Yeah, Malmo, I think am getting it - I just was trying to avoid the next objection which was going to be that the difference wasn't that significant. Humidity reduces oxygen (for practical purposes) in two ways - literall somewhat fewer oxygen molecules per unit of air - the money quote:
"What Avogadro discovered leads us to conclude that if we added water vapor molecules to our cubic foot of air, some of the nitrogen and oxygen molecules would leave — remember, the total number of molecules in our cubic foot of air stays the same."
..but also because it decreases the density, so the "proportion" of oxygen will be somewhat different (but not too greatly), but the overall number of oxygen molecules will be less because there are less of all molecules in less dense air.
Anyone who lives somewhere humid knows this. When the dewpoint is 70+, I might go out for an easy day and I will not be terribly comforable averaging in the 7:10's on an easy day (I have had runs lately with the first mile around 7:40!). Have a cold front go through and you will be jogging around very comfortably easy at 6:50 pace feeling a hell of alot better and wondering how you suddenly got fit Don't believe your subjective impression? You can see what it does on a HRM - when the dewpoint is high, your HR is higher FROM THE GUN - before you even have time for your blood volume to lower.
People in the blessed parts of the country without humidity just won't ever get it. In Omaha, I run regularly when the dewpoint is in the low-mid 70's. I ran once a few weeks ago when the dewpoint was 80! This place has to have the worst weather of anywhere in t e country - July is worse than Atlanta while January is almost as bad as North Dakota. And it smells bad too. At 8:52am, the temp here was 80 and the dewpoint 73. I live by a large lake and it was frozen solid from mid Dec. - early March. Now I may as well be in South Carolina.