Technical Abstract: Endotoxins are derived from Gram-negative bacteria and are a potential respiratory health risk for animals and humans. To determine the potential for endotoxin transport from a large open lot dairy, airborne endotoxin concentrations were determined at an upwind location (background) and five downwind locations on three separate days. The downwind locations consisted of the edge of the lot, 200 and 1,390 m downwind from the lot, and downwind from a manure composting area and wastewater holding pond. When the wind was predominantly from the west, the average endotoxin concentration at the upwind location was 24 endotoxin units (EU)/ cubic meter, while at the edge of the lot on the downwind side it was 338 EU/cubic meter. At 200 and 1,390 m downwind from the edge of the lot, the average endotoxin concentrations were 168 and 49 EU/cubic meter, respectively. Airborne endotoxin concentrations downwind from the composting site and wastewater holding pond, and 1,390 m from the edge of the lot, were not significantly different than the upwind location. In addition, there were no significant correlations between ambient weather data collected and endotoxin concentrations over the experimental period. The downwind data show that the airborne endotoxin concentrations decreased exponentially with distance from the lot edge. Decreasing an individual¿s proximity to the dairy should lower their risk of airborne endotoxin exposure and associated health effects.