Absolute rating is a measure of how efficient the filter element is at removing particles (eg. dust, pollen, etc.) from the air stream. It is typically measured in microns. It is determined by an industry standard test based on a Variable Air Volume (VAV) system and reported in the filter element's initial efficiency.
Normal rating is a calculated design value that quantifies the filter element's performance under normal conditions. It is determined by taking the absolute rating and factoring in environmental variables such as humidity, temperature, and air velocity. Normal rating is typically reported in MERV (Minimum Efficiency Reporting Value), which is a number between 1 and 16. Higher MERV numbers indicate more effective filtration.
