Onboard air: Every breath you take

1 Mar 2006 by BusinessTraveller
Emirates flights undergo enhanced cleaning

The air you’re breathing inflight is not as it seems. Sarah Maxwell investigates the controversy over the quality of oxygen in the cabin

As long as we reach our destinations safely and on time, most air travellers are happy to leave the finer points of aircraft engineering to the experts. But where there’s mystery there’s myth, and if there is one aspect of air travel that is prone to speculation it is the quality of the air on board.

We no longer have to inhale other people’s cigarette smoke on most airlines, but recirculated cabin air has been accused of stifling our oxygen, spreading disease and drying us out like prunes.

In a specially conducted survey on businesstraveller.com, 77 percent of respondents believed that cabin air is bad for our health.

But is it?

The answer depends on whom you ask. Airlines are not required to make public the statistics for their on-board air and the industry regulations are far from comprehensive.

The Joint Aviation Authorities (JAA) and the US Federal Aviation Administration (FAA) both set requirements for the levels of carbon dioxide, carbon monoxide and ozone (see table) but there are no rules for particle or bacteria levels, temperature or humidity.

In its 2000 report Air Travel and Health, the UK House of Lords expressed surprise, given the media speculation about cabin air quality, that airlines had not attempted to reassure passengers by releasing data on the cabin environment.

And this reluctance has continued, even after widespread concern over the role of air travel in spreading the Sars virus in 2003. The result is that most air travellers remain starved of information (if not oxygen) regarding the quality of cabin air.

Many business travellers will vouch they have developed a cold or flu after a work trip, and when you have just spent half a day in a cramped space, surrounded by strangers, the aircraft cabin seems the likely culprit. Most airlines use about 50% external and 50% reecirculated air, and it’s the latter part that feels “secondhand”.

Farrol Kahn, director of the charity Aviation Health Institute in Oxford, is convinced that a move away from recycling air would benefit both passengers and airlines: “We would like to see a return to 100% fresh air – it’s pure alpine air, which would come through the engine and then through filters. Airlines can then advertise their service as having ‘purer than alpine air’, and they would have a marketing advantage.”

But is recirculated air really to blame?

Surprisingly, in the few studies where recirculated and outside air have been compared, there is no evidence to suggest it is. In one US study, researchers found that 20% of passengers contracted a cold within a week of stepping off a plane. However, the results were virtually identical for passengers who had flown on aircraft using 100% external air and those in a plane using part-recirculated air.

Many medical experts point out that in any enclosed environment, from buses and offices to airport lounges, people are exposed to germs. Michael Bagshaw, visiting professor of aviation medicine at King’s College London explains: “If someone gets on a plane and suffers from a disease, they take it with them – it’s a fact.

“ But if you’re sitting next to someone on a plane (the risk) is no different to sitting next to someone on a bus – not worse, but not less.”

Perhaps even more surprisingly, there is no evidence that recirculated air helped to spread Sars.

According to the World Health Organization, there were four flights on which Sars was transmitted between people. Dr Mark Popplestone, head of passenger health services at British Airways, says: “Flying clearly has a part to play in Sars travelling around the world, and there was one well-documented case of a dozen people scattered through an aircraft who went on to develop Sars.

But they had had close contact on the ground as well – there was nothing unique about the aircraft environment in terms of where they were.”

Another disease that has been transmitted during air travel is tuberculosis. The US Center for Disease Control documented two cases of transmission in which passengers went on to show exposure to the disease (though they didn’t develop active TB). Here too it seems person-to-person contact was to blame rather than the air system. In one case, a flight attendant infected a fellow attendant after 12 hours of exposure, and in the other a passenger passed it to nearby passengers on an eight-hour flight.

Popplestone has a theory: “There was a paper published a long time ago that said that people who travel get more respiratory infections than people who don’t – and that applies to all travellers regardless of how they travel. When you travel you’re exposed to different germs that are new to you and to your immune  system. It’s plausible that you’re going somewhere new so you’ll be more likely to pick up a cold.”

Another suggestion is that on long-haul flights, the combination of low humidity, time zone changes, jet lag, stress at the airport, pressure changes and other factors can leave you more susceptible to illness.

Aircraft ventilation systems are designed to minimise the risk of transmitting airborne diseases. Inflow and outflow ducts at every row draw air from the top of the cabin to the floor to restrict movement of germs up and down the cabin. But Richard Johnson, environmental control systems engineer at Boeing, admits: “The ventilation system can’t overcome the proximity aspect.” In other words, if you’re sitting next to someone with a cold, you’ll have to take your chances with the laws of nature – or try and change seats.

To remove particles and germs from recirculated air, aircraft use high-efficiency particulate air (“Hepa”) filters, which catch bacteria, fungi and almost all viruses.

The filters are over 99.9 percent efficient at removing viruses and bacteria.

Airlines insist that air that has passed through Hepa filters contains far fewer contaminants than many other environments – including most office buildings.

Bagshaw agrees: “Hepa filters are exactly the same as the filters used in hospital operating theatres.

The conditioned air in a cabin is cleaner than in your office or my office.”

There are no regulations stating the type of filters that aircraft must use or how often they must be changed, but most aircraft entering service in the last 10 years are equipped with the Hepa variety. Of course, it’s up to airlines to maintain the filters and change them as necessary.

Boeing’s Richard Johnson says: “We have an evaluation programme to recommend when they should be changed – we work on the conservative side so we recommend changing them when they have half or a third of their life still left. But each airline has its own maintenance programme and they have to show to the regulators that they have this programme – they can’t arbitrarily pick and choose what they do. They have a schedule that they submit and they have to abide by it.”

But there is more than just germs in cabin air to worry about, according to a study published last year. Researchers measured oxygen in the blood of long-haul travellers and found for over half of passengers it fell by 6%, a level at which hospital doctors would consider prescribing oxygen. The team concluded that “these falling oxygen levels,together with factors such as dehydration,immobility and low humidity, could contribute to illness during and after flights.”

The study dismayed aviation experts.Michael Bagshaw says: “I was surprised that such well-known physiological facts were misinterpreted. Any textbook of aviation physiology confirms that as you ascend to 8,000ft (the altitude at which most cabins are pressurised) the partial pressure of oxygen reduces, but the oxygen saturation is still 90% in the blood and this causes no problem.

Many people live and work at these altitudes so there’s nothing to worry about. The only people who should take advice are those with respiratory disease or heart disease. For frequent travellers who are healthy the body copes perfectly well. If you can walk 45.7m without getting breathless, you’ll cope well with cabin altitude.”

If it feels hot or stuffy during the flight, it may be because the pilot has switched off one of the air conditioning units to conserve fuel The controversy surrounding this aspect of cabin air is encapsulated in lively debate on the website of Diane Fairechilde, a former flight attendant who now offers health advice to air travellers (www.flyana.com). She reveals controversial insights into aircraft practices, including that “pilots receive 10 times more oxygen than passengers”.

She suggests if it feels hot or stuffy during the flight it may be because the pilot has switched off one of the air conditioning units to conserve fuel (elsewhere this has also been described as a ruse to make passengers sleepy and more compliant).

Two long-haul pilots we spoke to denied that such a practice had been used in their experience. Boeing’s Richard Johnson insists it is not possible to make in-flight alterations to the air conditioning:“Our normal procedure is to set the pack on – you set it and forget it, unless there’s a failure in the system. In the cockpit there’s a switch that says ‘auto’.The pilot can’t say ‘I only want 80% of it’ – it’s either off or on.”

Airbus says that air conditioning settings vary across its aircraft – in the A320 and A319 for example, there are high, normal and low settings, while in the A321 there is normal or economy. The normal flow position is only changed during the flight if there is a system problem, when it would be switched to high. The low setting can be used if the number of occupants is below a certain number.

But Airbus adds: “Any low setting would be set at the beginning of the flight and the pilots would have no cause to change this setting during a normal flight.”

Johnson admits that pilots do receive more outside air than passengers, but says there are technical reasons for this.

The cockpit is packed with electronic equipment, which generates heat, as well as huge windows that give rise to a “high solar load”. To compensate, it receives a higher proportion of colder, outside air than the rest of the aircraft.

Pilots receive about four times more external air than passengers, which keeps the cockpit at a steady temperature. Outside air also helps to maintain pressurisation. Johnson says: “We want the cockpit to be slightly higher pressure (than the rest of the aircraft) for smoke evacuation purposes – if there was smoke we would always want it to flow out of the cockpit.”

Following the House of Lords report, a two-year research study was commissioned to investigate cabin air. The EU-backed Cabin Air project, which began in 2000, brought together airlines, manufacturers, ventilation and filtration experts from 16 organisations and seven countries. Cabin air was sampled on 50 flights with three major carriers (British Airways, SAS and KLM) to establish for the first time a full picture of the cabin air environment.

Samples were measured for a range of parameters including carbon dioxide, carbon monoxide, temperature, relative humidity, ozone, particles, dust, volatile organic compounds and microbiologicals (eg bacteria and fungi). With few regulations to use as a benchmark, the researchers compared the aircraft environment with other indoor spaces such as offices and other buildings.

The study found that levels were all within acceptable safety limits – not only that, but levels of contaminants were lower than in other internal environments. It concluded that,“none of the measured values of indoor (aircraft]) air quality are at levels of concern for health and safety of passengers or crew”.

That’s not to say there isn’t room for improvement. Levels of airborne pollutants and bacteria were found to be slightly higher when aircraft were on the ground, prompting researchers to recommend that outside air should be filtered during the period before take-off.

What lies ahead?

The Cabin Air project results have been used as a basis for drafting voluntary guidelines to fill the current regulatory void.

Project manager Dr David Ross says: “Because there were no regulations, we have developed a European-wide standard that can be used by the industry and gives people something to refer to. The regulations will eventually catch up but this gives us something to work to.”

Don’t expect anything to change too suddenly. As Ronald Ashford points out, it is costly and impractical for airlines to redesign existing planes: “You can’t continually modify older planes to the standards of the newest.”

But in the next generation of aircraft there are some major design changes that may improve the cabin environment.

The new composite fuselage developed for the Boeing Dreamliner 787 will mean the maximum cabin altitude can be reduced from 8,000ft to 6,000ft. Says Richard Johnson: “Healthy individuals may not notice a difference but immuno-compromised individuals will be more comfortable. A lower

cabin altitude makes it easier for your body to assimilate oxygen, so it will make it more comfortable for potentially health-compromised people, elderly people and infants.”

In the meantime, aircraft filtration manufacturer Pall Aerospace has worked with Airbus Industrie to develop a new filtration system which is interchangeable with the existing systems – thereby requiring no alterations – but incorporates a combined particle and odour filter element. The first stage is a standard Hepa filter, followed by an odour adsorber that removes odours and volatile organic compounds. A Pall spokesperson said it was developing a filter to purify external air: “Most aircraft have the filters located in the recirculation loop but for future and existing aircraft, consideration needs to be given to providing adequate purification of the outside air as well as the recirculated air.”

It may be little comfort to those business travellers waking up with a streaming cold after a long-haul flight to point out that you’re just as likely to have picked it up from a colleague in your overseas office than on board a plane. But it might bring some relief to know that normal, healthy individuals are not taking their health in their hands by sucking in deep lungfuls of recirculated air.

For more information, visit boeing.com/ commercial/cabinair; ba.com/travel/health; aviation-health.com; and asma.org/ publications to download Medical Guidelines for Airline Passengers.

Parameter                     Regulator            Rule

Cabin pressure                FAA/JAA            <8,000ft

Carbon monoxide            FAA/JAA            <50 parts per million

Carbon dioxide                JAA                   <3%

FAA                   <0.5%

Ventilation                       JAA                   10 cubic feet per min/crew member

FAA                   0.55lb per min/occupant

Ozone                             FAA/JAA               <0.1 parts per million (over any 3hr interval) above 27,000ft


1. Myth: Recirculated air is “stale”

Fact: The nature of the system means that air is constantly being diluted, so the same air does not endlessly circulate. When “used” air is drawn out of the cabin, half of it is exhausted from the plane while the rest is filtered and then mixed with an equal amount of outside air.

2. Myth: Older aircraft have poorer quality air

Fact: Older aircraft are not necessarily pumping out dirtier air. A UK-based study recently measured the air on two older aircraft types (the BAe 146, with 100 percent mask outside air, and the Boeing 737-300 with 50% recirculated air) during flights of up to three hours. They found that levels of air particles, bacteria, dust, carbon monoxide and carbon dioxide were well below regulatory levels and were comparable with the air on newer aircraft.

3. Myth: If you are feeling short of breath on board, you can request the use of an oxygen canister

Fact: British Airways told Business Traveller that if a passenger is feeling unwell, the crew are trained to deal with the situation and if necessary contact with a medical advisory centre at ground level. Oxygen is not handed out on request.

4. Myth: It’s a good idea to wear a surgical mask to protect you from infections on board

Fact: Dr Mark Popplestone of BA says: “no more or less than it’s worth doing in any public place.” Wearing a surgical mask protects those around you from infections you may be carrying but does not keep you safe from others’ germs.

5. Myth: Cabin air is drier than it used to be

Fact: Recirculated air may have a bad reputation but it does perform one vital service, and that’s raising humidity levels on board. Before recirculated air was introduced the atmosphere on aircraft was even drier than today. Says Richard Johnson, environmental control systems engineer  for Boeing: “In systems with 100% outside air it’s around 5 percent humidity while in Economy Class using recirculated air it can be 15 to 20%, which is still low but relatively it’s much better.

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