he'd have to permanant the phosphorus transfiguration anyways, though, since he doesn't want any bits of inhaled smoke to turn into wood inside people's cells.

This would most likely require the Philosopher’s Stone to be in contact with the transfigured matter for several minutes (see chapter 111), which is impossible:

"Professor," Harry said, "if the worst happens in a case like that, is there any way of maintaining the Transfiguration?"

"No," Professor McGonagall said flatly. "Sustaining a Transfiguration is a constant drain on your magic which scales with the size of the target form. And you would need to recontact the target every few hours, which is, in a case like this, impossible. Disasters like this are unrecoverable! "

(chapter 15; emphasis mine)