Back on the planet, the colonists have taken shelter underground. Data believes that the refractory metals in the rock strata will protect them. Crusher is concerned that they and the settlers will run out of air soon.
Data, Riker, Dr. Crusher and a number of colonists are sealed into an underground cavern, and they start to run out of oxygen.
The Android Lieutenant Commander Data also had a functional respiratory system with the purpose to maintain thermal control of his main systems. Data explained this to Doctor Julian Bashir after he noticed that Data was breathing. (TNG: "Birthright, Part I")
Why couldn't Data reprogram/use his respiratory system to filter out the carbon in the air and exhale oxygen? Even if it was a percentage of what was needed, it would extend the time in the cave by some degree, I'm sure.