Dr Karl S. Kruszelnicki – Aircraft, Oxygen and Jet Setting Germs

From Dr Karl’s book ‘Dis Information and Other Wikkid Myths’

Last time, I talked about the common (but wrong) belief that because commercial jet planes are the most squashy place that people will pay to be herded into, the germs that the passengers breathe out will infect everybody else on-board.

In the early days of pressurising airliner cabins, around WW II, the air was fed in, and then dumped, so that the passengers breathed the air only once. This was very good for the passengers, but it cost fuel, energy and dollars.

So the plane makers introduced recycling of some of the cabin air, so that they would need to bleed less air from the engines.

The advantage of recycling air is that it saves the airlines about $US 60,000 per jet plane each year. The disadvantage is that the passengers breathe air with higher than normal levels of carbon dioxide. Other factors (such as low humidity) make the air not quite as lovely as rain forest air.

The flight crew control how much of the air is recycled, and hence the quality of the air you breathe – which helps explain why sometimes you come off a plane feeling OK, while other times you might have a slight headache. By the way, the flight crew in the cockpit usually get totally fresh air that has not been recirculated – for safety reasons.

Back in 1970, a passenger in an average aircraft of the day would get 7 litres/sec of outside air. By 2000, this had dropped as low as 2.8 litres/sec, and occasionally, 1 litre/sec.

Today, the recycled air is cleaned with filters that can remove bacteria. These filters are present in about 85% of the planes that carry over 100 passengers, but usually not in the smaller planes.

So what about germs swirling around in the cabin air of a jet with air filters?

It turns out that you are probably more protected from germs in a plane than if you work in an office. The plane’s air con system is divided into blocks that service about 5 rows of seats. The air tends to stay in its little “block”, and to not flow along the length of the plane.

The air in the plane’s cabin comes out of these vents in the ceiling, flows over the passengers, and then gets back into the air con system via vents at floor level. Each cycle takes about 3-5 minutes. About 63% of germs are removed on each cycle, thanks to the combination of some fresh air being admitted on each cycle, and the filters. So the germs from one person do not get spread along the length of the plane by the air-con system – they stick in the block of 5 seats. However, if a sick person walks along the aisle, coughing all the way, they might spread germs right down the plane.

Certainly, it’s not impossible to get sick on a plane. One study predicted that you could halve the infection rate on a plane if you doubled the ventilation rate. But this would cost about $1 in fuel per passenger on an 8-hour flight, and most airlines earn less on their investment than they would get, from putting the money equivalent to their investment in a bank, and earning bank interest.

Dr. Ron Behrens, a consultant in Travel Medicine at the London School of Hygiene and Tropical Medicine, said in New Scientist magazine that departure lounges at airports were far riskier than the actual flight. You can be infected by the air-con on a plane, but only from people near you – which is the same as going to the supermarket, or travelling on a bus.

But there is one major infectious diseases risk associated with planes – and that is spreading diseases around the world.

Each year, over one billion passengers fly on planes, with about 50 million passenger flights to the poorer countries (where many new diseases start). Since the 1960s, influenza epidemics have followed major air transport routes. Back in the 14th century, the Black Death took about 3 years to get from Southern Italy to Great Britain on the backs of rats. Today, a passenger on a jetliner can carry infectious diseases anywhere in the world in a few hours – but because of how the air con is set up, probably won’t infect most of the other passengers on the jet.

© Karl S. Kruszelnicki Pty Ltd 2005