The Minimum Efficiency Reporting Value (MERV) rating for an air filter is a measure of how well it prevents dust and other contaminants from entering the air stream. MERV ratings range from 1 to 16, with higher ratings trapping small particles more effectively than lower ratings. A more efficient filter will be better at removing particulates from the air as they enter the air conditioning system, improving the air quality in your home and keeping your lungs healthy. It's important to note that the relationship between filtration efficiency and air flow resistance is not linear. That is, as filtration efficiency increases, air flow resistance does not necessarily increase at the same rate.
For example, the Essential filter from Second Nature's is approximately 450% more effective at capturing particulate matter than a cheap, low-efficiency fiberglass filter, but its airflow resistance is only 20% higher, a marginal difference. MERV stands for Minimum Efficiency Reporting Value. In short, it tells you what size air particles an air filter will trap. A higher rating means that the filter can trap smaller air particles. The higher the MERV rating, the more efficient the filter will be at removing pollutants from the air.
For instance, a filter with a MERV rating of 8 will capture at least 70% of particles between 3.0 and 10 microns in size, while a filter with a MERV rating of 11 will capture at least 85% of the particles in that size range. Ironically, the effectiveness of low-efficiency air filters increases as the filter is filled with dirt and dust. During this short period of time you get a small benefit from your low-efficiency filter, but you are more than offset by the problems it creates. As air passes through a building's air conditioning system, air filters trap and collect large and small particles, such as dust, allergens, and microorganisms. HEPA filters are more efficient than filters with a high MERV content at capturing smaller particles, but they also have greater resistance to airflow. If you leave a clogged filter unchanged for a long time, that's when you can start to see problems with airflow resistance. The higher MERV rating means an air filter will capture the most dust, while the lower rating means a filter will capture the least amount of dust.
It is recommended to change filters in this category every two to four weeks, which may be difficult for some to maintain. The MERV scale is not linear; the difference between a MERV 6 and a MERV 8 is almost double in terms of percentage of particles captured. Although ratings don't exceed those of the MERV 16, high-efficiency particulate air (HEPA) filters trap the smallest particles at a faster rate than even the MERV 16 and are typically used in surgical operating rooms, clean rooms, and other contexts that require absolute cleaning. When used in a multi-filter system, prefilters trap dirt and large particles before they reach the final filters downstream which then remove the smallest particles. This helps ensure that your home's air quality remains healthy and clean.
