Typically, air filters are rated by the size of the particles they can remove. Most good furnace filters can easily remove particles larger than 10 microns in size, but the best filters are able to remove particles smaller than 3 microns. One micron is one millionth of a meter or approximately 1/25,000 of an inch. For comparison, a human hair is about 70 microns thick.
Most pleated filters have a performance rating that is assigned a number to indicate the level of effectiveness. Generally the higher this number is, the higher the performance of the filter.
Different manufacturers use different performance ratings but 3M's MPR rating is emerging as the industry standard. MPR stands for Micro-particle Performance Rating. The 3M MPR rating focuses on the smallest and most troublesome particles - those between 0.3 and 1.0 Microns in size. Generally speaking, a filter with an MPR of 1000 is twice as effective at removing those tiny particles as a filter with a 500 MPR.
The flat panel fiberglass filter, such as this one,
is the lowest cost filter available under $1.50 retail each. This is the least effective filter in terms of particle filtration, but is good for protecting equipment if changed regularly. Flat panel filters are designed to catch only larger particles, allowing smaller particles to pass through. Flat panel filters have no performance rating due to their low efficiency. The recommendation is to replace flat panel filters every 30 days.
The second type of filter is the basic pleated filter
like this 3M 300 Dust Reducing filter
. Compared to flat panel filters, the pleated filter is 5 times more effective in catching the airborne particles than flat panel filters allow to pass through. The retail for a basic pleated filter is under $5.00 each. Because they catch smaller particles, they are more efficient than fiberglass filters. They reduce energy costs by keeping your unit cleaner, as well as keeping your air quality at a higher standard. Basic pleated filters, based on testing of residential systems, need to be replaced every 30 days. The MPR rating on basic pleated filters ranges from 300 to 500.
Electrostatic filters have media that is charged with electric current during the manufacturing process, allowing the filter to attract smaller particles; much like a magnet attracts metal. 3M electrostatic filters have both positive & negative charges to attract dust no matter which charge it holds. Electrostatic filters are up to 18 times more effective than flat panel filters and have MPR rating of 600 to 2200. The average retail on this type of filter is from $7.86 to $ 10.96 each. These filters are designed to be replaced every 90 days. Considering the lifespan of these electrostatic filters, they not only do a better job of cleaning the air in your home but the cost less on a monthly basis because they don’t have to be changed as often.