In a vehicle moving at a speed <30 km/hr with windows open and AC off, the cough-jet from an infected passenger in the back seat produces an aerosol plume that initially heads towards the front. It returns along the airflow, quickly spreading throughout the entire confined space. At higher speeds, droplets do not travel far while aerosol moves far and leaves the car via rear windows.

Model #2: Aerosol and Droplet Behavior in Car Interior Space

In a vehicle moving at a "moderate" speed with AC on and windows closed, the cough-jet from an infected person in the back seat can form droplets and a plume of aerosols (avg speed: 10 m/s; ACH < 60 per hr; temp: <25 C; relative humidity < 50%). The plume of aerosols first spread towards the front and turned backwards as it got carried away by the AC streamline. This increases the risk of infecting everyone in the car.

Model #1: Aerosol and Droplet Behavior in Car Interior Space

- Model #1

- Model #2

(The numbering does not indicate prioritization)

Smith College

- cabin of airplane

- interior space of car

- dormitory-type space of healthcare or isolation center

Effect of Confined Space on SARS-CoV-2-laden Aerosol and Droplet Behavior

The air exchange rate depends heavily on vehicle speed, ventilation setting, window positions mainly control the air exchange rate (ACH)


Parked Car:

- AC off: 0.42 – 1.09 per hour

- AC on:  1.96-3.23 per hour

- AC off and fans on: 8.7-10.7 per hour

- windows closed and no mechanical ventilation: 1.0 – 3.0 per hour

- ventilation set on recirculation: 1.8-3.7 per hour

- windows closed and fan set on fresh air: 13.3-26.1 per hour

- windows open and no mechanical ventilation: 36.2-47.5 per hour

Moving Car (28 km/hr):

- window open and no ventilation: 71 per hour

- window closed and ventilation on: 60 per hour

- windows closed and no ventilation: 4.9 per hour




Aerodynamics in Car Interior Space

A cough expelled at 10 m/s can cause droplets to travel four to five seats ahead and one seat behind. This experiment predicts that 5-10 people could get infected due to droplet spread with an infected person on board. Aerosol  plumes may affect far more but the extent to which aerosol plumes can infect susceptible hosts is still poorly understood. 

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