How I would define airborne: ........My personal opinion ....... The number of seconds/minutes/hours the aerosolized particle/virus can stay suspended in air: It could be dependent on the air: humidity, temperature, air pressure, oxygen content, co2 content, air speed, closed vs open spaces, and other factors. Hence the chance that someone could inhale X number of particles could be dependent on the time it takes (t) for the particle to reach the ground. It also depends on the size and density of the particle/aerosolized viral particles. Covid19 becomes suspended in air for longer times in closed spaces due to lack of the wind factor when compared to outside air quality.