Harmful?

Well, it depends. There's a ozone generator I've heard of that's used for killing mold in basements. You would NOT want to be around when it's running. I've seen them used to kill paint smell in newly painted houses, too.

But even if you're running the hottest model out there, it doesn't actually work through walls. I know there's a company that claims their ionizer works with radio waves, but that's bogus. Radio waves just don't carry that much wattage.

Mine has a timer mode that will run full blast for an hour and then shut down.

It's a good idea, if yours doesn't have a timer, to run it with an extension cord from outside the room... that way you can unplug it without entering the room being treated. Open the door and walk away.