One wears a face mask for two reasons. Even a loose one protects others from aerosols you exhale, a good idea in general because you don’t know if you are shedding the virus but staying well (a carrier). Second, you wear a tight mask to keep from breathing invisible droplets from others speaking. If aerosol virus escapes from a evaporating droplet, it just hangs around in the air, especially in those African buses that rely on open windows for air flow but get stuck in traffic. A mask doesn’t particularly help for incoming aerosols — just for keeping you from spraying them as you speak.