Under the context of the COVID-19 pandemic contingency, several economic activities have to undergo difficult days. Public transportation must reduce the level of occupation, which affects the economical feasibility of the activity. To increase the occupation a bio-security protocol must be applied and some actions were taken. Users use masks to reduce the droplets from talking and coughing activities, and there were everyday cleaning routines to reduce the risk of contaminated surfaces. However, aerosols' contagious vector starts to be important when there is a space occupied by people under tight proximity. To reduce the contagious risk, the space must be ventilated to maintain low aerosols concentration. This paper present a methodology that evaluates the ventilation system of several vehicles of public transport, we used Computational Fluid Dynamics (CFD) and tracer experiment to calculate the Residence Time Distribution (RTD) to analyze the air conditioning system as a way to reduce the concentration of aerosols and as a result, reduce the contagious risk of airborne disease in high occupancy spaces. The evaluated ventilation systems present different kinds of characteristics. It is important to improve the design of the air conditioning in public transport services to reduce the contagious risk. We show some ventilation designs that have good air renovation time, concluding that they are desirable to prevent aerosols as a contagious vector.