Safety of cutting Polyurethane varnished wood.

I currently have a business where I engrave wood cases imported from China. The factory have told me they use a polyurethane varnish on the wood. Does anyone have any information on if it is safe to engrave this wood that has been prevarnished. I have read some material about PU varnish containing isocyanates that are potentially cancerous. Does anyone know more about this and whether it is safe to engrave?

The 60W machine is in a garage with the door open and has a ventilation pipe running outside as far as possible, I wear a 3M 4279 mask, and I have a large gym fan blowing towards the door. My concern is although I have all these precautions in place, I can smell the wood when I open the machine probably from the dust that has settled.

Let me know your thoughts on whether this is safe ?

Many Thanks
Tom