Keeping an outside dryer vent clean is essential to dryer efficiency and to prevent moisture problems. Most outside dryer vents have slats or "doors" that are forced open when the dryer is running to allow moisture and lint to escape. If these become clogged with lint, the moisture will remain in the dryer hose and make your dryer work harder or allow mold and mildew to grow.
Check the vent while the dryer is running to make sure the slats are opening freely. If not, turn off the dryer and open the vent manually. Use a long, thin brush to remove the clogging lint.
Regular cleaning is especially important in humid climates where moisture can cause lint to cling to the vent.