If the unit is considerably dusty this can be very dangerous. Most furnaces run in a negative air pressure in order to pull in combustion air. There are no filters to filter this air. So any lint or dirt that is in the air, is free to come into the furnace and be collected within the furnace. This lint is flammable. If there was ever a roll-out flame, it could ignite the lint and cause a fire.

As it is almost effortless to clean the lint from the furnace, you will want to to remove this fire hazard from your home. Using a damp rag and a vacuum cleaner, remove all visible lint from inside the furnace when the furnace is off.