I've had a good look on-line since my last post and there are lots of references to lasering polystyrene, mostly the hard stuff rather than the foamed. Most comments are about it not lasering very well and the edges rounding rather than noxious gases. I do have a good home-brewed extraction system to the outside through a hole in the house wall, uses the 3020 fan, a £70 in-line fan bought off e-bay, and a cooker hood (don't ask!). Very little smell in the room provided when I take material out of the machine I put it under the cooker hood for a few minutes until material has cooled and stoppped vapourising.
I suppose a good common-sense approach is 'if you can smell it then it's probably doing you harm'.
PS. Initally fitted cooker hood to extract fumes from airbrushing models.
LS3020 used for hobby - making model railway buildings/signals etc.