Global O2 Levels

I was recently asked by a friend if our oxygen levels are lower now than they used to be. He thought it was lower because of all the destruction of the forests, and dumping in the ocean of toxic wastes. Anyone know if there could be any truth to this? Is there any measure of global O2? What would be the effects to us living of a lowered O2 level?
I think probably from a quick Google scan no one really knows the global O2 apparently. Appears NASA has recently launched a satellite that will study this along with Co2 and global warming issues. So, probably these numbers will be available in the near future.
I'm not sure what the global O2 levels are, but there's nothing to worry about. People can easily survive on less than half of the current levels...and they do it all the time.

The normal atmosphere is about 20% O2, and normal sea level pressure is around 30.00 in Hg. People living/travelling above 18,000 feet live with about 15.00 in of pressure, and presumably the same ratio of O2. Thus, half the O2. I've climbed to 19,300 feet and was able to breath just fine. Who am I kidding, it was very hard to breath, but I survived. Mountain climbers know that 8000 meters is the magic point that cannot sustain life with out Oxygen. You can go above that altitude without O2, but your body is in a constant state of decay. Standard pressure at that alitude is about 10.00 in Hg. Its amazing what the body can adabt to.
From what I've learned, the O2 levels have been dropping naturally for hundreds of millions of years. The theory behind this is that the initial boom of terrestrial plant life caused a spike in the O2 and then as animal life spread and plants consumed the excess CO2 the O2 levels gradually dropped. I'd imagine the levels are still dropping, but at an ever lessening rate. It has to be more stable than the CO2 levels at least.