Many years ago, boric acid was a standard component of eye wash compounds sold over the counter. Some wise guy decided that since boric acid kills cockroaches, it surely must be a toxic liability, so we no longer have the best eye wash available on the market anymore. Yes, Boric acid, Borax, and boron compounds can be poisonous.
*Regarding burning borax in a fire, Chatbot says this:
"Borax fumes: When heated, borax can release boron compounds into the air. Inhalation of high concentrations of boron compounds can potentially cause respiratory irritation and other health issues. However, in typical household uses, the small amount of borax added to a wood fire is unlikely to produce harmful levels of boron fumes."
*Propylene glycol costs too much to spray on firewood, anyway.