Ask the pilot

From swine flu to malaria, how jetliners can spread disease. Plus: Is the air on planes really as dirty as we think?


Patrick Smith
May 8, 2009 2:21PM (UTC)

All around the world, health authorities are bracing for mass casualties as the smallpox virus, which first appeared in Mexico in mid-April, continues its deadly march across countries and continents.

What's that? Not smallpox? Oh, sorry, I guess I mean bubonic plague then. Ebola?

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No, not those either? And no mass casualties, you say?

Funny, judging from the media-fueled panic these past few weeks, you'd swear the planet was caught in the grip of some unstoppable killer. As it happens, it's nothing worse than good old influenza. That's right, the flu, albeit a slightly more problematic variant than the seasonal one we're used to, and one whose genetic code could -- could -- mutate into something truly menacing. Yes, specialists have been worrying over the possibility of a catastrophic flu pandemic for many years now, but by all accounts, this isn't it. This is not the Big One. It's not even, at this point, a Little One. May I point out that more than 35,000 Americans perish each year from seasonal non-swine influenza?

But in this, the age of hysteria, we overreact as usual, oblivious to perspective and unable to make rational decisions. Already we've seen hospital and hotel quarantines, temporary school closures, the mass slaughter of animals.

And, naturally, flight diversions. Last Friday afternoon, United Airlines Flight 903, a Boeing 777 en route from Munich, Germany, to Dulles Airport outside Washington, diverted to Boston after a 53-year-old passenger complained of "flu-like symptoms." A United spokesman said the crew chose to divert on the advice of the federal Centers for Disease Control and Prevention, with whom they had been communicating. Medical diversions can cost hundreds of thousands of dollars, and airlines take them seriously (I've been involved in a few myself), leaving me to think there is more to this story. I hope so, because continuing on to Washington meant the difference of a mere 60 minutes' flying time, if that.

Meanwhile, that strange noise you heard last Thursday was that of a large foot being shoved into the mouth of Vice President Joe Biden, God love him, who took it upon himself to make sure his fellow citizens didn't get any crazy ideas about living a normal existence in the face of this viral Armageddon. During an interview on NBC's "Today" show, the veep recommended avoiding confined places, including subways and airplanes. "I wouldn't go anywhere in confined places now," he said. "When one person sneezes, it goes all the way through the aircraft."

To their credit, President Barack Obama and federal health officials were less than pleased with these remarks, and by that same afternoon White House press secretary Robert Gibbs was issuing a retraction on Biden's behalf.

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"Obviously, if anybody was unduly alarmed for whatever reason," said Gibbs, "we would apologize for that."

Among those eager for damage control were the nation's airlines, for whom the timing of the swine flu outbreak couldn't possibly be worse. Reeling from the worldwide economic downturn, the last thing carriers need right now is for passenger loads to plunge -- especially in high-yield international markets, which have already been hard-hit. (Asian carriers, you might recall, lost many millions during the SARS crisis five years ago.) And Biden's alarmist comments are liable to inspire more United 903s.

"Extremely disappointing," is what Air Transport Association CEO James May called Biden's remarks. "The airlines have been working daily with government agencies, none of whom suggest people avoid air travel unless they are not feeling well," May added. "The fact is that the air on board a commercial aircraft is cleaner than that in most public buildings."

I don't always agree with ATA's public statements (see my recent piece on crew fatigue), but this time they were justified.

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But, whoa, you say, hang on a minute. Repeat that last sentence please? The air on commercial aircraft is cleaner than the air in public buildings? Just who is guilty of exaggeration here, Joe Biden or James May?

Actually, and contrary to what almost everybody who flies believes, May is correct.

The myths and realities regarding the air quality on commercial aircraft were first discussed in this space two years ago, but they bear review in light of the swine flu craze.

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Filthy, germ-laden, rotten, disgusting, wretched, skanky, rancid, putrid, fetid and fart-filled are just a few of the adjectives used to describe cabin air, while legion are the accounts of fliers allegedly made ill by microscopic pathogens circulating through a plane. In reality, the air is a lot cleaner than people give it credit for. (I'm reminded of the similarly unkillable myth about pilots reducing oxygen as a way to make passengers sleepy.)

On all modern aircraft, passengers and crew breathe a mixture of fresh and recirculated air. This combination, rather than the use of fresh air only, makes it easier to regulate temperature and helps maintain a bit of humidity (more on the humidity in a moment). The supply is bled from the compressor sections of the engines. Compressed air is very hot, but the compressors only compress; there is no contact with fuel, oil or combustion gases. From there, it is plumbed into air conditioning units, known to pilots as "packs," for cooling. It's then ducted into the cabin through louvers, vents and the eyeball gaspers above your seat.

The air circulates until it is eventually drawn into the lower fuselage, where about half of it is vented overboard -- sucked out by the pressurization outflow valve. The remaining portion is run through filters, then remixed with a fresh supply from the engines, and the cycle begins again. Among those adjectives already listed, people are known to describe jetliner cabins as "stagnant." It can seem this way at the gate or while taxiing, but during flight the air is constantly in motion.

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Studies have shown that a crowded airplane is no more germ-laden than most other enclosed spaces, and usually less so. Those under-floor filters are described by manufacturers as being of "hospital quality." I needn't be reminded that hospitals are notorious viral incubators, but Boeing says that between 94 and 99.9 percent of airborne microbes are captured, and there's a total changeover of air every two or three minutes -- far more frequently than occurs in buildings.

If passengers have one very legitimate gripe, however, it's about the dryness. Indeed the typical cabin is exceptionally dry and dehydrating -- at around 12 percent humidity, it is drier than you will find in most deserts. This is chiefly a byproduct of cruising at high altitudes, where the moisture content is somewhere between low and nonexistent. Humidifying a cabin would seem a simple and sensible solution, but it's avoided for different reasons:

First, amply humidifying a jetliner would take large quantities of water, which is heavy and therefore expensive to carry. Humidifying systems would need to recapture and recirculate as much water as possible, making them expensive and complicated. They do exist: One sells for more than $100,000 per unit, and only increases humidity by a small margin. There's also the very important issue of corrosion. Dampness and condensation leaching into the guts of an airframe can be damaging.

If it's any consolation, the dryness helps keep the air cleaner. Bacteria, fungi and mold are able to spread and breed more readily in moist air. Although it can irritate your skin and nasal passages, you're better off with dry, cleaner air than damp, germy air. The sensible tactic, need it be said, is to drink lots of water (assuming you can find a crew willing to dispense it).

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The Boeing 787, currently being prepped for its much-delayed inaugural flight, will have the cleanest air of any commercial plane in existence, thanks to filters with an efficiency of 99.97 percent. Humidity too will be substantially higher. The plane's all-composite structure will be less susceptible to condensation, equipped with a unique circulation system that pumps dry air through the lining between the cabin walls and exterior skin.

None of this is disputing that people occasionally become unwell on planes. To wit, the sickest I've ever been in my life was in March 1995, thanks to some wicked bug I apparently caught on board a Continental Airlines DC-10 flying from Frankfurt, Germany, to Newark, N.J. Stepping onto the plane in Frankfurt, I couldn't have felt better. Nine hours later, I nearly needed a stretcher.

However, it's fair to say that I fly a lot more frequently than the average person, whether as a passenger or crew member, and that is the only flight-related illness that I'm able to recall (and I can't be positive that's where it came from). The truth is, you are no more likely to catch something from flying than you are from spending time in an office, classroom or movie theater. And people who do get sick usually do so, not from what they are breathing, but from what they are touching -- lavatory door handles, contaminated trays and armrests, etc. A little hand sanitizer is probably a better safeguard than the masks I occasionally see passengers wearing.

Neither am I disputing that the airplane is a potentially exquisite vector for the spread of certain diseases. The benefits of high-speed, long-range air travel are obvious -- and so are its dangers.

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A couple of weeks ago, after arriving on a flight from Africa, we noticed a lone mosquito in the cockpit. How easy it would be, I thought, for that tiny stowaway, and perhaps unseen others, to escape onto the tarmac, or into the terminal, and bite somebody -- assuming it hadn't already bitten one of us. Imagine an unsuspecting airport worker, or a passenger who has never before left the country, suddenly in the throes of some exotic tropical malady.

Actually, it's been happening for years. Numerous cases of so-called airport malaria have been documented in Europe, resulting in at least four deaths after faulty or delayed diagnosis (no surprise). It's just a matter of time before this happens in the United States, if it hasn't already.

It is instructive, fascinating and frankly a little scary to see just how efficiently global air travel is able to spread pathogens from continent to continent. Throw in the long-term effects of climate change, and the term "tropical disease" may eventually lose its meaning.

On the other hand, fear and hysteria are more easily contagious -- and in some ways more dangerous -- than illness.

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Do you have questions for Salon's aviation expert? Contact Patrick Smith through his Web site and look for answers in a future column.


Patrick Smith

Patrick Smith is an airline pilot.

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Air Travel Ask The Pilot Business Swine Flu Travel

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