Modeling airborne virus concentrations in filtered swine barns with negative-pressure ventilating systems

Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically significant pathogen in the swine industry that can spread through the air. Many swine gestation and farrowing barns with negative-pressure ventilating systems filter the inlet air to manage airborne PRRSV transmission using MERV 8 pre-filters in series with either MERV 14 or MERV 16 filters. Recent research reported air infiltration rates for a new 3,000-sow gestation/farrowing swine barn at several static pressure levels. The barn infiltration data and supplier-provided airflow versus pressure drop data for the filters, a fan, and an evaporative cooling pad were used to model steady-state virus particle concentrations inside a well-mixed barn. Other model inputs included the inside temperature, design ventilating rate, a fan performance factor, filter area, a filter airflow reduction factor due to particulate matter accumulation, and ambient virus particle concentration distributions. For the conditions used, model results indicated that higher barn virus concentrations were obtained with lower mechanical ventilating rates and higher barn infiltration rates. Improved fan performance reduced the number of fans needed but had little impact on barn virus concentrations. Increasing the filter area reduced the pressure drop that the fans had to overcome at higher ventilating rates and correspondingly reduced the unfiltered infiltration rates and barn virus concentrations. Reduced airflow due to particulate matter accumulation on the filters increased the system pressure drop, increased the number of fans running, and increased the barn virus concentrations. Model results indicated that filter combinations that reduced overall virus penetration reduced barn virus concentrations by 57% to 80% for the conditions modeled. More work is needed to assess the model results and the importance of the adjusted factors for other barn and equipment conditions.