I would suspect it'd be the binder, urea formaldehyde resin. Having said that, I don't know if that particular chemical is what's released, or how it's affected by the heat of cutting with power tools.
For information on dust collection, I'd strongly suggest looking at Bill Pentz's excellent site:
http://billpentz.com/woodworking/cyclone/Index.cfm
If memory serves me right, he notes that many DCs are really just chip collectors, as the really dangerous (small) particles pass straight through the filters, and back into your workshop.
I work with MDF too, and I'm certain my DC is little more than a chip collector, so I always wear an MDF certified canister mask (and eye protection). You may end up looking like Darth Vader's brother, but you only get one pair of lungs.