Boron
Boron is a chemical element, number 5 on the periodic table.
Boron was needed by Tesnians to breathe. They could also survive in an oxygen-nitrogen atmosphere, but needed to breathe boron at least six hours a day. (ENT: "Shuttlepod One")
In 2373 a runabout carrying Benjamin Sisko and Michael Eddington ran into a pocket of boron gas which made it rock violently. (DS9: "Blaze of Glory")