Newer airplanes recirculate part of the cabin air (up to 50%) to save
fuel, in contrast with older planes, which use all fresh air
ventilation. There have been reports of passengers and (more
frequently) flight attendants complaining about headaches caused by
"stale air".

There have been two recent studies of cabin air quality that measured
carbon dioxide (CO2) levels. Although CO2 is nontoxic at ten times
ordinary levels, high CO2 levels are a sign of insufficient fresh air.
The normal CO2 level in outdoor air is 300 parts per million (0.03%).
The American Society of Heating, Refrigerating, and Air-Conditioning
Engineers has set a "comfort threshold" of 1,000 ppm, above which a
building is considered to suffer from stale air.

   1. Consumer Reports tested 158 flights on 20 airlines covering 44
      different kinds of planes (July 1994). They found that 25% of 
      flights had stale air at some point in the flight, with 13% 
      maintaining inadequate fresh air levels throughout the flight. 
      Boeing 757 planes were among the worst -- as high as 5 times
      higher than normal outdoor air -- while newer Boeing 747-400
      planes had the freshest air. All planes use HEPA (High Efficiency
      Particle Air) filters to remove dust, bacteria, and viruses,
      but the Boeing 747-400 and Boeing 777 planes use higher
      efficiency HEPA filters. See "Breathing on a jet plane",
      Consumer Reports, August 1994, pages 501-506, for details.

   2. Consolidated Safety Services of Virginia conducted a spot check
      of cabin air quality on 35 flights without notifying the
      airlines or crew (May 1994). Tests were conducted on Boeing 757
      and 727 and McDonnell Douglas DC-9 and MD-80 aircraft. This
      study found an average CO2 level of 1162 ppm. Note that this
      study was sponsored by the Air Transport Association (ATA), 
      which represents the airline industry.

Nevertheless, the most likely cause of the often-reported symptoms
is dry air, not a lack of fresh air (though stale air can contribute
to the problem). Cabin air on both newer and older planes has very
low humidity levels (15-20% relative humidity), due to very dry air
being brought in from outside at high altitude. (The air outside the
plane is very cold, and thus has a very low absolute humidity, which
translates into a very low humidity level when warmed.)

So if you suffer from sore throats, dry eyes, headaches, allergies,
itchy nose, or general fatigue when you fly, it might be due to dry
air. To alleviate these symptoms, try the following:

   +  If you wear contact lenses, take them out for the flight,
      especially if it is longer than an hour or two in duration.

   +  Drink lots of liquids, but avoid alcohol and caffeine, which
      tend to dehydrate you. Drink before and during the flight.
      Drinking water is good. Drinking a balanced electrolyte
      solution, such as gatorade/powerade, is better, as the
      carbohydrates in them let your body absorb them faster than water.

   +  If you suffer from dry skin, bring along a water sprayer and
      spray yourself on the mist setting. Hand lotion can also help.
      Some people feel that spraying water on your face or skin can
      make your skin even drier, if not chapped. Ask your
      dermatologist for advice.

   +  Don't take a decongestant before the flight, since this dries
      out your nose.

   +  If the air smells bad or feels stuffy -- a sign of stale air --
      complain to the flight attendant. On some planes pilots can
      control the mix of fresh and recycled air. (The pilots aren't
      affected by the stale air problem, because the cockpit has a
      separate ventillation system, as mandated by FAA regulations.)

Likewise, if you suffer from asthma, chronic bronchitis, empysema,
severe allergies, or impaired immunity, consult your doctor for his or
her advice.