Helium

Luke and Ben Skywalker wear in order to protect themselves from Dorin's helium-rich atmosphere. — Luke and Ben Skywalker wear breath masks in order to protect themselves from Dorin's helium-rich atmosphere.

"Hey, there's something I've been meaning to do for you. I've been practicing while you weren't around."
"Go ahead."
[Takes a deep breath of helium-rich air] "♪Where fields once grew, a road runs through, and buildings hide the sun♪"
"Ben, don't."
"♪Where grass of green could once be seen, are only gray and brown♪"
"I hate that song under normal circumstances."
"♪My childhood home, while I did roam, became a place of sadness♪"
"I'll just wait until you pass out."
"♪Now I return, my heart does yearn for times of light and gladness♪"
"You'll make your throat sore."

―Ben Skywalker, teasing his father on Dorin^[1]

Helium was an elemental gas found on many worlds, including the atmosphere of Dorin where it coupled with Dorin gas.

Mynocks were known to have an allergic reaction to helium causing them to be become bloated and die.