Skin diving with self-contained underwater breathing apparatus (scuba) has hazards that distinguish it both from free diving during short periods of voluntary apnea and from traditional diving with helmet, pump, and connecting tubes. It is also necessary to distinguish between the type of scuba in which the expired carbon dioxide is absorbed chemically, the needed oxygen is added, and the nitrogen is rebreathed from the type in which the expired air is simply discharged into the water as bubbles. Although the latter form is less dangerous, it has its hazards, including air embolism, decompression illness, nitrogen narcosis, oxygen intoxication, rupture of eardrums, and squeeze. Squeeze, the pressing of body tissues into air-filled spaces in the diving costume, is particularly damaging to the eyes. The bulging of the eyes into goggles is hard to avoid, but if a mask is worn instead of goggles the danger can be decreased by discharging air from the nose into the mask whenever pressures increase.