AMBIENT air contains nearly constant amounts of nitrogen (78% by volume), oxygen (21%), and argon (0.9%), with
varying amounts of carbon dioxide (about 0.03%) and water vapor
(up to 3.5%). In addition, trace quantities of inert gases (neon,
xenon, krypton, helium, etc.) are always present. Gases other than
those listed are usually considered contaminants. Their concentrations
are almost always small, but they may have serious effects on
building occupants, construction materials, or contents. Removing
these gaseous contaminants is often desirable or necessary.