USDA officials recently announced that “dairy dust” is not likely to pose a health hazard to people who live or work near a dairy farm. Research has indicated that the dust stirred up by wind and restless cattle at dairies does contain bacteria, fungi and small bacterial remnants such as endotoxins. But these potentially problematic particles are not found at high levels far beyond the barnyard.

Agricultural Research Service microbiologist Rob Dungan investigated dispersal patterns and transport of these bioaerosols. In one study, which has been published in a peer-reviewed academic journal, Dungan and his colleagues set up three sampling sites at a 10,000-cow open-freestall dairy to measure airborne endotoxins and culturable microorganisms like bacteria and fungi during fall, spring and summer. The researchers found that overall average inhalable airborne endotoxin concentrations were 5 endotoxin units (EU) per cubic meter of air 655 feet upwind of the barn—their "background" levels"—and 426 and 56 EU per cubic meter of air 165 and 655 feet downwind of the barn, respectively.

Close to the barn, endotoxin concentrations at night were significantly higher than morning concentrations and similar to afternoon concentrations. The scientists attribute the higher levels to increased animal activity and lower windspeeds during these times. But at the other two sites, endotoxin concentrations did not vary significantly over 24 hours. Samples of bacterial concentrations showed a similar pattern, with the highest counts—84,000 colonies per cubic meter of air—measured near the barn. The other two sites had less than 8,000 colonies per cubic meter of air. As with the daily endotoxin concentrations, bacterial concentrations near the barn increased significantly at night, but concentrations farther downwind did not.