Whether you opt for a hood or an over-the-range microwave oven, be sure it's installed according to the manufacturer's directions—typically, 18 to 30 inches above the burners or elements. That gives you working room while helping to prevent steam from escaping to the sides. Vent outdoors, if possible, using the largest-size solid, smooth-walled metal ducting that fits. Keep duct runs short and minimize bends to maximize airflow. A wall or roof cap outside the house prevents back drafts. Wash or replace the filters every one to three months—or more often if you cook frequently.