There are many different filters being produced by many companies around the world. This can make the decision to choose just one difficult. Scott Dee has written in this Pigletter about some of the filters he prefers. The scale for filters is called a MERV (minimum efficiency reported value)  which is a measure of how effective a filter is. The higher the MERV ranking the better the filter is, most filters used in research and in commercial barns are MERV 16, which is capable of stopping all particles over 0.3 microns in size. Although the PRRS particles are 0.05 microns, they travel on larger particles of dust or liquid that are usually 0.5-100  microns in size, making a MERV 16 filter effective. Dee has found through his research that a filter called Camfill Farr by the company Filtration Systems Inc. out performs any other filters from North America and the world. One drawback is that they can be expensive and may impede air flow if they are not a good fit with a barns current air flow system. Another suggestion by Dee is filters made by a company called Noveko. The unique thing about these filters is that they are coated with chemicals so that when contaminated particles move through the system they are killed. Understanding of filters is important for ridding a barn of PRRS, but sanitation should not be overlooked at all levels of production.