By the time you see that fine layer of dust on top of your host, you can be certain at least 3 times that amount of dust has been sucked in thru the vents and holes outside, settling into its internal components that are used to run the host. It will definitely become dangerous -- causing your host to run at high temperatures, and by then, the damage will be done. Overheating is the actual "cause of death" in most electronics and computer items.