Contentious subject, cabin air. While some studies suggest it is no more germ-laden than the air in, say, a large office, other research has found it to contain toxins linked to cancer, chronic fatigue and neurological problems.

Such studies are not as contradictory as they seem; it is, for example, possible to catch the flu on a flight without breathing in toxins – and vice versa.

To understand why is to understand where the air you’re breathing at 35,000 feet comes from; half is “recycled” from the cabin and half is “fresh air” drawn in from the engines (all of which is pushed through filters to remove any microbes and toxins).

Cabin air is replaced every two or three minutes, according to Boeing Credit: GETTY

There is some debate as to how effective those filters have been; according to the World Health Organisation (WHO), in 2015 more than 3.5 billion passengers and 500,000 pilots and cabin crew were exposed to low levels of engine oils in the air. Plane manufacturers, however, maintain cabin air is safe to breathe.

Either way, researchers claim the best way to avoiding catching a cold from a fellow passenger is to keep the air vent above your seat open and allow yourself to be buffeted by filtered air.

“For airborne viruses, it is incredibly important to ventilate, since ventilation becomes your main means of control besides isolating the affected person,” Dr. Mark Gendreau, an expert on the spread of infectious diseases at the Lahey Medical Center-Peabody, told Travel + Leisure Magazine.

Boeing suggests this isn’t entirely necessary. It claims that between 94 and 99.9 per cent of airborne microbes are captured by a plane’s air filters and that the cabin air is completely refreshed every two or three minutes – far more frequently than in an office.

Perhaps the biggest risk of contracting airborne diseases on a plane, however, is when a flight is delayed on the ground and the ventilation systems are turned off.

“Some of the best-known instances have occurred on the ground – such as a large flu outbreak following a prolonged delay on the ground – with ventilation systems switched off,” said Dr Richard Dawood, Telegraph Travel's travel health expert.