Originally Posted by MS Fowler

A word of warning about brake cleaner.
Do NOT ever use brake cleaner around heat. When heated, brake cleaner, unlike carb cleaner, will emit phosgene gas. Nasty stuff.
Spraying some on a hot engine may hit the exhaust and make just enough of that deadly gas to be bad news.
I read a safety tip from someone who used it instead of carb cleaner for a pre-welding cleaner. Some brake cleaner remained in one of the rust pits. When the guy hit it with the torch, it vaporized and he inhaled enough to nearly kill him.