[DelaWhere]

Not even close to 21% - the atmosphere is composed as follows:

PERMANENT gases in the atmosphere by percent are:

Nitrogen 78.1%
Oxygen 20.9%

(Note that these two permanent gases together comprise 99% of the atmosphere)

Other permanent gases:

Argon 0.9%
Neon 0.002%
Helium 0.0005%
Krypton 0.0001%
Hydrogen 0.00005%

VARIABLE gases in the atmosphere and typical percentage values are:

Carbon Dioxide 0.035%
Methane 0.0002%
Ozone 0.000004%
Water Vapor varies

[djf]

OOps! Knew I was off by at least one decimal point!