Chimney sweeping with a long metal-bristled chimney brush is done to clean out the inside of a chimney to remove anything which could cause a chimney fire (for example: any built-up creosote, soot or ash from the burning of wood in your fireplace.) As well as being flammable, creosote is toxic to breathe. A long-unused chimney flue should be swept to remove any bird/squirrel nests, spider webs or moss within.
Chimney Sweeping