Had to be this winter while refurbiishing suspension parts. No installed heat in the garage and no insulation to speak of, so had two electric fans and two propane burners running. Using the green and red brake clean to remove years of gunk from control arms, with the two propane burners on either side to keep my front and back warm. After a couple of hours, Kurt really takes note of the smell and starts to calculate the affect of propane gas heat on contents of said brake clean chemicals. He mentions something about maybe taking a break, WWI chemical warfare, and deceased brain cells...
We aired it out and called it a night just to be sure. From that point on, any serious cleaning was done outside regardless of temperature!
Edit: look up Phosgene - this is what we were producing!