The issue with trichloroethylene (TCE) brake cleaner is that it decomposes to phosgene under UV light from welding. In Washington, which is a pretty green state, we can still buy "chlorinated" brake cleaner as end users. It is a vapor degreaser, and is very useful in the shop. Environmentally, it is persistent in ground water as provided by thousands of dry cleaners across the country, all of whom have shuttered their doors long ago. On its own, it is separated from the bloodstream by the liver and kidneys relatively easily. I have seen shops with busy welding booths use TCE nearby without realizing its hazards. Phosgene does not play nice with the tissues that make up your lungs. It smells like fresh cut hay and feels like straight chlorine gas when you inhale it. Small amounts do damage, but you're probably not going to get a lethal lungful from welding. Just be aware.
