Wood dust can cause fire and explosion. The minimum explosible concentration for "wood flour" is 60g/m3 and a dust cloud at this concentration would resemble a very dense fog.
Many fires start in the extraction system, so it should be constructed such that sources of ignition, fire and explosion are removed. Consideration should also be given to the location of the fan and air cleaner.
Information sheets & Circular saw poster
Approved Code of Practice and Guidance (ACOP)
DVD: Health and safety in the woodworking industry
BS EN’s for woodworking machines