Don't "think". Someone like yourself who engages in such activity can hurt himself and others.
To answer your question, ketones are not harmful. In fact they're a great source of energy.
I don't know about that...Ketones can be oxidized into formaldehyde...this is why our bodies excrete it in urine and waste. By the way, if your body is so out of whack that you are producing ketones...you're probably in pretty bad shape.