Re: Could humans breath in an atmosphere composed of 100% oxygen?

Dear Bryan,

The short answer is yes - but not for long! Breathing pure oxygen rapidly 
induces light-headedness followed by unconsciousness. Oxygen is in fact 
toxic to all living organisms principally because of the production of 
reactive free radicals such as superoxide. 
Superoxide is easily converted to a host of other oxidizing molecules such as 
hydrogen peroxide, hypochlorous acid (bleach) and the highly 
reactive hydroxyl radical. Superoxide is a by-product of normal metabolism 
and is normally kept at safe levels, but as the oxygen concentration 
increases, superoxide levels overwhelm the body's natural defenses and 
start damaging proteins, the lipid membranes of cells, and DNA. 

One popular theory of aging is that the slow accumulation of such oxidant-damaged  
molecules over time is the cause of aging, even in our normal atmosphere 
of 21% oxygen. Although it is a rather old reference, the chemistry behind 
all this is very well explained by Barry Halliwell in a review article 
"Oxygen toxicity, oxygen radicals, transition metals and disease" in the 
Biochemical Journal (1984) 219:1-14.