to a lower pressure, then the extra gas comes out slowly until they are back to their normal amount of dissolved gas. But if they move quickly to a lower ambient pressure, then the gas comes out of our blood and tissues violently, in large bubbles, in the same way that quickly removing the cap from a bottle of soft drink produces far more bubbles than slowly opening the bottle. Types of dysbarism Different types of illness result from increases in pressure (for example, descent during an underwater dive, descent during a plane flight), versus decreases in pressure (for example, coming up from a caisson, or ascending a mountain). Dysbarism comprises several types of illness: Decompression sickness (DCS) Decompression sickness, also called caisson workers' disease and the bends, is the most well-known complication of scuba diving. It occurs as divers ascend, and often from ascending too fast or without doing decompression stops. Bubbles are large enough and numerous enough to cause physical injury. It is quite possible that all divers have microbubbles in their blood to some extent, but that most of the time these bubbles are so few and so small that they cause no harm. When DCS occurs, bubbles disrupt tissues in the joints, brain, spinal cord, lungs, and other organs. Symptoms vary enormously. DCS may be as subtle as unusual tiredness after a dive, or an aching elbow, or a mottled skin rash. Or, it may present dramatically, with unconsciousness, seizures, paralysis, shortness of breath, or death. Paraplegia is not uncommon. Nitrogen narcosis Nitrogen narcosis is also called “L’ivresse des grandes profondeurs” or "rapture of the deep". Nitrogen comprises 79% of the air, but at surface pressures it has no sedating effect. At greater depths, however, nitrogen affects the brain in the same way as nitrous oxide (also known as laughing gas) and other anaesthetic gases. The effect is similar to the effects of alcohol, and to some extent there is cross-tolerance. Unlike alcohol, the onset and disappearance are near instantaneous. A diver may be quite clear-headed at 20 meters, and yet giddy and silly at 30 meters. Ascending to 20 meters will almost instantly clear the head. High pressure nervous syndrome (HPNS) Barotrauma Barotrauma is injury caused by pressure effects on gas spaces. This may occur during ascent or descent. The ears are the most commonly affected body part. The most serious injury is lung barotrauma,