Earth’s atmosphere, apart from water vapor and pollutants, is composed of 78 percent nitrogen, 21 percent oxygen, and less than 1 percent each of argon and carbon dioxide. There are also traces of hydrogen, neon, helium, krypton, xenon, methane, and ozone. Earth’s original atmosphere was probably composed of ammonia and methane; 20 million years ago the air started to contain a broader variety of elements. The atmosphere weighs approximately 5 million billion tons. It exerts an average of 14.7 pounds per square inch (PSI) of pressure on the surface of the planet.