MERV stands for Minimum Efficiency Reporting Value. It is a measurement scale that measures a filter's capacity to remove bigger particles with a diameter of between 0.3 and 10 microns (m). The rating was obtained from a test procedure designed by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) in 1987. The grade of minimum efficiency reporting value is determined by subjecting a controlled testing environment to introducing particles ranging in size from 1 to 12. To calculate the minimum efficiency reporting value, a laser particle counter takes readings of the air both before it enters the test filter and after it exits the filter.