: It is an emergency: call 911. Generations Wellness is an outpatient center and does not provide emergency hyperbaric treatment. Gas embolism and other emergency indications.
ATA (atmospheres absolute)
The total pressure inside a chamber, counting the normal pressure of the atmosphere. Sea level is 1 ATA, so 2.0 ATA means twice normal atmospheric pressure. The UHMS defines HBOT as treatment at 1.4 ATA or more, and hard-shell chambers commonly treat at 2.0 to 2.4 ATA.
Why it matters: ATA is the number that separates medical HBOT from low-pressure soft chambers. See the conversion table above.
Barotrauma
Injury to an air-filled space in the body, most often the middle ear or sinuses, caused by a pressure difference that has not equalized. Middle-ear barotrauma is the most common complication of HBOT, reported in at least 2% of patients in monoplace series.
Why it matters: Clearing your ears early and speaking up about pain are the best ways to prevent it. Barotrauma and other side effects.
Boyle's law
The physics rule that, at a steady temperature, the volume of a gas shrinks as pressure rises and expands as pressure falls. It is why your ears feel full as a chamber pressurizes, and why air trapped in the chest is dangerous as pressure is released.
Why it matters: It is the reason for ear clearing and for lung screening before treatment.
Carboxyhemoglobin (COHb)
Hemoglobin that is carrying carbon monoxide instead of oxygen. A blood test measures the carboxyhemoglobin level when carbon monoxide poisoning is suspected.
Why it matters: Carbon monoxide poisoning is an emergency: call 911, or California Poison Control at 1-800-222-1222. Carbon monoxide poisoning and HBOT.
CHT (Certified Hyperbaric Technologist)
A professional credential for hyperbaric technologists awarded by the National Board of Diving and Hyperbaric Medical Technology (NBDHMT). It is one of several credentials that staff who operate hyperbaric chambers can hold.
Why it matters: When comparing centers, it is reasonable to ask what training the chamber staff have. How to choose a hyperbaric clinic.
Compression (pressurization)
The first stage of a session, when chamber pressure is raised gradually to the prescribed treatment level. Divers call this descent. It is when ear fullness is most noticeable.
Why it matters: This is when you actively clear your ears. How to clear your ears.
Confinement anxiety
Anxiety or panic caused by being in an enclosed space. About 25% of adverse-event reports in one review involved confinement anxiety, and one series recorded about 8 cases per 10,000 treatments.
Why it matters: Tell the team in advance. You can ask to stop a session at any point. Managing claustrophobia in HBOT.
Contraindication (absolute and relative)
A condition or factor that makes a treatment unsafe or calls for caution. An absolute contraindication rules treatment out; for HBOT, the only one is an untreated pneumothorax. A relative contraindication, such as COPD, a cold or pregnancy, means a physician must weigh the risks and benefits first.
Why it matters: This is what your physician assessment screens for. Who should not use a hyperbaric chamber.
CPT 99183 and HCPCS G0277
The billing codes used for hyperbaric oxygen therapy in the United States. CPT 99183 covers physician attendance and supervision for each session. HCPCS G0277 covers the facility's chamber time and is billed per 30-minute interval.
Why it matters: You may see these codes on estimates or insurance paperwork. Information for referring physicians.
CRAO (central retinal artery occlusion)
A sudden blockage of the main artery that supplies the retina, causing sudden vision loss in one eye. It appears on the UHMS list of hyperbaric indications and is managed urgently in hospital settings.
Why it matters: Sudden vision loss is an emergency: call 911 or go to the nearest emergency room. Emergency hyperbaric indications.
Glossary
Hyperbaric Terms D–H
Decompression (depressurization)
The final stage of a session, when chamber pressure is lowered gradually back to normal. Divers call this ascent. Your ears usually clear on their own; breathe normally and never hold your breath.
Why it matters: It is why stopping a session early takes some minutes rather than seconds. What happens if you want to stop.
Decompression sickness (DCS)
A condition in which gas bubbles form in the body after a rapid drop in surrounding pressure, most often after scuba diving. It is on the FDA's list of cleared uses for hyperbaric chambers and is treated in hospital-based chambers.
Why it matters: Diving emergencies need immediate care: call 911. DAN's emergency line is +1-919-684-9111. Decompression sickness and HBOT.
Delayed radiation injury
Damage to soft tissue or bone that appears months or years after radiation therapy. It includes soft-tissue injury, such as radiation cystitis, and bone injury (osteoradionecrosis). Delayed radiation injury is an FDA-cleared use of hyperbaric chambers.
Why it matters: If you had radiation in the past and now have bleeding or a wound in that area, ask your doctors whether it could be related. Radiation injury and HBOT.
Diabetic foot ulcer (DFU)
An open wound on the foot of a person with diabetes. Select non-healing diabetic wounds of the lower extremities are an FDA-cleared use of hyperbaric chambers, and Medicare coverage requires a Wagner grade 3 or higher ulcer.
Why it matters: HBOT is used alongside, never instead of, standard wound care. Diabetic foot ulcers and HBOT.
Equalization
Balancing the pressure in your middle ear with the pressure around you by letting air through the Eustachian tube. Swallowing, yawning, moving the jaw and gentle nose-pinch techniques are common ways to equalize.
Why it matters: Equalizing early and often during compression prevents most ear pain. Equalizing step by step.
Eustachian tube
The narrow passage that connects the middle ear to the back of the throat. It opens briefly when you swallow or yawn, letting air in or out. A cold or allergies can make it swell shut.
Why it matters: If yours does not open easily, tell your physician, because it affects how pressure changes feel.
FDA clearance vs. approval
Two different FDA terms: medical devices such as hyperbaric chambers are cleared for specific uses, while drugs are approved. The FDA has cleared hyperbaric chambers for a specific list of conditions and states that HBOT has not been cleared for many other conditions it is marketed for.
Why it matters: Be cautious when HBOT is described as broadly FDA approved; ask which specific use is cleared. FDA-cleared indications.
fsw and msw
Feet of seawater (fsw) and meters of seawater (msw) are units that express chamber pressure as an equivalent diving depth. About 33 fsw, or 10 msw, adds one atmosphere of pressure, so 2.0 ATA equals about 33 fsw.
Why it matters: Diving and emergency protocols often use these units instead of ATA.
HBOT (hyperbaric oxygen therapy)
Breathing near-100% oxygen inside a chamber pressurized to at least 1.4 atmospheres absolute, as defined by the Undersea and Hyperbaric Medical Society. Every UHMS-recognized indication calls for 2.0 ATA or more. You may also see it written as HBO or HBO2.
Why it matters: At Generations Wellness, HBOT is delivered in a hard-shell monoplace chamber at a physician-prescribed pressure. HBOT at our Modesto center.
Henry's law
The physics rule that the amount of a gas that dissolves in a liquid rises with the pressure of that gas. Under hyperbaric pressure, extra oxygen dissolves directly into the blood plasma instead of traveling only on red blood cells.
Why it matters: It explains how HBOT can deliver more oxygen to tissue where circulation is compromised. How HBOT works.
Hyperoxia
A higher-than-normal level of oxygen in the blood or tissues. HBOT creates hyperoxia on purpose, for a limited time and at a prescribed dose.
Why it matters: Hyperoxia is the intended effect of treatment, but too much for too long causes oxygen toxicity, which is why dosing matters.
Hyperoxic myopia
Temporary nearsightedness that can develop over a course of hyperbaric sessions. It is the most common effect of HBOT on the eyes. Vision usually improves within about six weeks after treatment ends, though it can take up to a year.
Why it matters: Ask your physician before changing your glasses prescription during a course. Vision changes and other side effects.
Hypoxia and ischemia
Hypoxia means tissue is not getting enough oxygen, and ischemia means it is not getting enough blood flow. Tissue with poor blood flow is usually short of oxygen too, which can slow healing.
Why it matters: HBOT is used for certain conditions in which a lack of oxygen gets in the way of healing.
Glossary
Hyperbaric Terms I–N
Indication
A condition for which a treatment is recognized as appropriate. For HBOT, the key references are the FDA's cleared uses, the UHMS indications list, and the conditions Medicare covers under NCD 20.29.
Why it matters: Whether your condition is an indication affects both the strength of the evidence and whether insurance may cover it. Conditions HBOT is cleared for.
Investigational (off-label) use
Use of HBOT for a condition that is not FDA-cleared and not on recognized indication lists, such as long COVID or brain injury. Some of these uses are being studied, but the evidence has not established them.
Why it matters: Any conversation about an investigational use should start with an honest assessment by a physician. What the research does and does not show.
Marx protocol
A hyperbaric schedule of 20 sessions before and 10 sessions after a dental extraction in a jaw that has had radiation, classically at 2.4 ATA for 90 minutes. It was designed to lower the risk of osteoradionecrosis. A 2019 randomized trial (HOPON) found no benefit for this preventive use, so its role is debated.
Why it matters: If your dentist or oncologist mentions it, ask how the current evidence applies to you. Radiation injury and the pre-extraction protocol.
mHBOT (mild hyperbaric therapy)
Treatment in a low-pressure chamber, typically about 1.3 ATA, usually breathing room air. It does not meet the UHMS definition of HBOT, and low-pressure fabric chambers are FDA-cleared only for acute mountain sickness.
Why it matters: Mild hyperbaric therapy and medical-grade HBOT are not interchangeable. Mild vs. medical-grade HBOT.
Monoplace chamber
A hyperbaric chamber built for one person. Monoplace chambers are pressurized with oxygen, and fire codes classify them as Class B. Generations Wellness uses hard-shell monoplace chambers.
Why it matters: Because the chamber is filled with oxygen, rules on clothing and products are strict. Types of hyperbaric chambers.
Multiplace chamber
A larger hyperbaric chamber that holds two or more people, and can include an attendant inside. It is pressurized with air while patients breathe oxygen through a mask or hood. Fire codes classify these chambers as Class A.
Why it matters: Both monoplace and multiplace chambers can deliver medical HBOT; the experience differs. Comparing chamber types.
NCD 20.29
Medicare's National Coverage Determination for hyperbaric oxygen therapy, which lists the 15 conditions Medicare covers and the criteria for each. For diabetic foot ulcers, it requires a Wagner grade 3 or higher wound, at least 30 days of failed standard wound care, and re-evaluation every 30 days.
Why it matters: It sets the coverage rules at facilities that bill Medicare. Medicare HBOT coverage.
NFPA 99
The National Fire Protection Association's Health Care Facilities Code, which includes a chapter on hyperbaric facilities. It sets fire-safety requirements and classifies chambers as Class A (multiplace) or Class B (monoplace).
Why it matters: Its rules are the reason for 100% cotton clothing and no electronics in the chamber. What to wear and bring.
Glossary
Hyperbaric Terms O–S
Osteoradionecrosis (ORN)
Bone damage caused by radiation therapy, most often in the jaw. It is a form of delayed radiation injury, and Medicare and the UHMS recognize HBOT as an adjunct for established osteoradionecrosis.
Why it matters: HBOT is used alongside dental or surgical care, not instead of it. Osteoradionecrosis and HBOT.
Oxygen toxicity
Harm caused by breathing high levels of oxygen for too long. Central nervous system toxicity can cause a seizure, which occurs about once every 5,000 to 10,000 treatments and typically stops once normal air is breathed. Pulmonary toxicity irritates the airways and can cause a cough or chest discomfort.
Why it matters: Tell your physician about fever, a seizure disorder or disulfiram use, which raise the risk. Oxygen toxicity explained.
Physician-directed
Care in which a physician assesses each patient, decides whether a treatment is appropriate, and sets the treatment plan. At Generations Wellness, HBOT is directed by Dr. Rajwinder Singh Bahia, MD, and a physician assessment is required before any course begins.
Why it matters: It is the difference between a medical treatment and a self-selected wellness service. Our standards.
Pneumothorax
Air in the space between the lung and the chest wall, which causes the lung to collapse partly or fully. An untreated pneumothorax is the only absolute contraindication to HBOT, because trapped air expands as pressure is released and can become a life-threatening tension pneumothorax.
Why it matters: This is why your lung history is reviewed first. HBOT safety and contraindications.
PVHO-1
The American Society of Mechanical Engineers (ASME) safety standard for pressure vessels for human occupancy. It governs the design and construction of the chamber itself.
Why it matters: It is one of the engineering standards behind a medical-grade chamber. Chamber standards.
Refractory osteomyelitis
A bone infection that persists despite appropriate surgery and antibiotics. Chronic refractory osteomyelitis is an FDA-cleared use of hyperbaric chambers, where HBOT is used as an adjunct to surgery and antibiotics.
Why it matters: HBOT is added to infection treatment, never substituted for it. Osteomyelitis and HBOT.
Sham treatment
A control condition in a clinical trial designed to mimic the experience of hyperbaric treatment without delivering the treatment dose. How closely a sham matches real treatment varies between studies, which affects how the results should be read.
Why it matters: When reading HBOT research, check what the comparison group actually received. HBOT research.
Soft (portable) chamber
A fabric or inflatable chamber that typically reaches about 1.3 ATA. It is usually run on air rather than near-100% oxygen and does not meet the UHMS definition of HBOT.
Why it matters: A lower price often reflects this lower pressure, not the same treatment. Hard-shell vs. soft-shell chambers.
SSNHL (sudden sensorineural hearing loss)
A rapid loss of hearing caused by a problem in the inner ear or the hearing nerve. It is an FDA-cleared use of hyperbaric chambers. A 2019 clinical guideline says HBOT with steroids may be offered within 2 weeks of onset, or as salvage therapy within 1 month.
Why it matters: Timing matters, so sudden hearing loss needs prompt medical attention. Sudden hearing loss and HBOT.
Standard wound care
The established basics of treating a chronic wound, managed by your wound-care team before and alongside any adjunct treatment. Medicare requires at least 30 days of standard wound care with no measurable signs of healing before it covers HBOT for a diabetic foot ulcer.
Why it matters: HBOT is an adjunct: it is added to good wound care, never used instead of it.
Glossary
Hyperbaric Terms T–Z
TcPO2 (transcutaneous oximetry)
A noninvasive test that uses sensors on the skin to estimate how much oxygen is reaching the tissue. Wound-care teams may use it to assess the blood supply around a wound.
Why it matters: Your physician or wound-care team may use it when deciding whether HBOT is appropriate. For referring physicians.
Treatment table
A standardized schedule that sets the pressure, duration and breathing gas for each stage of a hyperbaric treatment. Emergency tables, such as U.S. Navy Treatment Table 6 for decompression sickness, are carried out in hospital-based chambers.
Why it matters: Outpatient courses follow a profile prescribed by your physician rather than an emergency table.
UHMS (Undersea and Hyperbaric Medical Society)
The professional society for hyperbaric and diving medicine. It publishes the recognized list of hyperbaric oxygen indications, now in its 15th edition, and runs a facility accreditation program.
Why it matters: Its definitions, such as the 1.4 ATA minimum for HBOT, are the reference point used across this site.
Wagner grade
A 0 to 5 scale that describes how deep and severe a diabetic foot ulcer is. Grade 0 is intact skin at risk; 1 is a superficial ulcer; 2 reaches tendon, joint capsule or deep fascia; 3 is a deep ulcer with abscess, bone infection or joint infection; 4 is localized gangrene; and 5 is gangrene of the whole foot. Medicare covers HBOT for grade 3 or higher when its other criteria are met.
Why it matters: Ask your wound-care team for your grade, because it affects coverage decisions. Diabetic foot ulcers and HBOT.
Side by Side
Chamber Types at a Glance
| Chamber type | Who is inside | What you breathe | Typical pressure | Fire-code class |
|---|---|---|---|---|
| Monoplace (hard-shell) | One patient | The chamber is filled with oxygen | Commonly 2.0–2.4 ATA in clinical protocols | Class B |
| Multiplace (hard-shell) | Two or more people, which can include an attendant | Oxygen through a mask or hood; chamber filled with air | Medical treatment pressures | Class A |
| Soft (portable) | One person | Usually room air | Typically about 1.3 ATA; below the UHMS definition of HBOT | Not a medical HBOT chamber under the UHMS definition |
For a fuller comparison, see hard-shell vs. soft-shell chambers and how to compare your options.
Research Literacy
Reading a Study: The Terms You'll Meet
HBOT headlines often cite studies. These terms help you judge what a study can and cannot tell you.
- Randomized controlled trial (RCT). Participants are assigned by chance to treatment or a comparison group, which reduces bias.
- Blinding. Participants, and ideally the researchers measuring results, do not know who received which treatment. Double-blind means both are unaware.
- Sham or placebo control. A comparison designed to feel like treatment without the active dose. See sham treatment above.
- Crossover design. Each participant receives both the treatment and the comparison, at different times.
- n (sample size). The number of participants. Small studies can produce results that do not hold up in larger ones.
- p-value. A measure of how likely a result this large would be if there were no real effect. A low p-value does not show that an effect is large or important.
- Effect size. How big the difference between groups actually was.
- Systematic review and meta-analysis. A structured summary of all eligible studies on a question, sometimes combining their data. It is only as strong as the studies it includes.
To see these terms applied to real questions, visit our research pages, which explain uses that are not FDA-cleared.
Important
A physician assessment is required before beginning HBOT at Generations Wellness. HBOT complements, and never replaces, the treatment prescribed by your doctors.
Questions
Hyperbaric Terms: Frequently Asked Questions
Have a term we have not defined? Call (369) 222-0979 or browse our FAQ.
What does ATA mean in hyperbaric therapy?
Atmospheres absolute: the total pressure including normal air pressure. Sea level is 1 ATA, and hard-shell chambers commonly treat at 2.0 to 2.4 ATA.
What does HBOT stand for?
Hyperbaric oxygen therapy: breathing near-100% oxygen in a chamber pressurized to at least 1.4 ATA, as defined by the Undersea and Hyperbaric Medical Society.
What is the difference between a monoplace and a multiplace chamber?
A monoplace chamber holds one person and is filled with oxygen. A multiplace chamber holds two or more people, is filled with air, and patients breathe oxygen through a mask or hood.
What is a Wagner grade 3 diabetic ulcer?
A deep ulcer with an abscess, bone infection or joint infection. Medicare requires grade 3 or higher, among other criteria, to cover HBOT for a diabetic foot ulcer.
What is a sham chamber?
A control condition in a clinical trial meant to mimic hyperbaric treatment without delivering the treatment dose. How closely it matches real treatment varies between studies.
Sources
- Undersea and Hyperbaric Medical Society. Hyperbaric Oxygen Therapy Indications. uhms.org
- Undersea and Hyperbaric Medical Society. Position Statement on Low-Pressure Fabric Hyperbaric Chambers. uhms.org
- Undersea and Hyperbaric Medical Society. Standards and Codes (safety and technical FAQ). uhms.org
- U.S. Food and Drug Administration. Hyperbaric Oxygen Therapy: Get the Facts. fda.gov
- U.S. Food and Drug Administration. Is It Really "FDA Approved"? fda.gov
- Centers for Medicare & Medicaid Services. National Coverage Determination 20.29, Hyperbaric Oxygen Therapy. cms.gov
- Noridian Healthcare Solutions. Hyperbaric Oxygen (HBO) Therapy billing guidance. noridianmedicare.com
- National Board of Diving and Hyperbaric Medical Technology. nbdhmt.org
- Wound Care Education Institute. Wagner Ulcer Classification System. wcei.net
- StatPearls. Hyperbaric Complications. National Library of Medicine. ncbi.nlm.nih.gov
- StatPearls. Hyperbaric Oxygen Therapy Contraindications. National Library of Medicine. ncbi.nlm.nih.gov
- Shaw R, et al. HOPON trial: hyperbaric oxygen for the prevention of osteoradionecrosis. International Journal of Radiation Oncology, Biology, Physics, 2019. redjournal.org
- American Academy of Otolaryngology–Head and Neck Surgery Foundation. Clinical Practice Guideline: Sudden Hearing Loss (Update), 2019. aao-hnsfjournals.onlinelibrary.wiley.com
- U.S. Navy. U.S. Navy Diving Manual (treatment tables). navsea.navy.mil
This page is educational and not medical advice. Definitions are general and do not describe your individual treatment. A physician assessment is required before HBOT. See our full medical disclaimer.
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