Clean air is a mixture of 10 gases: nitrogen, oxygen, argon, carbon dioxide, neon, helium, methane (CH4), krypton, hydrogen, and xenon. Five of these travel alone as atoms, while the rest have atoms bonded together forming molecules. About 78.09% of the air is made out of nitrogen atoms, 20.95% is oxygen, and argon is about 0.53%. Carbon dioxide is about 0.04% (392.39 ppm in 2010; NOAA, 2010) and rising continually. Psychrometry is the science that deals with the study of this mixture, essentially the relationship of these gases with water vapor. This is important because the amount of water vapor in the air affects thermal comfort. The graphic representation of these relationships is the psychrometric chart. Figure 3.1 shows the relationship of these variables with each other in the American Society of Heating, Refrigerating, and Air-Conditioning Engineers’ (ASHRAE’s) psychrometric chart at sea level.