Before you walked away from the dryer with your bundle of warm T-shirts, did you clean all the fuzzy fibers off of the lint screen? Sure, sometimes it might just slip your mind, but leaving your lint behind is rude. No one wants to clean up your old fuzz. Aside from social graces, though, there are also practical reasons for cleaning the lint screen after each completed dryer load: dryer efficiency and reducing the risk of starting a laundry-related fire.
A clogged lint filter will make a dryer work harder as it tries to exhaust hot, moist air from the dryer drum to the outside. Over long periods of time, it's also possible that fabric softener residue may build up on the screen and reduce the air flow through the vent. Any time the lint screen is clogged, the dryer's efficiency drops.
While you may be quick to blame the bad luck of a laundry-related fire on an electrical problem, it's actually human failure to clean out the lint that's causing the most combustions [source: NFPA]. Ninety-two percent of laundry-related fires are caused by clothes dryers. It's those fires that spark in commercial spaces, such as laundromats, that typically have more reported injuries than those in residential buildings [sources: NFPA, West Bend, FEMA]. Lint, it turns out, is a highly flammable combination of fuzzy fibers from fabrics, such as cotton, and also hair (and fur) and any other dirt and debris that hitched a ride on your clothes. The dryer will run longer — sometimes more than an hour — to get the job done, and in doing so will generate a lot of heat for a considerable length of time. Too much heat over too much time may ignite too much lint.