Today's airplanes provide less fresh air than recommended, giving germs a free ride
By Thomas J. Moore, Globe Correspondent, 8/13/2000
In the Middle Ages, the bubonic plague traveled primarily by ship, as rats carrying infected fleas leaped ashore to decimate new cities. In the modern day, the commercial airliner has become the vehicle for many aspiring diseases.
Although the modern air traveler is more likely to get a respiratory virus than a deadly plague, onboard defenses are limited and may be further weakened.
Scientific studies are few and precautions minimal even though most travelers can remember getting sick soon after a crowded flight, especially in winter.
By definition, airplane cabins provide a great opportunity for diseases to acquire promising new targets. People from around the world are packed tightly into a more poorly ventilated space than any sizable group would otherwise normally inhabit.
Unfortunately, some of these people are likely to be sick. Given studies that report that typical adult comes down with the flu or a cold about twice a year, here are the risks: A typical flight last year had 93 passengers and five crew members. On the average, about five of those passengers will have a cold or the flu. In a six-abreast aircraft, that means a better than 50 percent chance that someone who is sick will be sitting within one row.
In light of these risks, what most people would want is a lot of clean, fresh air. However, what passengers get is one-half or less the fresh air found in a typical modern office building. And that's under ideal conditions. There are other times - at the gate, waiting for takeoff, or during descents - when there is even less fresh air, sometimes much less.
Tests monitoring air during flights have found unpleasant (but not unsafe) buildups of carbon dioxide during some portions of flight. The engineers describe this as a buildup of ''bioeffluents and volatile organic compounds.'' We would perceive it as body odor.
But high levels of ''bioeffluents'' are likely to signal the presence of other hazards of poor ventilation, including increased risk of disease transmission and higher concentration of chemical pollutants.
Ventilation is measured in cubic feet of fresh air available to each person per minute - a measure known as cfm. A modern office building provides about 20 cfm of fresh air. The minimum standard is 15 cfm. That is also the voluntary standard for transportation vehicles, including airliners, buses and subway cars, according to ASHRAE, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers.
Newer jetliners provide 5 to 10 cfm of outside air taken from the engine intake compressors; it is considered fresh, although some have worried that it may sometimes contain chemical pollutants from the engine. Airliners provide an additional 10 cfm of recirculated and filtered air. The use of engine and recirculated air results in greater fuel efficiency, Boeing has said. The FAA, however, noted that it would cost only nine cents per passenger per hour to provide 100 percent fresh outside air.
Indeed, for years, aircraft provided 100 percent fresh outside air. But as manufacturers built airplanes more fuel efficient and cheaper to run, outside air was reduced. The Boeing 727 was the last jet made to provide 100 percent fresh outside air.
Although modern jet airlines do not meet the 15 cfm standard, so far the proposed solution has been to lower the allowable standards to 5 cfm for commercial aircraft, rather than recommend more ventilation on new airplane designs. That idea was backed by Boeing and the chairman of the special ASHRAE panel convened to consider the aircraft issue.
Not everyone is happy with that solution. Flight attendants and others oppose lower standards and have criticized ASHRAE for loading the panel with pro-industry members. The engineering group responded to criticism by including more members without industry ties, and saying the ventilation standard is still undecided, subject to the views of the newly constituted panel.
One panel member - air quality consultant Douglas Walkinshaw of Ottawa - has completed research suggesting that a densely packed airliner could require more ventilation than an office building, not less.
Walkinshaw decided to measure the bad stuff - bioeffluents - rather than good stuff, fresh air, which is usually studied. Even given the same amount of fresh air per person, he found that the level of bioeffluents was two to three times higher in a crowded aircraft environment, compared to a typical office. For brief periods when people first board, the levels could be eight times higher.
Given how many passengers worry about getting sick from a flight, surprisingly little research has been done. For example, the medical literature includes just one case study of flu transmission on aircraft - a 1979 study of an Alaskan flight in which 72 percent of the passengers came down with the flu, but only after being marooned together for three hours on the ground.
A handful of studies have focused on flights with a passenger severely ill with tuberculosis. (Only a few other passengers were apparently infected, but the total included not only people nearby but also some sitting many rows away.)
Even elementary engineering questions have not yet been adequately investigated. One key defense against disease transmission is that the recirculated airplane air goes through a high-efficiency particulate filter, which is capable of capturing most bacteria and many virus particles.
To get some base-line data for a study of how to reduce microbial contamination, John Moorehead of the Battelle Memorial Institute got permission to inspect the filters of six aircraft being brought in for scheduled servicing. (The filters are supposed to be replaced after 12 months.) In five of six planes, a noxious mixture of yeast, fungus, and bacteria had grown all the way through the filters.
As a result, the downstream ducts ''were dirtier than a shower room floor and worse than rest room door handles,'' he concluded. Moorehead noted that a properly installed and sealed filter is very effective against micro-organisms. But he said that this accidental finding shows that filters need to be serviced more frequently - although how much more frequently he could not say from this small sample. It is also surprising that the aircraft industry has not done the studies to determine how often these key filters ought to be replaced.
Also conspicuously missing are basic ventilation and disease studies to guide the design of future aircraft. The greatest disease risk to passengers, Moorehead noted, likely comes from someone sitting nearby.
However, the present practice of locating the air exhaust ducts on the floor, at the fuselage, draws air laterally across the seated passengers. A different design could result in an airflow that was straight up, or straight down, so that fewer passengers were exposed to air in which another may have coughed or exhaled. But would this reduce disease transmission? It ought to, but again there are no studies.
Meanwhile, the authorities have been combating public concerns with public relations instead of science. For example, the Federal Aviation Administration responded to concerns about the high levels of carbon dioxide by saying that levels were much lower than the limits of the Occupational Safety and Health Administration.
That was true, but the OSHA standards were for bakeries and breweries, where fermenting yeast produces unusually large amounts of carbon dioxide.
Boeing noted that its airliners provided more ''air exchanges per hour'' than surgical operating rooms. But because so many more people are sharing that fresh air on an aircraft, the supply per person is less than half the minimum recommended for jail cells.
It is long past time to begin a program of independent scientific research. We need to know how frequently diseases are transmitted through air travel, and what diseases provide the greatest risk. We need to study how they are transmitted, whether through recirculated air, between passengers, or from the seats and other objects people touch.
With this base-line data it would be possible to develop better aircraft designs. Help could come from other policies, such as allowing people to reschedule flights without penalty when sick with a cold or the flu. Our society has developed a splendid system to prevent the most obvious danger of air travel - a crash. The same scientific tools should be used to reduce the risk of getting sick.
In the meantime, Moorehead says, people who are seriously concerned about not getting sick on the next flight - or who are immuno-compromised - should consider wearing a paper surgical mask. It is low-tech, cheap, and very effective. While such a precaution might draw some odd looks in the United States, he notes, in Japan people with colds routinely wear paper masks on public transportation.
