Resuspension of accumulated dust inside ventilation ducts is a serious problem in HVAC operation. Many countries have hygienic standards regarding how much dust is tolerable in ventilation ducts. The standards, however, diverge from each other. In this study, we established a quantitative method to evaluate the health impact brought by in-duct resuspension. A mass balance model was developed to incorporate the particle deposition and resuspension in both ventilation ducts and indoor environment, by which the resuspended particles from ventilation ducts could be evaluated. The quantity of the attached bacteria on the resuspended particles was estimated and LD QRMA model was employed in the health-concerning assessment. Suggestions for building management are provided.