Air trajectories and virus concentrations were modelled for all 77 Iowa cases in the 2015 US outbreak of highly pathogenic avian influenza H5N2, an event that reached 232 farms across 15 states, cost over 50 million birds and 3.3 billion dollars, and put 65 percent of the national bird loss in Iowa alone. Modelled airborne virus concentrations never exceeded the minimal infective dose for poultry. Even so, under worst-case assumptions on shedding rate, emission rate and half-life, 33 Iowa cases carried greater than 10 percent infection probability and three exceeded 50 percent.
The result that matters most for design is a number. Filtering incoming air at 75 percent particulate reduction lowered risk but still left seven cases in the medium category. The authors state that inlet filtration would need at least 90 percent reduction efficiency to bring risk down to low or extremely low. That is a published performance target for poultry inlet filtration, and the 75 percent figure came from their own field evaluation at a commercial laying-hen house, so the measured performance falls short of the modelled requirement. Combining filtration with depopulation inside 24 hours brought every Iowa case to the extremely low category.
One finding is pure ventilation engineering. Turkey farms were markedly more vulnerable than layer farms, 57 percent against 32 percent in the medium-to-high risk categories, and the authors attribute part of that to building form: turkey barns admit air through curtains along open sidewalls with no impaction in the path, while enclosed egg-type barns draw through eave and attic inlets that impact incoming particles and reduce what reaches the birds. Inlet geometry is already doing partial filtration before any filter is fitted. Two cautions travel with all of this. It is dispersion modelling rather than confirmed transmission, and the authors' own downwind sampling failed to isolate virus across 104 samples, which they read as evidence that a negative air sample does not establish a safe aerial environment.