The interaction between ambient airflow and human thermal plume may affect airflow distribution in human micro-environment and then personal exposure. This study aims to investigate this interaction and to determine what dominates personal exposure: ambient airflow pattern or local human thermal plume. A computational fluid dynamics (CFD) method was employed in a downward ventilated room with a standing person below the supply grill. The research has taken into consideration the effects of room height, supply air opening area on the interaction. Results showed that the airflow was dominated by the downward jet from supply grill when the downward jet velocity was higher than high turning point of 0.275 m/s, but was dominated by the upward human thermal plume when the downward jet velocity was lower than low turning point of 0.075 m/s in the Case where room height is 2.7 m and supply air opening size is 0.6 m × 0.6 m. Dual steady flows were found when the downward jet velocity was between low and high turning point in the same case, indicating that the airflow pattern was determined by initialized airflow field with same boundary conditions. Besides, a higher downward jet velocity, larger air supply opening area and lower height were more likely to produce downward airflow pattern. This study may help to develop appropriate ventilation strategies to reduce personal exposure and save energy. Display omitted •Supply air velocity higher than high turning point downward jet dominates airflow.•Supply air velocity lower than low turning point thermal plume dominates airflow.•Supply air velocity between low and high turning points, dual solutions were found.•Both room height and supply area had significant impacts on multiple solutions.