MERV stands for Minimum Efficiency Reporting Value - and is a scale measuring the effectiveness of air fliters. The MERV rating scores filters in HVAC (heating, ventilating and air conditioning) systems from 1 to 16 - the higher the number, the more efficient the filter.
Because the material used to make a true HEPA filter needs to be more dense than that of a less efficient filter, true HEPA filters are not normally installed in HVAC systems. A true HEPA filter in such a system would cause too great a resistance to airflow - but according to the EPA, filters with a MERV rating of 7 to 13 are almost as efficient at removing small particles from the air. Standalone HEPA filter units are often less expensive to run continuously than an HVAC system.
"True HEPA" filters have a MERV rating of 17-20.