Re: Breathing in pure oxygen?

Under the right conditions oxygen gas can form many kinds of reactive species such as the superoxide radical O2.- or singlet oxygen O. (O molecule with an added electron - the dot). Both of these species are radicals, meaning they contain a loose electron which makes them highly reactive and able to break the bonds of nearby molecules. As you could imagine, randomly breaking apart the molecules that make up your body would have deleterious effects. For the first billion or so years of life on the planet, this wasn't much of a problem - life evolved in a predominantly anaerobic atmosphere, or one with little to no oxygen gas. It wasn't until certain microbes developed the ability to create energy via photosynthesis that O2 appeared in significant quantities. O2 is a by-product of photosynthesis. The energy of light is used to break apart molecules of water creating protons (H+) and oxygen gas.

With this change in the environment, organisms exposed to the changing atmosphere had to find ways to adapt, or face emminent death. Many evolved specific enzymes and factors that would 'diffuse' oxygen and some of it's more toxic relatives. Some microbes even took advantage of this change by evolving more efficent means of generating energy, namely respiration - the biochemical process by which we get most of the energy from the food we eat. In respiration electrons are stripped from molecules of glucose, squeezed of their energy, then combined with protons and oxygen gas to create molecules of water. Note that this is the reverse of what happens in photosynthesis!

Ok.. enough random background..

Our atmosphere consists of ~20% oxygen gas. We can survive in this atmosphere with the defense mechanisms we have evolved - enzymes that degrade toxic oxygen species, and the body's use of anti-oxidants such as beta-carotenoids (what makes carrots orange) and vitamin E (tocopherol) that absorb free radicals.

However, breathing in pure oxygen creates more toxic species than the body can handle. The primary site of damage occurs in the lungs. The damage affects the airways, and in particular makes it more difficult for oxygen to diffuse from the lungs into the bloodstream. As an example, premature babies with immature lungs are often placed on respirators to insure that sufficient oxygen makes it into the blood. The increased levels of oxygen place them at risk for developing bronchopulmonary dysplasia. You can see the effects of the disease at the U. Iowa Department of Radiology

-L. Bry, MadSci Admin