By volume, dry air is 78.09 percent nitrogen and 20.95 percent oxygen, accounting for over 99 percent of the total volume. The remaining 1 percent is made up of argon, carbon dioxide, neon, methane, helium, krypton, hydrogen, xenon, ozone, nitrogen dioxide, iodine and trace amounts of carbon monoxide and ammonia.
Water vapor is also present in air. Under very humid conditions, air can contain as much as 5 percent water vapor, though the amount is more commonly in the 1 to 3 percent range, making it the third most common component of air. Together, nitrogen, oxygen, water vapor and argon account for 99.998 percent of air by volume.