Argonite was a substance. Seventeen parts per million was within safety margins for Deep Space 9's air supply.
In 2373, Julian Bashir told Benjamin Sisko he was concerned with how much argonite was seeping into the station's air supply. Though Sisko reminded him that it was still at safe levels, Bashir noted that it continued to rise. (DS9: "Doctor Bashir, I Presume")