What chemical

[astronaut]

What chemical would change the atmosphere as to make the air unbreathable?

[ll_billym]

Lots of chemicals would do this so a context is required for the question.

A probable answer is carbon dioxide.

[astronaut]

Thank you for your answer, II_billym, if concentrations were to make this this a reality, what would have to be done to either reverse or make right the damage? Thanks once again.