The bends first struck bridge builders working in pressurised underwater boxes
Long before scuba, the illness now called the bends was known as caisson disease. Nineteenth-century workers dug bridge foundations inside pressurised chambers below the riverbed, then came up and collapsed with a mysterious illness. It killed 15 men building the Eads Bridge and disabled the chief engineer of the Brooklyn Bridge, Washington Roebling.
The cause is simple physics. Under pressure, the body absorbs extra inert gas, mostly nitrogen, into its tissues, and the deeper or longer the exposure, the more dissolves, as Henry's Law predicts. If pressure then drops faster than the lungs can clear that gas, it comes out of solution as bubbles, much like a shaken soda opened too quickly. Those bubbles can lodge almost anywhere, so effects range from a rash or aching joint to paralysis or death.
Early descriptions sorted it by symptom: the bends for joint pain that left sufferers doubled over, the chokes for breathing trouble and the staggers for neurological effects. Joints dominate: pain accounts for about 60 to 70 percent of altitude cases, most often in the shoulder. In more than half of cases symptoms take at least an hour to appear. Susceptibility is oddly personal and can even change from one day to the next for the same person.
Divers are the classic victims, but not the only ones. Breath-hold divers making many deep plunges with short rests have been affected, and the condition may explain taravana, an illness known for centuries among South Pacific pearl divers. Astronauts stepping into low-pressure spacesuits and pilots at high altitude face the same risk. A US Air Force study found 87 percent of altitude incidents occurred at or above 7,500 metres.
High-altitude flight in the 1930s pushed designers toward pressurised cabins, mainly to fight altitude sickness, a separate oxygen problem, but the fix also tamed the bends. Airliners now hold cabin pressure equivalent to 2,400 metres, only 73 percent of sea level pressure. In 1889 Ernest Moir, working on the Hudson River Tunnel, pioneered an airlock chamber for treatment, the forerunner of today's recompression chambers.
Source: Decompression sickness
