Pressure points

Aircraft cabin pressure often receives blame for passenger discomfort, from mental and physical fatigue to nausea, headaches and malaise. And while some justification exists for such claims, the subject is complicated.

To be sure, cabin pressurization is an absolute necessity for any aircraft that flies for long periods of time higher than 10,000 feet above sea level. Without pressurization, cabin altitude equals aircraft altitude. Though humans can adapt to elevations above 10,000 feet-the highest town in the world is Wenzhuan, China,  at 16,728 feet-those of us who live at lower elevations will experience the ill effects of altitude at much lower levels.

With pressurization, cabin altitude atmospheric (or air) pressure is maintained at a lower level than the atmospheric pressure outside the aircraft. Although Americans typically think of pressure in pounds per square inch (for example, standard atmospheric pressure at sea level is 14.7 psi), cabin pressure is normally referred to as an equivalent altitude in feet, as this provides more meaning for most people.

Typically, the pressure inside an aircraft cabin flying at high altitude approximates the atmospheric pressure at 8,000 feet (about 10.9 psi), which is like sitting on the top of Mount Olympus (elevation 7,962 feet) in Washington. On the theory that a somewhat lower altitude enhances passenger comfort, some newer business jets provide maximum cabin altitudes of 6,000 feet. According to Bombardier, its Global 5000 and Global XRS business jet cabins have the lowest altitude-equivalent pressure of any modern jet-4,500 feet while flying at an altitude of 45,000 feet and less than 6,000 feet at an in-flight altitude of 51,000 feet. But even at this altitude, if your body is not acclimated to the atmospheric pressure at these heights, you will likely feel some effects during a long flight, if nothing more than slight drowsiness.

So why not maintain cabin pressure at sea level? Though it would be possible to build airplanes that could do this, it would require a stronger cabin structure (or "pressure vessel") and therefore greater weight, more powerful engines, higher fuel burn and so on. Since symptoms of altitude sickness commonly occur in most people above 8,000 feet, pressurizing aircraft cabins to this altitude is considered an acceptable compromise.

Does pressurization at this level really affect passenger comfort?

Though some have complained that cabin air lacks sufficient oxygen, a study by The Boeing Company seems to debunk this claim. In a typical modern airplane, Boeing noted in a report entitled "The Airplane Cabin Environment," about 20 cubic feet per minute of air per passenger is provided, resulting in a complete cabin air exchange every two to three minutes. More important, according to the study, humans at rest consume about 0.015 cubic feet of oxygen per minute, while the cabin ventilation system provides approximately 4.19 cubic feet of oxygen per minute per person-about 279 times more oxygen per minute than can be physically consumed.

Nevertheless, a study published in The New England Journal of Medicine in July 2007 suggests that aircraft cabin pressurization at 8,000 feet can affect passenger comfort. It noted that "acute mountain sickness [or "altitude sickness"] occurs in some unacclimatized persons who travel to terrestrial altitudes at which barometric pressures are the same as those in commercial aircraft during flight." The report said the sickness occurred in 7.4 percent of the 502 study participants, who experienced altitudes ranging from 650 to 8,000 feet.

Altitude Sickness
Though the levels of oxygen-about 21 percent-and the other gases that comprise air remain the same as altitude increases (and pressure decreases), the number of molecules of each gas decreases as altitude increases, resulting in "thinner" air.

At a cabin altitude of 8,000 feet, the partial pressure of oxygen is about 74 percent of what it is at sea level.

It is the partial pressure of oxygen in the lungs that forces it into the blood across the lungs' alveoli. As pressure decreases, so does oxygenation of the blood, which can cause altitude sickness.

Different people are affected by altitude sickness in different ways, though they usually exhibit one or more of the following symptoms: headache, fatigue, drowsiness, nausea, malaise, dizziness and difficulty sleeping. Passengers with existing health issues-such as high blood pressure, diabetes or recent surgery-may find those problems exacerbated. And those with serious respiratory conditions, such as emphysema, may have difficulty breathing in a pressurized aircraft cabin at altitude.

For people without health problems, however, research by the National Academy of Sciences concluded that "pressurization of the cabin to an equivalent altitude of 5,000 to 8,000 feet is physiologically safe-no supplemental oxygen is needed to maintain sufficient arterial oxygen saturation."

Other Problems
The most common complaint as cabin pressure changes is pain in the inner ear. This occurs because air trapped in the ear expands as the pressure changes, stretching the eardrum outward or inward. (The $10 word for this is "aerotitus.") As the aircraft climbs, cabin air pressure decreases up to the 8,000-foot level and higher pressure air inside the ear tries to force its way out. When descending, the cabin pressure increases, pushing the eardrum inward against the now lower pressure air of the previous higher altitude inside the inner ear. The accepted remedy is simply to yawn frequently, allowing the air to escape and equalizing the pressure. Babies often need a bottle to facilitate this yawning motion of the jaw. Chewing gum can help, too. However, these remedies may not work for individuals with severe nasal congestion, in which case a decongestant may be the answer. If you are really stuffed up, you risk a severe earache and sinus headache after landing.

Another problem that arises in pressurized cabins is that the air exchange system creates a dry environment in which humidity can be as low as 3 or 4 percent. This inhibits bacterial growth inside the aircraft but fosters dehydration in humans, which contributes to thickening of the blood. And this may increase the possibility of deep-vein thrombosis or aggravate blood-related health issues. One way to avoid dehydration is to stay away from alcohol and other diuretics, such as tea and coffee, and drink plenty of hydrating liquids.

Whether it's first class on a transatlantic flight or a business jet cruising cross country, cabin pressure and the overall cabin environment most certainly affect passenger comfort and may have an adverse affect on health.