Why can't scuba diving tanks be filled with pure oxygen?

Because oxygen is toxic to the human body at depth when it is in too high of a concentration. Pure oxygen would be extremely toxic as soon as you got down just a few meters, and you would die of oxygen toxicity. Doesn't sound like fun to me.

Pure oxygen is toxic under pressure , and mighty dangerous to handle. Risking explosion just by filling them. When you go deep enough, the oxygen percentage is only 2-3%. The air that we breath is 21% oxygen, 79% nitrogen, and some trace elements. Even when you go to an oxygen bar, you will not find anything over 29%. Because they consider it too dangerous, and potentially harmful to their clients.

how long you got? this i can answer 1 you know how a car engine works on compression causing heat? how dangerous is it to compress oxygen? its only safe to breath 02 down to 10M or plus 1 atmosphere. after that the ppo2 gets higer than .6 thus becoming toxic to the human body. convulsions and then death follow. in a rebreather, the scuba cylinder is filled with o2 adding a tiny squirt of o2 to replace that taken out of the air loop when breathing. co2 is scrubbed out of the loop by sodalime. o2 and co2 are carefully monitored by 2 computers but still more people die from computer malfunctions in rebreathers than drown in open circuit scuba

Because we don't breathe pure oxygen right now.

Also, oxygen has to go into a tank that has been specially cleaned to accept oxygen. If you put oxygen into "any ole scuba tank", you run the risk of starting a fire due to the high oxygen content and the impurities on the tank/regulator. Edit: Also, the following isn't completely correct: Shandra - "Because oxygen is toxic to the human body at depth when it is in too high of a concentration. Pure oxygen would be extremely toxic as soon as you got down just a few meters, and you would die of oxygen toxicity. Doesn't sound like fun to me." Actually, the oxygen toxicity will cause you to convulse. On the surface, this isn't a problem. Underwater it is a huge problem since during the convulsions you'd more than likely spit out the regulator and aspire water (i.e. drown). Yes, the oxygen toxicity contributed to the entire situation, but oxygen toxicity doesn't directly kill people.

do you breath pure oxygen on land?

because you'd die

breathing pure Oxygen underwater makes you fall asleep and gives you a lack of awareness.