Breathing mask
From Memory Alpha, the free Star Trek reference.
A breathing mask is a special device worn over the face to allow species that do not breath the local atmosphere to breath their own supply. They are sometimes also used for medical purposes to supply a patient with special breathing gas.
The Zaranites breathe fluorine and cannot survive in oxygen - so Zaranites on class-M worlds (including Earth) have to wear special masks. (Star Trek: The Motion Picture)
The alien foreman of a trellium mining colony had to breathe with the help of a mask because the toxic fumes had destroyed his lungs. His workers (as opposed to the miners) of his species also wore similar masks, presumably as a preventive measure. (ENT: "The Xindi")
An Ambassador concerned with Coridan matters at early talks for a Coalition of Planets wore a breathing apparatus over most of his face. (ENT: "Demons")
