As far as I can tell, chlorine is the easiest and quickly dangerous mistake to make. There are other concerning materials I’ve run across, but most of these, the debate isn’t whether the substance is dangerous, but in what quantity it is produced.
Basically all of the carbon materials we burn produce some amount of carbon monoxide and hydrogen cyanide. The HCN is 25 times more deadly than CO and at some concentrations can kill you in sixty seconds. But that’s the thing, both of them it’s the dose that makes the poison.
Polyurethane apparently produces HCN at a much higher rate than acrylic and wood when burned especially with low oxygen availability. But the question I haven’t figured out is whether the amount you normally burn during a laser cut is enough. But seems scary enough, I’m gonna wait until somebody measures it.
Chrome tanned leather can release chromium gas. Hexavalent chromium is particularly dangerous for you and the environment since it persists. Here again, the thing that isn’t clear is how much and how dangerous, but seems better to just avoid. The bummer is that this takes recycling leather goods off the table.