Let's start with the frightening part: There is considerable scientific evidence suggesting that airliner passenger cabins are places where infectious disease and respiratory illness spread among people more often, and more easily, than in other environments.
The comforting news, if it can be called that, is that the cabin's air circulation system, long targeted by frequent flyers as the source of their ills, is probably not to blame.
The proven transmission of SARS on aircraft adds new urgency to a question: are airliner cabins hot zones for disease?
On May 22, the World Health Organization (WHO) raised to 27 its estimate of the number of people worldwide who became infected with severe acute respiratory syndrome, or SARS, during an airline flight. Earlier estimates had put that number at 16. Twenty-two of those 27 were infected during one flight, Air China's Flight 112 from Hong Kong to Beijing on March 15, the WHO said.
Contrary to earlier reports that SARS infection was a risk only within two rows of an infected person, the WHO said passengers sitting seven rows in front of and five rows behind the carrier were infected. While WHO officials declined to speculate on the mechanism for the wider distribution of the virus, independent experts believe that transmission may have occurred as infected people moved around the cabin, or as flight attendants (four of whom are among the 27 infected in flight) unwittingly passed the germ among passengers.
No in-flight SARS transmission has occurred since March 23, due largely to preflight passenger health checks instituted by airports and airlines serving SARS-affected areas, according to aviation officials.
But the transmission of those 27 cases raises two nagging questions: Should -- or can -- the airlines do anything to prevent the onboard spread of the many other conditions that, like SARS, are transmitted between passengers by sneezes, coughs, touches and other unsavory germ-launching mechanisms? And does anyone really know whether pathogens are spread via airliner cabin ventilation systems?
The National Research Council, an arm of the National Academy of Sciences, looked into the aircraft cabin air quality issue two years ago, and in January 2002 issued a report urging the Federal Aviation Administration to impose stricter controls. The House Aviation Subcommittee has scheduled a hearing on cabin air quality for June 5.
The National Research Council offered no conclusive link between airborne pathogens and passenger health, but wrote, "Available exposure information suggests that environmental factors, including air contaminants, can be responsible for some of the numerous complaints of acute and chronic health effects in cabin crew and passengers."
Clean Air Acts
The cabin air in most commercial aircraft is a 50/50 mix of air taken from outside the aircraft during flight and recirculated air from within the cabin.
The outside air is first pressurized in the aircraft engine compressors, explains David Space, an air cabin quality research scientist at Boeing. That air is then mixed with recirculated air from within the cabin. The recirculated air normally passes through a high-efficiency particulate air (HEPA) filter, the same filters used in hospital operating rooms, before it is mixed with outside air, Space says. HEPA filters were designed in the 1950s for use in nuclear reactor facilities and "are basically 100 percent efficient," says William Nazaroff, an environmental engineering professor at the University of California, Berkeley, and an author of the NRC cabin air quality report.