Not all air filters need to be or should be HEPA or ULPA. In commercial HVAC design a HEPA filter would provide too much resistance to air flow and produce a large pressure drop in the HVAC system. The MERV rating system developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers – ASHRAE – categorizes air filters by the size of particles captured at ~95% efficiency; the particle range is from 0.1 to 10µm (micrometers). This rating is called Minimum Efficiency Reporting Value and represents a worst case performance of the air filter. A MERV rating of 14 captures 90-95% of cigarette smoke particles (~0.2µm) and MERV 8 filters used in many commercial buildings capture 90% of particles from 3 – 10µm. Medium efficiency filters – MERV 7-13 – capture most allergens while providing for efficient air flow. The MERV rating is only used to rate filters used in whole house systems or air handling in large buildings and does not apply to small room air cleaners. The MERV rating does not give information as to a filters ability to remove gases.