This review sorts common swine viruses by the distance over which airborne transmission has been demonstrated. Foot and mouth disease virus, PRRSV, influenza A, PEDV and pseudorabies virus are placed in the long distance category, with potential for transmission beyond 150 metres, which makes them inter farm threats. Classical swine fever virus and African swine fever virus are placed in the short distance category, from zero to 150 metres, which makes them intra farm threats.
The distinction is useful for framing what a filtration system is being asked to do. For the long distance group, the source of risk may be a neighbouring site, and the measured field evidence supports that framing: Dee and colleagues recovered infectious PRRSV at 4.7 kilometres in 2009, Otake and colleagues at 9.1 kilometres in 2010, and Alonso and colleagues detected PEDV RNA at 10 miles downwind in 2014. For the short distance group, the relevant geometry is the site itself and the movement of air between rooms and buildings. One caveat on corroboration: this review comes from the same research group as the 2023 ASFV aerosol case study by Li and colleagues, so the two should not be read as independent confirmation of each other.
For a producer, the practical reading is that the pathogen of concern determines what the boundary is. A herd whose principal exposure is PRRSV or influenza is defending a perimeter against air arriving from elsewhere. A herd concerned with ASFV or CSFV is managing air movement and contamination within its own site, where the pathways documented in the outbreak literature include dust, fomites and insects as well as air.