What you need to know
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.
What you need to do
Fires in woodworking extraction systems (Photographs courtesy of West Yorkshire Fire Service)
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.