Should decompression be accomplished on pure O2 or on an 80/20 mix? Some of the tech agencies teach deco on 80/20 because a sudden change in the weather or loss of buoyancy control due to emergency conditions can make deco on 100% O2 dangerous. They are also beginning to argue that breathing 100% O2 for extended periods can have harmful effects on lung tissues. George Irvine argues that decompression should be accomplished with 100% O2. His arguments, although presented as a "Baker's Dozen" actually boil down to just a half dozen palpable arguments: 1. Decompression with mixes less than 100% is inefficient. 2. Decompression with 80/20 allows divers to compensate for poor buoyancy control with a longer decompression. 3. The 80/20 mix is actually incorrect for 30 feet as that provides a pO2 of 1.52. 4. The 80/20 mix provides a pO2 of only 1.28 at 20 feet and 1.04 at 10 feet, which he claims is "worthless for decompression." 5. In a diving emergency that causes decompression to be shortened, 80/20 does not provide as large a safety margin as 100% O2. 6. If buoyancy control is a problem at 20 feet, why isn't it a problem at 30? In response to arguments that breathing pure O2 has measurable negative effects on the lungs, Irvine's followers argue that you can take occasional breaks from breathing the pure O2 by breathing a little backgas. This argument appears to negate the principal argument in that breathing backgas provides a large return dose of nitrogen. T'anks.