epilog, as other manufacturers, specifically list polyurethane as laser safe.
[That would be alarming, if true. Fortunately, it's not. If you click the "full list" of recommended materials on Epilog's site,
https://www.epiloglaser.com/laser-engraving/ there's no mention of polyurethane, as I suspected - companies are generally reluctant to poison their customers.]
the isocyanates and ‘hydrogen cyanide gas’ are at lower levels than cutting or engraving wood.
[Where'd you pull this "fact" out of? The only wood products known to generate these chemicals when burned are those treated with polyurethane or isocyanate compounds. Or do you have a source for this stuff?]
It’s about as accurate as people wittering on about formaldehyde, when you get more from one apple than in a 300x600mm sheet of 3mm mdf
[You eat your MDF, I'll have the apple...]
Just listing chemicals without reference to the amounts and, more importantly, their constituency, is fear-mongering.
[So enlighten us - how much isocyanate comes from burning a gram of polyurethane rubber, versus a gram of pine? Who is your "constituency" for this disregard of people's health?]
msds sheets tell you what happens when you burn something in a self-combusting situation - i.e. light it, stand back. That’s not what happens in a laser. PU burns at 220-250C - the laser at the point of ablation is between 600-800C - none of the chemical processes are the same at that temperature
[So you think they're entirely safe, then, as long as they don't reach a certain temperature? What temperature is that, exactly? Can you give us an accounting of all the chemicals generated. including their relative quantities, when a laser burns through a piece of polyurethane at 700C, for instance?]
PU leather is fine to cut and engrave as long as you, the operator, is not breathing in the output. Any decent extraction system will make it safe, for you. Whether you want to vent everything untreated is down to you. I think that, at least, a water bath is advisable when cutting plastics.