The Bends


    How do scuba divers get "the bends", and just what are they? Kids, you can feel some of the effects of pressure in a swimming pool. Down just a few feet underwater your ears begin to hurt. This is caused by pressure on your eardrums.

    Where does that pressure come from? At the surface of the water, a column of air weighs 14.7 pounds per square inch, or "one atmosphere". When you go underwater, you add the weight of the water to the atmospheric pressure. A 10-meter (33-foot) column of water also weighs 14.7 pounds per square inch, so at a depth of 10 meters the pressure is two atmospheres: half from the water and half from the air above it. Pressure influences how divers use air. At ten meters, the increased pressure means that lungs hold twice as much air as they do at the surface - and divers breathe air from their tanks twice as fast. This is why divers can stay down only a short time, for example, 15 minutes at 50 meters.

    "The bends," or decompression sickness, is a health hazard associated with pressure changes. The longer you stay down and the deeper you go, the more nitrogen gas dissolves into your body tissues. Nitrogen comes from the air we normally breathe, which is about 80% nitrogen and only 20% oxygen. If you ascend too rapidly, the dissolved nitrogen comes out of solution too quickly and forms bubbles in your tissues. You could experience severe pain in joints, dizziness, blindness, paralysis, and convulsions.

    Divers learn they must ascend slowly, and sometimes take "decompression stops" on the way up. This allows the dissolved nitrogen to come out of the body safely. Sometimes a hyperbaric chamber has to be used to stabilize a diver in critical condition. These are chambers in which patients breathe 100% oxygen at greater than one atmosphere pressure using a mask. Flooding the body with pure oxygen helps to quickly and safely eliminate the nitrogen gas. Divers can also be certified to use different mixtures of air in their tanks, which are enriched in oxygen ("nitrox").


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    October 1996


    Reference: The Internet at: (Minnesota's PBS TV station KTCA produces "Newton's Apple", a national science program for kids and adults).