Dysbarism refers to medical conditions resulting from changes in ambient pressure. Various activities are associated with pressure changes. Scuba diving is the most frequently cited example, but pressure changes also affect people who work in pressurized environments (e.g. caisson workers), and people who move between different altitudes. more...
Ambient pressure is the pressure in the water around the diver (or the air, with caisson workers etc). As a diver descends, the ambient pressure increases. At 10 meters (33 feet) in salt water, it is twice the normal pressure on land at sea level. At 40 meters (the recommended safety limit for recreational diving) it is 5 times the pressure at sea level.
Pressure decreases as we rise above sea level, but less dramatically. At 3000 feet altitude (almost 1000 meters), the ambient pressure is almost 90% of sea level pressure. Ambient pressure does not drop to 50% of seal level pressure until 20,000 feet or 6,000 meters altitude.
Effects of pressure on the body
Direct effects on tissues
This is not of practical importance, because the body is mostly composed of barely-compressible materials such as water. People often wonder whether scuba divers feel their body being crushed by the pressure. The answer is no. Divers would have to reach depths of thousands of feet before their flesh began to suffer significant compression.
Air is very compressible. Humans have many air spaces: sinuses, middle ears, gas in our bowels, cavities in our teeth, and largest of all, our lungs. On land in our daily lives, the pressure in our air spaces is usually exactly the same as the pressure outside, because our air spaces are connected to the outside world. If there was a pressure difference between the outside world and one of our air spaces, then we experience painful pressure on the walls of that air space, as air “tries” to get from the higher-pressure side to the lower-pressure side. This is why we sometimes get painful ears on air trips.
A percentage of the gas we breathe (air) is always dissolved in our blood, like the gas dissolved in a soda bottle with the lid on. If we move to a higher ambient pressure, then the gas we breathe is at a higher pressure, and more of it dissolves in our blood and body tissues. If we move back to a lower pressure, and we move slowly, then the extra gas comes out slowly until we are back to our normal amount of dissolved gas. But if we move quickly to a lower ambient pressure, then the gas comes out of our blood and tissues violently, in large bubbles, like to the difference between slowly opening a bottle of soda (dropping the pressure in the bottle slowly down to sea level), versus ripping the cap off quickly.
Types of dysbarism
Different types of illness result from increases in pressure (e.g. descent during a SCUBA dive, descent during a plane flight), versus decreases in pressure (e.g. coming up from a caisson, or ascending a mountain). Dysbarism comprises several types of illness:
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