On all modern aircraft, passengers and crew breathe a mixture of fresh and recirculated air. Using this combination, rather than fresh air only, makes it easier to regulate temperature and helps maintain a bit of humidity (more on the latter 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 gasses. From there, it is plumbed into air-conditioning units, known to pilots as "packs," for cooling as needed. It's then ducted into the cabin through louvers, vents and the eyeball gaspers above your seat.
The air circulates through the cabin until eventually it is drawn into the lower fuselage, where about 50 percent is vented overboard -- sucked out, if you will, 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. (The flow is front to back, so if you're especially germophobic, try sitting in a forward row.)
The air is also a lot healthier than people give it credit for, though it somewhat depends on the aircraft model and, as you'd expect, how full it is. Studies have shown that a crowded airplane is no more germ-laden than most other enclosed spaces, and usually less so. Those underfloor filters are described by manufacturers as being of "hospital quality." I needn't be reminded that hospitals are notorious viral incubators and ideal places to get sick, but according to Boeing, between 94 and 99.9 percent of the airborne microbes are captured, and there's a total changeover of air every two to three minutes -- far more frequently than occurs in buildings, where the percentage of fresh air is also much lower. (Personally, only once do I blame a particular illness on something I picked up while flying -- it was a Continental flight from Frankfurt, Germany, to Newark, N.J., in 1995 -- and even then I can't be sure. By contrast, as both a youngster and an adult, I've been struck by classroom contagions many times.)
The strength of airflow is not always directly adjustable. Cockpit controls vary, and sometimes allow only for basic on/off positioning, along with some emergency shutoff options. Flow is regulated separately from each engine, but the switches are typically left at some standard setting and the system more or less takes care of itself. On the flight deck of the Airbus A320, one of the most common types, there's a "Pack Flow" switch on the overhead panel with three positions: Lo, Norm and Hi. At one airline, guidance is to use Norm unless fewer than a hundred passengers are on board, in which case Lo is selected.
Temperature, meanwhile, is controlled by separate switches, and can be fine-tuned as required.
Alas, with flow and circulation so dependent on engine power and pressurization, maintaining a comfortable environment on the ground is a lot more challenging than while aloft, especially on broiling tarmacs in the summer. "Stagnant": sometimes, yes. One or more engines are sometimes shut down during taxi, while at the gate it's usually the less powerful auxiliary power unit doing the work. (At some stations, external hoses are attached, supplying cold or hot air directly from the terminal or a portable air-conditioning unit.) Some aircraft have more effective packs, fans and gaspers than others, but I'm as mystified as you are as to why plane makers haven't engineered air-conditioning systems with a little more on-the-ground gusto.
Another common complaint is about dryness. Indeed, the air aboard commercial aircraft is exceptionally dry and dehydrating. The typical humidity level aboard a jetliner is around 12 percent. That's substantially drier than you will find in most deserts. This is chiefly a byproduct of cruising at high altitudes, where moisture content is somewhere between very low and nonexistent. That compressed, high-pressure bleed air from the engines also contributes. Humidifying a cabin would seem a simple and sensible solution, but it's avoided for different reasons.
First, to amply humidify a jetliner would take large quantities of water, which is heavy and therefore expensive to carry around. And because a portion of cabin air is constantly refreshed, humidifying systems would need to recapture and recirculate as much water as possible. Thus they'd be expensive, heavy and complicated. They do exist: One sells for more than $100,000 per unit and only increases humidity by a small margin overall. (Some aircraft have cockpit humidifiers, which, I'm told by pilots who have used them, are at best minimally effective.)
There's also the very important issue of corrosion. Dampness and condensation leaching into the structure of an airframe is never helpful.
If it's any consolation, the dryness, while irritating, actually helps keep the air clean. 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 and more germy air. The sensible tactic, obviously, is to drink lots of water (assuming you can find a crew willing to dispense it, or you're able to sneak some past the TSA scarecrows).
For those of you still skeptical, the new Boeing 787, set to debut next year, should make you happier. The 787 will have the cleanest air of any plane in existence, approximating the same microbial content of outside air, thanks to filters with an efficiency of 99.97 percent, and humidity will be substantially higher. The plane's all-composite structure will be less susceptible to condensation, and it will also be equipped with a complex circulation system that pumps dry air through the lining between the cabin and exterior skin, keeping out the moisture. Cabin pressure will be at approximately 6,000 feet during cruise, compared with the 8,000 feet standard, meaning more oxygen and less fatigue.
Until then, if you're the type who likes to fret over exaggerated dangers, forget microbes for a minute and consider microroentgens. As maybe you've read or heard, airline passengers are routinely exposed to increased levels of cosmic radiation. This is especially true in daylight hours at extreme latitudes, such as on transpolar routes, where natural atmospheric shielding is low. Add in those unpredictable solar flares, and radiation levels can approach or exceed those of a chest X-ray over the course of a single flight.
The risks this poses do exist, but they are small. On average, the hundred-thousand-mile frequent flier (or airline crew member) will receive a higher annual dose of radiation than a member of the general public, but exposure is well within the limitations normally set for nuclear workers and other industry specialists. The Europeans have been studying the issue closely, mandating that radiation levels be assessed and monitored for some crew members. One carrier, Scandinavian Airlines System, previously explored a connection between radiation and rates of crew-member cancer, but concluded there were no substantive links.
For an added thrill, do what I did, and combine your next high-latitude crossing with a day trip to Chernobyl.
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Do you have questions for Salon's aviation expert? Send them to AskThePilot and look for answers in a future column.
About the writer
Patrick Smith is an airline pilot. His column is archived here.
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