In this paper, the transmission characteristics of the spits excelled through coughing are examined in a calm environment. Three subject experiments were performed, and it shows that there are about 6.7mg spits expelled at a high speed up to 22m/s when coughing, and the spits can arrive further than 2m. Then the transmitting processes of the coughed spits of different diameters were analyzed using Lagrangian Equation. The results show that coughed spit's transmission process will be greatly influenced by the indoor airflow, and its transmission characteristics will change with its increase in size. The soils less than 30μm will be spread to the whole room by the airflow's movement, the spits between 50μm〜200μm will fall by gravity, and the spits greater than 300μm will become difficult to fall because of inertia. Moreover, it indicates that the couched spits probably cause droplet infection in close contacts of patients of infectious respiratory diseases.
