The Nordic Pure MERV 8 anti-allergen pleated air filters are made out of an electrostatically charged synthetic material that draws airborne particulates to it like a magnet. Electrostatic filters help trap airborne particles to protect your family from pollen, household dust, mold spores, and dust mites. with each pass through the pleated filter, the air in your living environment improves. Nordic Pures synthetic filter media will not permit the growth of bacteria or mold, therefore, making your environment healthier. This air filter will capture other large particle contaminants to keep surfaces free of dust longer in your home. MERV 8 air filters are comparable to air filters with the MPR rating of 500-600 or FPR rating of Green. Particles in indoor air are measured in microns, which are 1/1000 of a millimeter. A human hair is about 70 microns wide. A MERV 8 Nordic Pure air filter captures airborne particles as small as 3.0 to 10.0 microns. This high performance air filter will outperform non-electrostatic fiber-glass filters, washable filters, and poly disposable filters. Nordic Pure air filters have an actual size that is a fraction of an-Inch smaller than the nominal size. with the 4 and 5 air filters there is a wide variance between the nominal size and actual size. Almost ALL filters have a different actual size than the nominal size printed on the filter so it is best if you measure your filter for the EXACT size and compare it to our "Actual Size. This is why we ask you to measure your filter so you aren't surprised if the filter doesn't fit! Our MERV 8 four or five-Inch air filter should be changed at least every 6 months depending on your environment. Changing your air filter at the recommended time will help maintain a healthy environment as well as keeping your air conditioning and heating systems running efficiently.