Over the past 40 years hyperbaric oxygen therapy has been recommended and used in a wide variety of medical conditions, often without adequate scientific validation of efficacy or safety. Consequently a high degree of medical scepticism has developed regarding its use. The Undersea and Hyperbaric Medical Society approves use of hyperbaric oxygen for a few conditions for which there is thought to be reasonable scientific evidence or well validated clinical experience. In these conditions early referral is essential. #### Therapeutic uses of hyperbaric oxygen ##### Strong scientific evidence ##### Suggestive scientific evidence Hyperbaric oxygen has been shown ineffective in diseases such as multiple sclerosis and dementia, but it continues to be used despite the risks of the treatment. For conditions where its use remains unproved—for example, rheumatoid arthritis, cirrhosis, and gastroduodenal ulcer—hyperbaric oxygen should be used only in the context of well controlled clinical trials. At sea level the plasma oxygen concentration is 3 ml/l. Tissues at rest require about 60 ml of oxygen per litre of blood flow (assuming normal perfusion) to maintain normal cellular metabolism, although requirements vary between tissues. At a pressure of 3 atmospheres (304 kPa) dissolved oxygen approaches 60 ml/l of plasma, which is almost sufficient to supply the resting total oxygen requirement of many tissues without a contribution from oxygen bound to haemoglobin. This has advantages in situations such as carbon monoxide poisoning or in severe anaemia where difficult crossmatching or religious belief prevents blood transfusion. #### Cellular and biochemical benefits of hyperbaric oxygen