Spread of SARS prompts worries over safety of air travel

Experts say cabin filters curb airborne diseases, but they can break down

April 12, 2003|By Julie Bell | Julie Bell,SUN STAFF

When at least 13 people on an Air China flight from Hong Kong to Beijing developed a mysterious respiratory illness last month, health officials scrambled to track down the other passengers and warn them to call a hot line if they got sick.

On Wednesday, Hong Kong officials put out an alert for passengers on a Cathay Pacific Airways flight to Taiwan after another traveler contracted what appeared to be the same disease.

In places where severe acute respiratory syndrome (SARS) is spreading, officers are screening passengers before they board outbound planes. Airlines are offering protective masks at ticket counters.

In the United States, the Centers for Disease Control and Prevention hands out "health alert" cards to travelers returning from China, Singapore and Vietnam.

This frenetic activity has left air travelers with a critical question: Is there something about the design of airliners that promotes the spread of SARS?

Public health officials don't think so. They believe it's spread primarily by close contact with someone who has it - such as sitting within a row or two of someone on an airliner. But that doesn't mean the airplane is to blame.

"My understanding is at least the suspect cases which have occurred are as a result of being close to someone - in an adjacent seat or a seat ahead - who has been confirmed with SARS," said Dr. David Brandling-Bennett, deputy director of the World Health Organization's regional office in Washington.

Overly cautious

Public health authorities say that if anything, they're erring on the side of caution as they try to contain SARS, now suspected of killing at least 116 and sickening nearly 2,900 people worldwide.

Engineers familiar with airliner design said that many cabin ventilation systems are designed to eliminate small particles and impurities, including viruses. And their circulation pattern doesn't favor the spread of disease.

In many planes, filters can weed out particles as small as .3 microns, meaning all but the tiniest viruses are trapped before air moves back into the cabin. SARS droplets, according to Brandling-Bennett and other experts, are almost certainly too big to pass through.

David R. Space, a Boeing engineer working on industrywide cabin air quality standards, said Boeing planes typically circulate the air in a way that minimizes passengers' exposure to germs.

About 50 percent of the air is pulled in from the outside and ultimately exhausted, while the other half is recirculated.

The air comes in streams from nozzles above passenger seats that direct it left and right in a circular motion that hugs the fuselage - instead of running the length of the plane. The air flows out of the cabin through a filter under the floor.

"If someone is sick in row six, you don't spread the illness to row 15," Space said. "They're the same type of filters used in hospital operating rooms."

But others note research showing that filtration systems can contribute to the spread of disease if they break or they're poorly maintained.

Filter damage

John Moorehead, a chemical engineer and researcher with the Battelle Memorial Institute in Ohio, said he studied filters in a handful of Boeing 737 jets in 1999 and found they can become a source of contamination if they are not changed often enough.

In some cases, he said, yeast, fungus or bacteria growing on the intake side of the filter grew through to the other side after five to 15 months - allowing microbes to be blown into the cabin.

Likewise, a 2002 National Research Council study warned that the filters used to process cabin air "could be a source of microbes."

But it noted that "infectious agents that are transmitted from person to person generally grow poorly outside the human body, so contamination in an aircraft cabin is unlikely to be a source of them."

There is evidence that disease spreads more rapidly when filtration systems are turned off or aren't working properly.

One case mentioned in the American Journal of Epidemiology involved an Alaskan flight from Anchorage to Kodiak on which 54 passengers were stuck in the cabin for three hours after a mechanical problem crippled the ventilation system. Some 72 percent of the travelers contracted influenza within three days.

Close quarters

But overall, researchers say, it's the close quarters rather than any flaw in aircraft design that promotes the spread of SARS.

"Certainly you're at much greater risk if you travel than if you stay home," said Herbert L. DuPont, chief of internal medicine at St. Luke's Episcopal Hospital in Houston and the co-author of a textbook on travel medicine.

But he said, you would have a greater chance of getting hurt jogging on the streets of Baltimore or speeding to the airport than you would have of catching SARS on an international flight.

Sun staff writer Scott Shane contributed to this article.

Hopkins on alert

Because so many foreign visitors come to Johns Hopkins Hospital, the institution is warning visitors and patients to be on the lookout for symptoms of SARS.

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