Born over 350 years before the Clone Wars on the harsh world of Dorin, Plo Koon grew up in an atmosphere mixed with the substance known as dorin gas and also helium. Dorin Gas was absolutely crucial to the lives of the Kel’dor and was as essential to them as oxygen is to humans. If a Kel’Dor’s eyes were not exposed to this gas for a short period of time, he would slowly go blind and his eyes would bleed out… Definitely not a fun experience.
On top of this, because the Plo Koon and his species evolved in a place with no oxygen, the gas was horrifically toxic to them and could cause a very quick death if inhaled in a big enough amount. So in order to combat these major problems, when Dorin joined the Republic nearly 6000 years before the Clone Wars, each of the clans on the planet developed anti-oxmasks to protect their members and allow them to roam the galaxy without fear.