As a semi-enclosed space, mastering the transmission mechanism of aerosols in laboratories is crucial for understanding the spread of respiratory infectious diseases. This study uses Fluent software to simulate the impact of different outdoor air conditions on the particle distribution in the human breathing zone within laboratories, and employs a modified Wells-Riley equation to estimate the infection probability of exposed individuals. The simulation results show that when the horizontal outdoor air temperature reaches 28 ℃, the high-temperature outdoor air flows over the human body, effectively avoiding interference with the breathing zone, and the infection probability from the infected person to the exposed individual is 11.7% at this time. When the outdoor air temperature is 24 ℃, the interaction between the outdoor air and the exhaled airflow of the infected person causes trajectory deviation, and the infection probability of the exposed individuals decreases to 10.6%. When the outdoor air temperature is 22 ℃, the lower-temperature outdoor air causes the exhaled airflow of the infected person to flow toward the floor, thus the infection probability of the exposed individuals is further reduced to 9.8%. By investigating the conditions of rectangular and circular outdoor air outlets, it is found that although the air velocity of the circular outdoor air outlet is higher, its ability to penetrate the human thermal boundary layer is weaker, with most of the airflow bypassing the human body, resulting in little impact on the breathing zone and low disturbance degree. Comparing the mixed ventilation with upper supply and lower return and horizontal ventilation under the same temperature, the former is prone to forming air eddies in the breathing zone, increasing the infection risk. The latter forms a V-shaped semi-enclosed area, preventing complete closure of the airflow space and reducing the infection risk.