A Production Animal Disease Risk Assessment Program analysis was conducted in Canadian barns to determine causes of PRRSv contamination in barns with an air filtration system, and the corrections needed to avoid future re-occurrences. Literature and correspondences with American experts allowed Canadian and American results to be compared. The analysis found there that the contamination could come from aerosols, improperly installed or maintained air filters, introducing contaminated gilts or using contaminated semen, or biosecurity breaches by various personnel. The Canadian barns analysed had a higher rate of contamination than the American barns, which could be explained by the use of older antimicrobial filters, lateral air inlets, age of air filters, proximity of surrounding farms, or size of farms. Recommendations to improve biosecurity include increasing air tightness, reducing biosecurity breaches by personnel, updating air filters and changing to positive-pressure, and conducting regular audits on biosecurity.