I believe the decimal point is off by one place.
By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere.
Oh, yeah. That’s right. It’s 4/100ths of 1%. Thank you.