THURSDAY, March 10, 2005 (HealthDay News) -- As the global outbreak of SARS showed, dangerous infections can now spread from continent to continent in hours, thanks to modern jet travel.
That speed of transmission may be made even easier because of poor air filtration in many planes, new research suggests. In fact, experts say 1 in every 7 large commercial jets still doesn't have HEPA filters in place that can help prevent passengers from breathing in airborne pathogens.
"It's not mandated and it probably should be required," said David Ozonoff, a professor of environmental health at Boston University School of Public Health. He is also the co-author of a commentary on a new study on airplane air quality, published in the March 12 issue of The Lancet.
For any given flight, risks of airborne infection are probably restricted to a pasenger's ability to pick up that nasty cold from the guy sitting in 24G, said Dr. Mark Gendreau, co-author of the study and a senior staff physician at the Lahey Clinic in Burlington, Mass.
"Although the risk is present, it's very low," he added. "The current systems keep the risk of transmission to a minimum."
Still, he and Ozonoff agree that the threat of bioterrorism or a fast-moving pandemic, aided by air travel, remains a distinct possibility.
People are traveling more than ever before, they note, and to more disparate locations. Approximately 1.8 billion people traveled on aircraft in 2004, and the number is expected to increase 6 percent each year until 2008.
"There are a lot of people who travel, and more people traveling to developing countries where emergent infections seem to come out of," Gendreau said.
Back in the 1300s, it took three years for bubonic plague to get from southern Italy to Britain, Ozonoff noted. Today, it would take only hours.
Furthermore, the interior of the average plane represents a closed environment that in many ways is ideal for disease transmission. For one thing, air is re-circulating and travelers are mixing with people from a wide geographic area.
The result? Passengers are vulnerable for the duration of the flight. "You can't move, so when the person next to you has an explosive cough or sneeze, you're out of luck," Ozonoff said.
That much was confirmed by Gendreau's analysis of one planeload of passengers who were tragically impacted by SARS, the respiratory disease that killed more than 700 people and sickened more than 8,000 in a global epidemic in 2003.
Of 40 flights investigated for carrying SARS-infected passengers during the 2003 outbreak, five showed evidence of on-board transmission. One in particular -- a March 15, 2003 Air China flight from Hong Kong to Beijing -- accounted for 22 of the 37 people known to have contracted the disease following air travel, in a phenomenon the researchers called "superspreading."
That particular incidence was unusual, in that the flight was only three hours long and individuals sitting a full seven rows away from the "index" passenger became infected.
That's surprising, the researchers said, because airborne disease is most likely to be transmitted within two rows of a contagious passengers, on flights more than eight hours long.
In their review of the available literature, the authors unearthed documented cases of other airborne disease transmissions, including tuberculosis (the risk is about one in 1,000 when a symptomatic source is present), flu, and outbreaks of food-borne illnesses, which remain the most common.
No common cold outbreaks have been reported, probably because the illness is so widespread it would be difficult to trace, they said.
But according to Ozonoff, the literature is "surprisingly sparse," so airborne outbreaks are probably more frequent than reported.
The relative safety of high-flying air quality seems to be largely a matter of luck, he added.
That's because only 85 percent of commercial airlines carrying more than 100 passengers have High Efficiency Particulate Air (HEPA) filters, which are currently not required by U.S. government regulations, he said.
HEPA filters capture anything 0.3 microns or larger, Gendreau said. That means all bacteria as well as fungus, spores and pollutants. Viruses are smaller than 0.3 microns but they usually travel in clumps or on dust particles that are captured by the filters.
It remains unclear if the SARS-linked Air China flight had a HEPA filter on board, Gendreau's team note.
Part of the problem, Ozonoff explained, is the balkanization of infectious disease regulation on airplanes. For example, the U.S. Environmental Protection Agency regulates water supply on domestic flights, while the U.S. Food and Drug Administration regulates ice, he pointed out. The U.S. Department of Agriculture gets involved in vector-borne diseases that might affect animals or crops, but not humans.
"You've got this patchwork quilt of regulations, and lots of stuff goes through cracks," he said.
Things may be looking up for worried plane passengers, however: Gendreau believes the U.S. government will soon begin requiring HEPA filters on all planes and issuing regulations related to ventilation. Also, he said, the World Health Organization is now modifying its international health regulations to require more stringent reporting of disease outbreaks.
Disinfection of aircraft before disembarking would also help reduce the risk of disease transmission.
While international health regulations recommend disinfecting all planes flying out of areas endemic for malaria and other vector-borne disease, only five countries actually do so, Gendreau said.
Travelers can minimize their own risk by frequent hand washing, too.
And in the end, the Massachusetts researcher stressed, on any given day, air travel remains safe for the vast majority of passengers.
"When you think about how many people are traveling, the risk is present, but it's manageable and it's low," Gendreau said. "It's not anything to get hysterical about."
More information
For more on air safety, try the U.S. Federal Aviation Administration.
