The initial efficiency of a filter is the level of efficiency that the filter has when one first installs it. The efficiency is going to consider the cost of running and using the filter, as well as the amount of particulates it is able to keep out of the system. Over time, based on the use of the filter, you will find that the initial efficiency is not going to be static. The filter’s efficiency is likely to change and drop over time, which could necessitate a filter change. Even the best filters will need changing occasionally.