Dryers evaporate moisture from wet clothes and accumulates lint build up blowing high temperatures of heat from the dryer. The moisture and lint is expelled through the dryer vent, passed through the dryer duct and exhausted outside of the home.  Because of the need for a consistent airflow, dryer ducts accumulate lint build up and the moisture sticks to the ducts creating a layer of build up. So do not put the airflow at risk. Get it cleaned right away.