Just how healthy (or unhealthy) is cabin air on aircraft? How does it compare to city buses, shopping centres and air terminals? Offices and homes?

Its cleaner!! Cabin air has tenfold fewer bacteria than city buses, shopping centres and air terminals. Studies have shown that air in homes and offices is exchanged only every 5-12 minutes. Air on a plane is exchanged every 3-4 minutes while flying.

The air coming into the plane, passes through jet engines (250 degrees celsius) on its way into the cabin so is essentially sterile. However, since cooling the air down takes power, rather than all fresh air all the time, 50% of air is recirculated. there are special systems for ensuring air flows directly from the top of the cabin to the floor further decreasing the likelihood of disease.

The most risky time for disease transmission is when the plane is 'stuck' on the tarmac with the engines off.