BIBS and Oxygen Delivery Systems in Hyperbaric Chambers Explained

[ Chamber session ] Reference imagery; final photography in production

What BIBS means

BIBS stands for built-in breathing system. It is a set of masks or hoods plumbed into the chamber, so the person inside breathes a gas that differs from the chamber air. The clearest public definition comes from the US Navy. The Operating Standards for NOAA Hyperbaric Chambers, dated January 2017, reproduce the Navy Diving Manual chapter on recompression chambers. That chapter lists the basic parts of a chamber: the pressure vessel, an air supply and exhaust system, a pressure gauge, and a BIBS to supply oxygen to the patient.

The same text lists optional parts. These include oxygen, carbon dioxide, temperature and humidity monitors, carbon dioxide scrubbers, extra BIBS lines for air and other treatment gases, a BIBS overboard dump system, and heating and cooling. It says modern Navy chambers combine a BIBS, an oxygen dump, monitors and a scrubber, which together reduce the ventilation the chamber needs. Spec sheets in the same appendix list BIBS mask counts for each lock, an overboard dump, and oxygen analysis of the supply gas.

Navy chambers are a different class of vessel from a home unit, as our decompression vs hyperbaric guide explains. We use them here because their manual is public and spells out how a BIBS works.

Overboard dump vs exhaust to the chamber

A BIBS mask has two sides. Fresh gas comes in. Exhaled gas has to go somewhere. There are two places it can go: out of the chamber through a dump line, or into the chamber air. That choice decides how much oxygen builds up around the occupant.

The Navy manual shows the cost of the second option in its ventilation rules. Those rules are designed so that, when oxygen is in use, chamber oxygen does not exceed 25 percent. When people breathe air, it calls for 2 cubic feet per minute per diver at rest and 4 for a diver who is not at rest. When oxygen is breathed from the BIBS, it calls for 12.5 cubic feet per minute per diver at rest and 25 for one who is not. Those higher rates apply only when no BIBS dump system is installed. If a BIBS dump or a closed circuit BIBS is used for oxygen, the air-breathing rate may be used.

So the dump is what keeps exhaled oxygen out of the room the occupant sits in. The Navy predive checklist in the same text includes BIBS masks installed and tested and the BIBS dump functioning. The postdive list says to clean, dry and test the BIBS masks.

Masks, hoods and head tents

Three things carry oxygen to the face in an air-filled chamber. An oral-nasal mask, also called an oronasal mask, covers the nose and mouth. A hood is a clear cover over the head that seals at the neck. A head tent is the name one hospital uses for its own head enclosure. The sources below describe all three.

Townsville University Hospital's patient leaflet says patients are measured for a hood and neck seal to breathe 100% oxygen. In some cases the hood cannot be used and a mask system is used in its place. The nurse places the clear hood over the neck seal once the chamber reaches treatment pressure, and the hood comes off twice for breaks. The University of Maryland Medical Center says its chamber is pressurized with air and oxygen comes through a head tent, in two segments with a five-minute air break. In the Navy text, tenders who breathe oxygen hold the mask on their faces rather than fasten it to their heads. Our guide to what a session feels like covers the patient side.

Monoplace: the chamber is the oxygen

A hospital monoplace chamber often needs no mask at all. The K081506 510(k) summary describes a Class B monoplace chamber built for one patient up to 3 ATA, or 29.4 psig. It says the chamber uses 100% oxygen as the pressurization gas and the patient breathes the oxygen inside the chamber. Air pressurization with oxygen by mask is listed as an option.

In an oxygen-filled monoplace, the exhaust is the delivery control. The K140559 510(k) summary for two Sechrist monoplace models describes chambers that administer 100% oxygen up to 3 ATA. It has a ventilation, or purge, control that sets the rate at which gas exhausts from the chamber. It says that once the chamber reaches the desired pressure and oxygen concentration, flow can be reduced to save oxygen. It lists a purge range of 80 to 400 liters per minute at 15 psig, with an oxygen conservation range of 80 to 200.

The whole vessel is oxygen-rich in this design. That is why the fire rules for Class B chambers focus on what goes inside, as our NFPA 99 guide sets out.

Multiplace: air in the room, oxygen at the face

A multiplace chamber keeps the room on air and puts oxygen at each face. The K021689 510(k) summary describes a Class A chamber for up to 12 patients at up to 6 ATA, or 73.5 psig. It says the chamber uses compressed air as the pressurization gas and 100% oxygen as the treatment gas, given through properly fitting oronasal masks or head tents. It lists an oxygen analyzer installed to monitor the oxygen in the chamber continually, and two fire suppression systems, a water deluge and a handline.

Our monoplace vs multiplace guide compares the two on price, size and power. For delivery, the split is simple. In a monoplace, the gas you breathe and the gas around you are the same. In a multiplace, they are kept apart, and the BIBS and its exhaust do that work.

The 23.5% line and oxygen limits

Normal air is about 21% oxygen. The Des Moines Fire Department's 2024 appeal letter gives the figures: air is 20.95% oxygen, OSHA defines a concentration above 23.5% as a hazardous atmosphere, and NFPA 99 uses 23.5% in its definition of an oxygen-enriched atmosphere. It says items burn hotter and faster and ignite more easily with more oxygen.

We looked for a sentence in NFPA 99 that caps Class A chambers at 23.5%. We did not find one in the public documents we read. What we found is close to it. A UHMS safety FAQ on standards and codes, in an answer on chamber textiles, says Class A chambers typically maintain an oxygen concentration below 23.5 percent and have fire suppression, and that the opposite can be said for Class B chambers. The Navy manual sets its own line at 25 percent.

The threshold also shaped a code fight. In 2023, a proposal to NFPA 99 sought a lighter category for chambers under 5 psi with oxygen below 25 percent. The committee ballot rejected it 0 to 17. The proposer's packet included lab tests of soft chambers with concentrator oxygen at 4.0 liters per minute and about 4.1 psi. The lab letter says chamber oxygen did not exceed 23.5% with the mask on or off. The packet's fire analysis gives a high of 23.1% and says staff limit the concentrator to four liters. Those are the proposer's own submitted results, not an independent finding.

How chambers monitor oxygen

A Class A chamber watches its own air. The UHMS Clinical Hyperbaric Facility Accreditation Manual, fourth edition, uses the 2015 edition of NFPA 99 as a primary reference. Its survey items, each tied to an NFPA 99 clause, include these:

• Class A monitoring: oxygen levels are continuously monitored when patients or attendants breathe mixtures above 21% oxygen, or when flammable agents are present (14.2.9.4.2).
• Alarm: audible and visual alarms indicate oxygen above 23.5% by volume (14.2.9.4.2.1).
• Low oxygen: oxygen is monitored, with low oxygen alarms, whenever nitrogen or another diluent gas is added to lower the oxygen in the chamber (14.2.9.4.1).
• Above 23.5%: antistatic procedures apply, and the patient in a Class A or Class B chamber is grounded through a high-impedance conductive pathway (14.3.1.5.3).
• Exhaust: exhaust from a Class A or Class B chamber is piped outside the building, clear of hazards and of reentry (14.2.10.2).

The next edition keeps the exhaust rule in view. NFPA's 2024 first-revision report says exhaust from all classes of chambers shall be designed to vent outside the building, and that oxygen exhaust and its signage shall be inspected and documented at scheduled intervals. That report is a draft, not a published edition. The Navy text lists an oxygen analyzer among its chamber parts, and NOAA's predive checklist has a line for an oxygen analyzer functioning and calibrated.

Home soft chambers: concentrator, mask or cannula

A soft chamber has no BIBS in the Navy sense. Its compressor fills the bag with room air. Oxygen, when a seller offers it, comes from a separate concentrator through tubing to a mask or a nasal cannula inside the bag. The Minnesota State Fire Marshal's interpretation on mild hyperbaric oxygen therapy describes these chambers at 2 to 4.5 psi, with oxygen given by mask or nasal cannula at 90 to 95 percent.

With a mask or cannula in a sealed bag, what the occupant exhales and any leak at the face stays in the chamber unless something carries it out. One committee member wrote in the 2023 ballot that, from the proposer's own test data, supplemental oxygen by mask is used in addition to the pressurization air in these chambers. How concentrators work, and the flow and purity figures behind them, are in our concentrator guide.

What the FDA says about soft chambers and oxygen

The FDA has drawn a line. In its consumer update "Hyperbaric Oxygen Therapy: Get the Facts", the agency says it has cleared a large, zippered bag to treat altitude sickness only, that these bags provide pressure but do not attach to oxygen tanks, and that it has not cleared them for use with oxygen tanks or oxygen concentrators. It also says people have used these bags to build homemade setups, which can pose a risk of fire and suffocation. That page now returns a 404 on fda.gov; the link is the archived copy.

The UHMS position statement on low-pressure fabric chambers says they are not designed to be used with supplemental oxygen and are not designed to be pressurized above 1.4 atmospheres. The FDA's August 2025 letter to health care providers warns of a heightened risk of fire with oxygen at high concentration and asks for proper grounding. For what a clearance does and does not cover, see FDA cleared vs FDA registered.

Delivery methods compared

The table sets each method side by side, using only what the sources above state. Where a source is silent, the cell says so.

Sources read 22 October 2026 and linked above. Each row describes the device or setting named in its source. It is not a rule for other models.

What to ask a seller

Ask in writing and keep the answers.

• Is this chamber designed and cleared for use with supplemental oxygen?
• Does the mask exhaust outside the chamber, or into it?
• What oxygen level has been measured inside the chamber during use, at what flow, and by whom?
• Does the chamber have an oxygen monitor with an alarm, and at what level does it alarm?
• Which mask, hood or cannula does the maker supply, and how is it cleaned?

A good answer is not a reason to buy. It tells you the paperwork exists. Our safety guide covers the wider risks, and the scorecard shows how the chambers we track compare on what we can verify.

What we have not verified

We have not read NFPA 99 directly. The clause numbers above come from the UHMS accreditation manual, which works from the 2015 edition, and from a 2024 draft. We did not find a sentence in any edition that caps Class A chamber oxygen at 23.5%, and we did not confirm the clause numbers in the 2024 edition. We did not read the Navy Diving Manual itself, only the chapter as reproduced by NOAA. We did not find a published oxygen level measured inside a home hard chamber with a BIBS or exhaust line. The soft chamber oxygen figures come from the proposer's own packet; one letter in it says 22%, and the fire analysis says 23.1%. We did not find a published flow or purity for any hood or mask circuit sold for home use. If you hold a test report or a manual that settles one of these points, send it and we will check it.