Postoperative sepsis is a costly and potentially devastating problem in total joint arthroplasty. Airborne bacteria and other viable microorganisms shed from surgical staff are a source of deep prosthetic infection, and the density of airborne bacteria is correlated with the rate of postoperative joint sepsis in total joint arthroplasty surgery. Previous studies have also reported a positive relationship between the density of nonviable airborne particulate and viable CFU counts, both airborne and in the surgical wound, during surgery. The purpose of this study was to determine the extent to which a system that delivers a small field of local, directed HEPA-filtered air flow over the surgical field reduces airborne particulate and airborne bacteria during total hip arthroplasty. A minimum of 8 subjects per group provided 80% power (a = 0.05) to detect a =75% difference in bacterial density between groups. All patients who consented to undergo primary total hip arthroplasty were eligible. Thirty-six patients were prospectively randomized into three groups: directed air flow, air flow system present but turned off (sham), and control (standard) conditions. Airborne particulate and bacteria were continuously collected in consecutive 10 minute intervals within 5 cm of the surgical wound using an air sampling device. Data were analyzed using a generalized linear model for repeated measures. Particulate counts and bacterial density at the surgical site were 80% lower on average in the directed air flow group compared to the other two groups (p<0.001) (Figure 1). Density of particulate >10 μm in diameter was strongly related to bacterial density at the surgical site (p<0.001), as was staff count (p<0.001) and bacterial density at a control site that was remote from the surgical field (p<0.001). The directed air flow system's effectiveness in reducing bacteria appears to be related to its ability to reduce particulate that may carry and allow proliferation of bacteria. The directed air flow system is relatively simple to use and does not appear to hinder the function of the surgeon or operating room staff, impede access to the surgical site, or interfere with the surgical procedure. The directed air flow system can be used in any operating room environment to provide clean air equivalent to a properly-used, well-functioning laminar air flow system. The directed air flow system was effective in reducing airborne particulate and bacteria in the surgical field during total hip arthroplasty.