On my way to work, I often walk past a smoking area. While second-hand smoke can harm passers-by, cigarette smoke can at least be seen and smelled so the risk to others is apparent. Like tobacco smoke, the Covid-19 virus is also airborne: it is exhaled, floats around and can fill a room. But unlike smoke, its airborne particles are odourless and invisible.

The transmission mode of Covid-19 has been controversial. Some people still believe that the main routes of transmission are through touching contaminated objects that can carry infection such as doorknobs, or through coming into contact with mouth droplets.