Furnace filters can clean indoor air by trapping dust, pollen, mold spores, pet dander, and other indoor air contaminants. Air filter efficiency is defined in MERVs, or “Minimum Efficiency Reporting Value.” A typical 1-inch-thick spun fiberglass furnace filter has a rating of 1 to 4 on the MERV scale and can trap only large particles like sawdust, carpet fibers, dust mites, and pollen. High capture filters like those specified for Indoor airPLUS Certified Homes must be at least MERV 8. These filters can remove most cement dust, mold spores, animal dander, hair spray, and even smaller particles from air. Even higher MERV levels are available including HEPA filters with MERV ratings from 17 to 20 that can filter out particles smaller than 0.3 microns, including viruses. However, higher MERV filters restrict airflow. As a result, heating and cooling system air handlers need to be designed to for a filter size that meets air flow requirements.