The use of air filtration to prevent PRRSv contamination has been supported by a pilot study, but this trial looked at a larger scale trial. Ten treatment herds with either a MERV 16 or MERV 14 filter,  and 30 control herds with no filter were assessed monthly for 36 months. 28 out of the 30 control herds and 3 out of the 10 treatment herds were infected with PRRSv during the trial. Of the three treatment herds one used MERV 14 and two MERV 16 filters. The source of contamination for the three facilities was thought to be a contaminated transport truck, multiple biosecurity breaches, and an improperly installed fan protection. The results still show air filtration significantly reduced the risk of PRRSv infection in a herd.