MERV stands for "Minimum Efficiency Reporting Value". It is a measurement scale designed by the ASHRAE, the American Society of Heating, Refrigerating and Air-Conditioning Engineers. The independent scale allows for accuracy in measuring a filters ability to capture contaminates. This can help gauge needs for health and costs.
For example, according to "Residential Air Cleaners (2nd Edition): A Summary of Available Information", a HEPA filter is often impractical in central HVAC systems due to the large pressure drop the dense filter material causes. Experiments indicate that less obstructive, medium-efficiency filters of MERV 7 to 13 are almost as effective as true HEPA filters at removing allergens, with much lower associated system and operating costs. The table below shows various MERV ratings and their typical applications.