Pandora's atmosphere is a mixture of nitrogen, oxygen, carbon dioxide (>18%), xenon (>5.5%), methane, and hydrogen sulfide (>1%) and is about 20% denser than the atmosphere on Earth primarily due to the high percentage of Xenon; a heavy, colourless, odourless, and generally unreactive noble gas. The high concentration of carbon dioxide in the Pandoran atmosphere makes it extremely poisonous to humans, rendering them unconscious in about 20 seconds and causing death in about 4 minutes when they venture out unprotected by specialized breathing masks