Have you ever stood in the air conditioning filter isle at your local Lowe’s or Home Depot store wondering why some filters are priced around two dollars per unit while some are unbelievably twenty something dollars each?  Besides the packaging with some being eye-catching while others are generic looking, most filters – whether they’re at the low end of the spectrum or at the high end – all look the same.  They may vary in sizes to accommodate the type of unit you have in your home.  The inexpensive ones are mostly made of fiberglass; therefore cheaper, while pleated filters and permanent filters - for example the electrostatic filters - are at the high end of the spectrum.

One commonality among the filters is the MERV rating.  MERV stands for minimum efficiency reporting value, a standard developed by the National Air Filtration Association (NAFA), a group consisting of over 600 air filter distributors, manufacturers, and engineers.  This contaminant removal standard is prescribed by ANSI-ASHRAE 52.2/2007 “Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size”, which is very important to the IEQ Credit 3.1 and 3.2 under the NC LEED Rating System. 

But let’s put aside all the acronyms for now and look at how MERV can help us determine which air filter we should buy.  MERV allows end-users (consumers) to select the appropriate filters for their a/c unit based on efficiency and the removal of contaminants from the air.  Keep in mind that both factors - efficiency and removing particulates - is the key to having an effective air conditioning filter.  Air conditioning and heating systems operate best when there is little restriction in air flow.  When adding an air filter to the system it restricts that air flow which decreases the system’s efficiency.  But without the filter, air-born allergens and other contaminants would easily flow into the breathing areas of your home or office.  The best filters have the least restriction on air flow and remove the most particles.

MERV ratings range from 1 – 16 and are measured in microns (μm).  Pet dander, dust, pollen, and tobacco smoke are some of the more common household particles, while mold spores, bacteria, and even viruses can also be flowing through your vents if not filtered properly.  Most of the residential filters have a MERV rating of 1 to 4, which is not effective enough to capture particles less than 10 microns. The word micron derived from micrometer, which is a metric unit of distance equal to one millionth of a meter.

MERV 5 to 8 filters are found in many commercial buildings.  They capture particles as small as 3 microns.  MERV 9 to 12 filters are found mostly in industrial buildings and commercial buildings requiring more efficiency and effectiveness due to higher volume of particulates generated by manufacturing of products, higher number of occupants, and/or number of heavy equipments being used. The most efficient filters have a MERV rating of 13 to 16 stopping particles as small as .3 microns from entering in your breathable areas.  These filters can be found in hospitals where the buildings are most prone to bacteria and viruses.

Next time you buy a filter for your home, consider the information from the following table.  This is NAFA User's Guide for ANSI/ASHRAE Standard 52.2 - 2007.