Impinger and cyclone bioaerosol samplers were compared for detecting and measuring virus-containing particles in animal agriculture settings. Two results reach well beyond the sampler question. Infectious virus was detected in only 29 percent of the samples in which viral RNA was found, and only for particles 2.1 micron and larger. Separately, low flow rate samplers generally measured higher and probably more accurate airborne concentrations of both infectious virus and viral RNA than high flow samplers, even though the high flow samplers collected greater total quantities of material.
The isolation threshold is the same one reported in the 2015 chamber study by Alonso, Raynor, Davies and Torremorell, which found infectious influenza A and PRRSV in particles larger than 2.1 micron. That figure now has two published sources behind it from separate sampling campaigns, which is more than most numbers in this field can claim. It remains an Andersen cascade impactor stage cut point, so what it establishes is that infectious virus was not recovered below where the sampler divided, and the carrier particle argument rests on that rather than on a biologically derived boundary.
The 29 percent figure is the more uncomfortable one and it deserves to travel widely. Roughly seven in ten samples that returned viral genetic material yielded no recoverable live virus. Every RNA detection number in this literature should be read through it, including the field carrier size measurements taken during outbreaks and the recovery of viral RNA from spent barn filters. Detecting genetic material establishes that something arrived. It does not establish that what arrived could still infect an animal. The sampler finding carries a similar warning for anyone comparing concentration figures across studies: the number reported depends on the instrument that produced it, and a high flow sampler collecting more material can still report a lower concentration.