Non-pleated panel air filters are installed in HVAC systems to capture contaminants in the air stream. This improves indoor air quality and prevents contaminants from building up in the equipment and damaging it. These flat air filters are commonly used to capture large particles between 3 and 10 microns in size, such as cement dust. Non-pleated panel polyester air filters or fiberglass air filters are commonly used in furnaces and central air systems in residential and commercial buildings. They are designed to be discarded when they are spent. A filter's MERV rating measures how efficiently it removes contaminants from the air stream based on the size of the particles it can effectively capture. The higher the MERV rating is, the more effectively the filter traps small particles.