Decompression sickness (DCS), also known as the diver’s disease, the bends, or caisson worker's disease is characterized by severe joint pain after a person undergoes a severe reduction in pressure in a quick period of time, typical of a diver rising too fast on a dive or a person stepping out of a normally pressurized work environment, such as tunnel digging.
The pain is caused when inert gases (generally nitrogen) which are normally dissolved in body fluids, blood and tissues, come out of physical solution (i.e., outgas) and form gas bubbles which collect in the body's joints. The effect is similar to what happens when a bottle of soda is opened - the resulting change in pressure allows gas that was in solution to form as bubbles.
Pure oxygen will ameliorate the symptoms by allowing nitrogen that is still dissolved in the blood to eventually leave the body through the lungs. This will allow some of the gas bubbles in the body to eventually redissolve into the bloodstream. However, the only way to treat such a patient is to put them back into a pressurized environment. This forces the nitrogen back into the body fluids. At that point, the pressure can be gradually lessened to atmospheric pressure to allow the nitrogen dissolved in solution to naturally leave the body through breathing.
Decompression sickness can be crippling or fatal if left untreated. Washington Roebling, who oversaw the construction of the Brooklyn Bridge, was crippled by the disease, caused by his frequent trips to the pressurized chambers where the digging of the bridge's piers took place. The astronauts on Soyuz 11 had a rapid depressurization of their air atmosphere, resulting in the bends before the astronauts eventually succumbed to asphyxiation.