As the US swine industry moves towards regional control and elimination of PRRSV, a critical component is the ability to reduce the risk of the airborne spread of the virus between herds. Therefore,the filtering of incoming air to has been proposed as a means to reduce this risk. To test this intervention, a study was conducted utilizing 10 treatment (filtered) herds and 26 (non-filtered) control herds over a 24-month period involving large breeding herds in swine dense regions. Throughout the study period eight of the treatment herds remained free of infection; however, two herds experienced clinical PRRS secondary to the introduction of a new variant of the virus from an external source determined to be contaminated transport in one case and a personnel biosecurity breach in the other. In contrast, 24 of 26 (92 percent) of control herds experienced severe clinical episodes of PRRS secondary to the introduction of new variants. These results indicate that air filtration is an effective means to reduce the risk of external PRRSV introduction to large breeding herds located in swine dense regions.