My experiment on how to chemically remove mill scale ...the cleanest way.

[Claudio HG]

LOL I had to search in Wikipedia for "within cooee". Nice. Yes HCl acid is pretty aggressive, that's why I suggested to stay outdoor, or at least have a good ventilation with a fan to extract fumes. But it also depends by the concentration of the acid. 33% does release fumes but way less than higher concentrations (say 66% which is pretty harder to manage). When etching PCBs I use an even lower concentration at 8.5%, with 5% of H2O2, working at 40°C.
Also the rate of fumes released is dependent on the exposed surface: the narrower the vessel that contains the acid the less fumes are released. Often people (including me) use a large tray to etch PCBs making easier for the acid to release fumes. A better solution would be to use a narrow (a two, three centimeters) and tall container, and to place the board to be etched vertically.