I do not like thinking of how the air I breathe comes from the engines. It is supposed to be 100 clean, but it never is. There has been done numerous studies on this, and even lawsuits (for the 757) and thats why I book the 787 as often as I can because of its bleedless design. I will go out of my way to fly it. I realize the pressure of the cabin, and the cleanliness of the air does not necessarily have direct impact of the o2 levels, but nevertheless, I am concerned about the air quality on planes.