Re: How long can a human breath pure oxygen????

Hi Jake-

Some quotes state that 48 hours of breathing 100% oxygen will kill a healthy adult. I'll go into why this is so, and how we manage to survive in an atmosphere containing oxygen gas.

In the meantime - please note your comment concerning nitrogen is INCORRECT! People need nitrogen but they **do not** get it from the nitrogen in the air!! We receive nitrogen from the foods we eat, from things like proteins which contain amino groups-- chemically represented as -NH3. Many species of bacteria fix atmospheric nitrogen, that is they convert N2 gas into NH3 (ammonia) which plants use to make amino acids and other compounds.. we eat the plants, and that is how we obtain our assimilated nitrogen. Only species of bacteria such as Azotobacter, Rhizobium and cyanobacteria can carry out this important function. No plants or animals can fix atmospheric nitrogen on their own. Part of the reason is that nitrogen fixation can only take place under anaerobic conditions - that is in the absence of oxygen gas. Since we need O2 to survive, we can't breath oxygen and fix our own nitrogen at the same time.

I hope you'll set everyone straight in your school.. :)

You can learn more about nitrogen fixation by running a search at Altavista for any of the words I put in bold type.

Concerning the lethality of inhaling O2 gas - the effect relates to the fact that oxygen is a highly electronegative (imply => reactive) element. O2 gas is capable of creating free oxygen radicals which indiscriminantly cause damage to cellular tissues. How then do we manage to breath an atmophere containing 20% O2? First, this same reactivity of oxygen is what our bodies harness in the final processes of respiration - how our cells make energy. Also - the body has a few defenses to counter free radicals. Vitamin E and beta-carotenes can squelch oxygen radicals before they cause damage to something important such as cell membranes, proteins or DNA. At the levels of O2 found in the atmophere these defenses serve us well. However when breathing 100% O2, too many radicals are produced for the body to handle.. cellular damage, to the lungs in particular, ensues.

Cheers..

Lynn Bry, MadSci Admin