In short
Hyperbaric oxygen therapy (HBOT) involves breathing near-100% oxygen inside a pressurized chamber, at pressures typically 1.5 to 3 times normal atmospheric pressure, to dramatically increase the amount of oxygen dissolved directly in blood plasma.
Under normal conditions, the vast majority of oxygen in blood is carried bound to hemoglobin, which becomes saturated at a fixed maximum. Increasing ambient pressure and oxygen concentration, as HBOT does, forces substantially more oxygen to dissolve directly in plasma, delivering oxygen to tissue independent of hemoglobin saturation — useful in situations where tissue is oxygen-starved despite normal hemoglobin levels, such as decompression sickness, carbon monoxide poisoning, certain non-healing wounds (particularly diabetic foot ulcers), and radiation injury.
These are the FDA-cleared, well-established indications for HBOT, supported by decades of clinical use and are covered by many insurers for these specific diagnoses. Longevity-focused interest is more recent and centers on a specific published Israeli study (Hachmo et al., 2020, Aging, from Tel Aviv University and Shamir Medical Center) in older adults, which found that a series of HBOT sessions increased telomere length and reduced the proportion of senescent T-cells in blood samples, compared to a control group.
This is one real, published human trial with a real biomarker finding — it is not evidence that HBOT extends lifespan, reverses aging, or should be considered a general anti-aging treatment, and it has not yet been independently replicated at scale with clinical outcome endpoints. Longevity clinics offering HBOT for general anti-aging purposes are extrapolating from this early biomarker signal, and from HBOT's established safety profile in its approved indications, to a use case that has not itself been validated by an outcome-level human trial.