Dryer vents need to be cleaned regularly to prevent the accumulation of lint, which is a fire hazard. Clothes dryers are the top source of home fires, and the likeliest places for a fire to break out are in the dryer vent and the lint trap.
Cleaning out the lint trap at every use of the dryer helps, but lint still accumulates in the dryer vent hose. Signs that lint is building up to potentially dangerous levels include longer drying times for clothes, dampness of clothes after going through a full dryer cycle, higher than normal humidity in the laundry room and a burnt smell in the laundry room. Use brushes specifically designed for dryer vent cleaning to loosen lint from the dryer vent hose and clean each section of the vent hose separately, if possible.