(Inside Science) -- When officials at the World Health Organization (WHO) declared the new H1N1 strain of influenza to be a pandemic last week, they also offered advice on how to stay healthy. It is safe to travel, they advised, including on airplanes.
New research in the field of fluid dynamics, which may help further reduce the risk of infection in the future, shows how germs like influenza viruses spread on airplanes and other forms of public transportation.
Planes pack lots of people into highly confined spaces, often for hours at a time, and in them people share the same air over and over again. However, said Michael Plesniak, a fluid dynamicist and chairman of George Washington University’s mechanical and aerospace engineering department in Washington, D.C., airplane ventilation systems are highly controlled and designed to direct air flow and limit passengers' exposure to influenza and other dangerous airborne particles.
Commercial aircraft maintain good air quality by circulating cabin air through HEPA filters that remove germs and other dangerous particles, added Plesniak, who studies how germs circulate with the air on airplanes.
"Aircraft filtration systems are very effective," Plesniak said after having recently flown on a long commercial flight himself. Vents in a plane's ceiling direct air out and down toward the floor below the windows, creating a swirling flow pattern within each row of seats that aims to keep contamination to a single row or, at worst, its next-row neighbors.
Last year, Plesniak and his colleagues at Purdue University in West LaFayette, IN, reported the results of an experiment that showed how people moving up and down the aisle of an airplane can disrupt the carefully designed air flow pattern from the plane's ceiling vents and spread infectious particles farther. They found this out by building a scale model of an aircraft cabin and immersing it in a water-filled tank. Because both air and water are fluids, the flowing water in the tank simulated moving air through the cabin. By injecting fluorescent dye into the liquid, they could simulate the release of germs and map their flow through the cabin.
They found that moving passengers create eddies in their wakes as they walk that can “entrain” and spread contaminants much farther. "If someone happens to cough or sneeze while a person is walking by, the walker’s wake can carry the germs seven to eight rows," said Qingyan Chen of Purdue University, who collaborated with Plesniak.
Swirling eddies can create stagnant zones where air doesn’t flow as freely. Germs may be trapped at just the wrong place -- at the nose and mouth level where seated passengers breathe, said Plesniak.
This may explain the spread of SARS on a plane during the outbreak in 2003. One person on a flight from Hong Kong to Toronto infected 20 other passengers, four of whom later died from the disease. Passengers as far as seven rows away were infected.
After the SARS outbreak, Chen's laboratory began working with the Federal Aviation Administration to study how infections spread through airplanes. Could they improve the airflow pattern by delivering the air from somewhere else than the ceiling?
One idea was to create personalized air distribution systems that would supply air from the seat back in front of each passenger. Another was a displacement system under the floor that would supply the air from the level of the passengers' feet and drive contaminant particles with the flow out the top of the plane.
These systems would also help protect individual passengers from their nearest neighbors’ viruses. In fact some infectious disease specialists believe that because modern plane air-filtration systems are so effective, most infections caught on planes come from a near neighbor, before the air has been re-circulated through the plane filtration systems.
The problem of flow patterns isn't restricted to airplanes. "Particles, especially smaller ones, will be transported by the flow pattern in [any enclosed space]," said Shengwei Zhu, a researcher at Harvard University in Cambridge, MA.
Last year, Zhu studied the air quality on Harvard university shuttle buses, taking measurements of carbon dioxide every 10 seconds for one day as one bus went along its route through the Cambridge campus. He found that when the doors were closed, the carbon dioxide levels increased very quickly. This may indicate that the buses’ ventilation systems can’t sufficiently mix new air with old -- which may increase the risk for influenza transmission.
Research like this may help in the improvement of air handling systems for future public transportation systems. For now, scientists recommend staying away from sick people and keeping clean.
The World Health Organization advocates the following simple practices to prevent catching influenza:
- avoid close contact with people who show influenza-like symptoms
- avoid touching your mouth and nose
- regularly wash your hands thoroughly with soap and water or an alcohol-based cleanser
- reduce the time you spend in crowded settings if possible
- improve airflow in your living space by opening windows
- get adequate sleep, eat nutritious food, and keep physically active