of decompression illness. Oxygen therapy The administration of oxygen as a medical intervention is common in diving medicine, both for first aid and for longer term treatment. Normobaric oxygen administration at the highest available concentration is frequently used as first aid for any diving injury that may involve inert gas bubble formation in the tissues. There is epidemiological support for its use from a statistical study of cases recorded in a long term database. Recompression and hyperbaric oxygen therapy Recompression treatment in a hyperbaric chamber was initially used as a life-saving tool to treat decompression sickness in caisson workers and divers who stayed too long at depth and developed decompression sickness. In the 21st century, it is a highly specialized treatment modality found to be effective for treating many conditions where the administration of oxygen under pressure is beneficial. Hyperbaric oxygen treatment is generally preferred when effective, as it is usually a more efficient and lower risk method of reducing symptoms of decompression illness, but in some cases recompression to pressures where oxygen toxicity is unacceptable may be required to eliminate the bubbles in the tissues in severe cases of decompression illness. Medical examination for fitness to dive Fitness to dive, (or medical fitness to dive), is the medical and physical suitability of a person to function safely in the underwater environment using underwater diving equipment and procedures. Depending on the circumstances it may be established by a signed statement by the diver that he or she does not suffer from any of the listed disqualifying conditions and is able to manage the ordinary physical requirements of diving, to a detailed medical examination by a physician registered as a medical examiner of divers following a procedural checklist, and a legal document of fitness to dive issued by the medical examiner. The most important medical examination is the one before starting diving, as the diver can be screened to prevent exposure when a dangerous condition exists. The other important medicals are after some significant illness, where medical intervention is needed there and has to be done by a doctor who is competent in diving medicine, and can not be done by prescriptive rules. Psychological factors can affect fitness to dive, particularly where they affect response to emergencies, or risk taking behaviour. The use of medical and recreational drugs, can also influence fitness to dive, both for physiological and behavioural reasons. In