While the likely dangers seem modest enough to cause few people to reconsider their flying plans, the transmission of disease through commercial airline flights has received a bare minimum of attention from the medical research establishment. Furthermore, the statements from various aviation authorities suggest that reassuring the passengers may be a higher priority than accurately measuring or reducing the actual risks.

   For example, a Federal Aviation Administration declared in a fact sheet that pollution from carbon dioxide measured on commercial air flights "were well below the maximum levels recommended by the Occupational Safety and Health Administration (OSHA)."

   While true, what the FAA statement didn't mention was that those standards were set for breweries and bakeries. Measured carbon dioxide levels on commercial flights suggest ventilation problems, especially before takeoff and after landing, according to studies by air quality specialists.

   In another case Boeing noted that the air is exchanged in the airliner cabin more often than in some office buildings. This is true, but by the key measure of fresh air per occupant, most Boeing planes provide about one half that recommended for jail cells.

   The first question is what do researchers know about the risks? One of best-studied cases of disease transmission on airliners is a nightmare scenario. It is one of the relatively few cases were public health investigators had the time, money and focus to overcome the practical obstacles to studying people who gather briefly on airline flights and then scatter to the winds. Here is what happened, according to a 1996 medical study:

   A 32-year-old woman tourist from Korea with a serious and contagious case of tuberculosis boarded a flight from Honolulu to Chicago, then proceeded to Baltimore. She stayed in Baltimore with friends for a month and then made similar return flights. Eight days after she got back to Honolulu, she coughed up almost a quart of bright red blood, and soon died of respiratory failure and lung hemorrhage.

   So how many fellow passengers were infected with her tuberculosis? In this case the federal Centers for Disease Control and Prevention launched an investigation, contacting 802 passengers and crew on the four flights and recommending skin tests for tuberculosis.

   The most unambiguous results occurred on the final long flight home to Honolulu, with 249 passengers on board, and when the woman's illness was most severe. Fifteen passengers had positive tests suggesting a prior tuberculosis infection--either on this flight or possibly elsewhere, including six passengers with clear evidence of new infection.

   The six clearest cases included not only passengers sitting immediately behind and across the aisle from the infected woman, but also two people sitting 10 or more rows away, and one in a separate cabin. Overall 4 percent of the passengers tested on all four flights had positive tuberculin tests but investigators concluded that many were probably exposed to the disease earlier in life rather than on those particular flights.

   When a flight attendant-rather than a passenger-has tuberculosis, the risks of transmission can be higher. In case the CDC reported in 1995, a flight attendant a developed progressively severe tuberculosis lung infection that may have begun three years earlier. When investigators tested the crew who worked the same flights during the previous six months, 26 percent had evidence of tuberculosis infection, compared to 4 percent of crew randomly selected from other flights.

    Among a small number of passengers checked from frequent flyer records, 7 percent had evidence of tuberculosis infection, but again some may have be exposed to the disease elsewhere.

   These and other studies suggest that the risks for getting a serious disease are scary, real, but nevertheless very small. With about 25,000 cases of tuberculosis reported each year in the United States and fewer still in a highly contagious state, the chances are very small of sitting near a tuberculosis carrier nearby among the 421 million passengers who board 4.5 million flights each year.

   For other respiratory diseases, however, the risks are much higher, but the consequences not nearly so severe. The case in point is one of the most resourceful, slippery, and prevalent of all viruses: the flu. Every year it kills by the thousands, infects by the millions, and moves swiftly from city to city by airplane. This is likely what happened to Florence and Gerald Berman on their flight to Washington.

   However, despite the thousands of flu studies in the scientific literature, just one report concerns an airline flight, and it occurred in 1977. Nevertheless, it is a memorable account of a very bad airplane trip: A Boeing 737 was on the runway in Homer, Alaska, ready to proceed to Kodiac, Alaska, with 49 passengers and five crew members on board. As the airliner accelerated down the runway for takeoff, the left engine failed. The pilots reacted in time, and the aircraft screeched to halt.

   For the next four-and-a-half hours, a majority of passengers waited in the plane, and a few remained in the terminal. Finally a smaller aircraft was found to take most of the passengers to their destination; the remainder were sent via Anchorage, arriving in Kodiac a day later.

   Of all those on the plane when the engine failed, 72 percent came down with the flu. What epidemiologists call "the index case" was a 21-year-old woman who boarded the flight in Homer. The chances of getting the flu rose astronomically the longer the passengers remained on the same plane. Among those who deplaned when the plane first reached Homer, none got sick. But among those who spent one hour or less on the plane with the index case, half got the flu. However, for the 29 passengers who spent three hours of more on the plane, 86 percent got the flu.

   This report has been seized on by the optimists about disease transmission and airplanes. One of them is Russel Raymond, executive director of the Aerospace Medical Association, and he believes the overall risks are minimal. "If there is person-to-person transmission in the airline cabin," he declared in a safety assessment, "it is probably do to a passenger with an infectious disease who is sitting in a nearby seat or row. The same could occur in an office or in a room."

   He and others blamed the flu cases on the fact that the Homer, Alaska passengers had to spend more than three hours on the ground without the cabin ventilation system in full operation. However, this overlooks the fact that half of those who left the flight immediately after the plane returned to the terminal still got the flu. On the other hand, the rate of infection is so high that it suggests many fewer people than normal had immunity to this particular strain of the flu.

   While there may be only one scientific study, there are a lot of flu cases in any given winter. A typical epidemic, according to CDC, lasts about 10 winter weeks and can produce 25 to 50 million cases of the flu, 100,000 hospitalizations and 20,000 deaths. For those flying during the peak of flu season when 10 the 20 percent of the population is infected, it doesn't take an epidemiologist to calculate the risk-and to consider getting a flu shot. For those who escape the flu, the possibility of other respiratory infections nevertheless remains, notably the many viral variants of the common cold.

   The two best community health studies suggest that the average person gets from 2.5 to 6 respiratory infections every year, and they last an average of 7 to 13 days, requiring from 14 to 23 tissues per day. (These figures, however, include the flu.) As most parents have already learned, the colds are most frequent in preschool children (5 or more a year) and decline to less than 2 a year after age 40.

   The prevalence of respiratory infections is so high that among the 93 passengers on the typical airline flight, an average of five or more are likely to have a respiratory infection. This estimate uses the most conservative figure that each person will get 2 infections per year, each nine days long. If one doubts the large number of cases of respiratory illness in any large group, consider what takes place at practically any concert or play: During those brief intervals of unexpected silence a chorus of coughing can invariably be heard.

   Given a significant chance of sitting near someone with the common cold, what are the chances of getting the infection? The best clues come from other studies of transmission in other environments. The person in the next seat is most likely to be your spouse. And in the Cleveland Family, study the overall transmission rate between partners was 38 percent. However, transmission was rare unless the couple spent most of the day together and one partner had a moderate to severe case-a situation that does not occur on airplane flights.

   In one simple experiment with college students, transmission of a common cold virus was quite rare in a single exposure. When nine volunteers played cards and talked loudly for several hours with five subjects with a cold, none became infected. After kissing, only one of thirteen brave volunteer students became infected. (In fact, other studies show transmission through saliva is rare.)

   These simple transmission studies do not measure the additional risk of collecting people from many different environments and packing them tightly into adjacent seats. Exactly this does occur, however, in schools, which are in fact notable hotbeds for the transmission of colds and the flu.

   For example, in a study of respiratory disease in Tecumseh, Michigan, researchers found respiratory illness rates spiked for a couple of weeks every year in the fall just after school began. It is also clear, that people regularly exposed to these high-risk environments acquire immunity. Consider the stories of two airline flight attendants:

   Dan Sampey will not forget his first year of regular flying as a cabin attendant for US Airways. "From Janaury to December I had 15 colds," he said. "I'm getting these different medical tests done. They're looking at my sinuses. Then we started talking about what I do for a living, and the doctor said, 'That's it.' But after the first year the frequent colds disappeared.

   JeanAnne Griffith had a similar experience when she started out, also for US Airways. "The first year I got sick like 10 times. I was like a walking illness."

   Depending on how often they fly and are exposed to groups of people, a passenger may have resistance to infection that could range from the vulnerability of kids in the first week of school to the heightened immunity of veteran flight attendants. But especially on winter flights, the chances are good that some of the passengers will have respiratory illnesses.

   From the perspective of keeping our whole society healthy, the jet aircraft ought to be one of the places where the barriers to disease transmission are most effective, disease transmission epidemiologists note

   "Transmission by airplane can have the effect of amplifying the infection, and that is a more serious issue," noted James Koopman, an epidemiologist at the University of Michigan School of Public Health. "If you compare the number of people who might get infected if the person was at work, compared to the airplane, the issue is not only just who gets infected but how the infected individuals get connected to other people. This gives extra significance to the transmission of disease in the airline setting."

   Given these risks of confining approximately 100 strangers in tight quarters for a few hours, what does the cabin ventilating system contribute to raising or lowering those risks? Airliner cabin air is a unique environment, providing something different from what people breathe on the street, at home or in the office

   Once a modern jet aircraft gets to cruising altitude, the air the passengers get is at much lower humidity and somewhat lower pressure than typically seen on the ground. Also, cabin air is 50 percent recirculated, and about 50 percent "fresh" meaning drawn through the jet engine air intakes, compressed, and diverted or "bled" to the cabin. In addition, the fresh and recirculated air is routed through filters that can capture large droplets and small particles of all kinds. The air flows into the cabin from ducts in the ceiling and is sucked out through vents along the floor at the cabin wall.

   Compared to conditions on the ground, jet airliner provides lower humidity than practically anywhere except the driest desert cities such as Phoenix, and at a little lower pressure (equivalent of 5,500-7,000 feet) than the highest altitude cities such as Denver and Mexico City. Generally, the 17 percent humidity in aircraft cabins is less than half the 40 per cent or greater that people find most comfortable. On long flights humidity can drop to as low as 3-4 percent.

   From a disease transmission perspective the difference is mostly positive: the very dry air is tough on bacteria and fungi and any microbe that rides on tiny air droplets. On the negative side, the low humidity air dries out the nose and throat-especially on long flights--and might render people more vulnerable to infection.

 The main controversy over cabin air surrounds the decision of airlines and manufacturers to reduce the amount of fresh air to save money on fuel, and to rely on the engines to supply it. Older jetliners, such as the Boeing 727, provided 100 percent outside air ventilation and the supply was not obtained from the engines. New Airbus and Boeing aircraft provide only 50 percent "bleed" air from the engines and 50 percent recirculated cabin air.

   Furthermore, the 50-50 mix is provided under ideal conditions at cruising altitude. Less fresh air is available while taxiing, descending, taking off or at the gate. Less fresh air is available if the captain turns off one of the two air conditioner packs to save money. Less fresh air is available if the pilot does not turn on the auxiliary power unit in the tail while the aircraft is at the gate, or taxiing.

   In fact, most passengers can remember an aircraft flight where the cabin air failed the "sniff test" and smelled stale or unappealing. (What people smell is mainly body odors.) The scientific equivalent of the sniff test is to measure carbon dioxide concentrations. And the evidence shows that for portions of many flights measured carbon dioxide levels are high enough for passengers to notice a ventilation problem.

   According to the standards used by heating and ventilating engineers, most people are quite content with air quality when carbon dioxide levels do not exceed 700 ppm, or parts per million. This is a standard routinely used in the design of office buildings.

   These standards are not consistently met on airliners, according to air quality studies. For example a team from Harvard's Department of Environmental Health sampled the quality of the air on an Airbus 320. Carbon dioxide concentrations averaged around 1500 ppm-double the recommended level-and spiked to more than 2000 ppm while the aircraft was on the ground. Another study by heating and ventilation specialists found the average carbon dioxide concentrations on domestic flights was 1,613 ppm, or more that double the level when people begin to find the air unpleasantly stale.

   Not only do airliner cabins have higher than recommended levels of pollutants such as carbon dioxide, not unexpectedly the cabins have lower than recommended amounts of fresh air. The most straightforward measure is the number cubic feet of fresh air, per person, per minute, or cfm. The current minimum standard for offices, public places, theaters and transportation vehicles is 15 cfm for each person and many buildings provide much more. A few environments, including game rooms, bowling alleys, kitchens and jails require 20 or more cfm.

   Commercial jet airliners provide 10 cfm at cruising altitude, but a little less while descending, and sometimes substantially less while taxiing, taking off or at the gate. This is one third less than the 15 cfm recommended for transportation vehicles by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) the professional group that sets ventilation standards.

   But given that modern airliners don't meet ASHRAE standards, the solution has not been higher ventilation standards for future aircraft design, but rather to lower ASHREA standards. At least that is where a special ASHRAE panel is headed.

   Declaring that it was "seeking to improve the comfort of airline passengers," an ASHRAE panel has proposed reducing the aircraft ventilation standard to 5 cfm per minute, or one-third the fresh air ventilation for office buildings. The panel, however, has not yet taken final action on the new proposed standard for aircraft, and the Association of Flight Attendants union complains that the panel is loaded with 10 airline industry members and just 2 with other perspectives.

   The reason for reducing fresh air flow is to save money on jet fuel. These savings, depending on how stated, look either substantial or pitifully small. Boeing, for example, stated that 40 million gallons of fuel have been saved by using 50 percent recirculated air in its fleet of 767 airliners since they began flying. However, the Federal Aviation Administration calculated that each passenger would have to pay an extra 10 cents an hour to get 100 percent fresh air.

   Can we be sure that more fresh air ventilation on airliners would reduce disease transmission? One office building study of tuberculosis suggests that it might. The Massachusetts Department of Public Health studied the ventilation in an office building in which 40 percent of the workers had been infected with tuberculosis by co-workers. The study estimated that if the ventilation rate had been doubled the number infected would have been cut by 50 percent.

   But San Diego research architect Hal Levin noted that the disease transmission problem on aircraft may not be that simple. In a crowded cabin, he said, it would not be practical to provide enough fresh air flow to protect passengers from the cough or sneeze nearby without creating an unacceptable draft.

   His thought: instead of blowing air down on passengers and across the seats to the exhaust ducts on the floor at cabin wall, the ventilation pattern should be reversed. If ceiling ducts sucked the air out of the cabin instead of blowing it in, the opportunities for transmission might be reduced because the circulation pattern would immediately pull the contaminated air up and away from people. However, Levin noted that the problem simply had not been studied in enough depth to say with confidence what's wrong with current systems, and what a better design might look like.

   In the meantime, what can you do? No precautions have been thoroughly tested, but a few simple steps might cut the odds. The window seat has a better ventilation flow than the aisle seat, Levin noted. It is a good place to sit if you're worried about getting sick, or giving your infection to someone else.

   Also, wash your hands and watch what you touch. While respiratory viruses are launched into the air on sneezes and coughs, they are readily transmitted by the objects on which the droplets first land. (In fact, one study found the transmission rate was 10 times higher from hand-to-hand contact than from the same air.)

   In the winter season, extensive travel plans could be another reason to consider a flu shot. People with compromised immune systems, or who have a dangerous infectious disease can use a paper face mask, which are simple, cheap and quite effective.

   If you really need complete protection, consider driving the plane yourself. The pilot's compartment is blocked off from the passenger air space, and has 400 cfm of fresh air instead of 10 cfm.