4 June 2003 12:00 GMT

by Alex Venter

Airline passengers and crew need a comprehensive system to protect against infectious diseases such as severe acute respiratory syndrome (SARS), say experts. Long after a contagious passenger has got off a plane, germs left behind may cling to surfaces, hide in biofilms, and re-contaminate the cabin air.

The risk of acquiring an infection during air travel is considered low. But with stress, fatigue and madding crowds, susceptibility to disease increases. Travelers have caught tuberculosis during long flights, and other respiratory diseases also have the potential to be spread on planes.

Talking, coughing and sneezing are all ways to transmit respiratory pathogens, says Richard Wenzel, chairman of the Department of Internal Medicine at Virginia Commonwealth University Medical Center.

When someone coughs, a hoard of droplets enters the air, Wenzel explains, much like "hot air balloons encasing an organism." Mycobacterium tuberculosis (the cause of tuberculosis) and the viruses that cause influenza and measles all go airborne in this way; hitching a ride in droplet nuclei. At ten micrometers or less in diameter, droplet nuclei can stay suspended in the air for hours. Meningicoccal meningitis, conversely, is spread by larger droplets, which quickly fall to the floor. Whether or not SARS can go airborne is not clear.

In the wake of SARS, some airlines have begun offering surgical masks to passengers. Such masks are protective as long as they fit tightly, says Wenzel, which can make it difficult for some passengers to breathe. There is also evidence that respiratory pathogens can infect via the eyes.

Although the air in a plane's cabin flows from ceiling to floor, and planes are commonly equipped with HEPA filters (to remove airborne pathogens), people seated as far as 15 rows away from a passenger with an airborne disease could be at risk. Passengers within two rows of an infected individual will be most at risk, says Wenzel.

On modern airplanes, about 50% of cabin air is recycled. Air exchange is considered more frequent (every three to four minutes) than in offices, homes and probably most city buses. But "clean air doesn't help if you've got a dirty airplane," said Graham Bruce, a product development manager with the Canadian aviation manufacturing and services company AeroMechanical Services (AMS).

AMS has been collaborating with biofilm experts from the University of Calgary on research and development of AirQ, an air quality and decontamination system for airplanes. The researchers have identified several pathogenic bacteria in samples collected from air ducts, as well as numerous non-pathogenic microorganisms. Microbiologist Merle Olson at the university notes that the samples also contained fungi that could contribute to asthma and allergies.

"We find that even though we have biofilms (of non-pathogenic bacteria), viruses and fungi can get embedded," said Olson.

The AirQ system will include UV light to help sterilize air ducts, as well as targeted biocides for various surfaces.

Some airlines have already replaced cloth seats with leather ones, which are more durable and easier to clean. "The only way you can deal with this is a multi-pronged approach," said Olson.

Infection risk may be low, says Bruce at AMS, but it's a "risks and stakes problem . . . when (a disease is) potentially fatal, the stakes are that much higher."

Airline representatives were unavailable for comment.