Tracking Down Onboard Contaminants

On May 12, an American man with tuberculosis flew to Europe, possibly exposing passengers and crew to an extremely drug-resistant form of the disease. Authorities are only now in the process of contacting those who sat closest to the infected man so that these individuals can seek medical attention.

Imagine if those exposed could have been notified before the man even got off the plane. While there are many hurdles to overcome before the widespread use of sensors for tuberculosis and other diseases becomes practical, researchers have now solved one part of the problem by creating a mathematical model that pinpoints the source of a contaminant to an area the size of a single airplane seat.

“Our model really gives you that kind of precision,” says Qingyan Chen, principal director of the Federal Aviation Administration’s Air Transportation Center of Excellence for Airliner Cabin Environment Research and professor of mechanical engineering at Purdue University.

Zeroing in on the source of a contaminant is key for public safety because it’s those closest to the problem who are most likely to be affected. For example, authorities from the Centers for Disease Control and Prevention are most interested in speaking to those passengers who sat in the two rows surrounding the man who recently flew while infected with tuberculosis.

The spread of Severe Acute Respiratory Syndrome (SARS) from one region to another, sometimes by plane, further shows how important such a system could be for public health. “In the Air China flight from Hong Kong to Beijing in 2003, twenty passengers were infected with SARS, and four of them died,” Chen says. The faster contagious individuals can be identified, the easier it will be to stop the disease from spreading.

The terrorist attacks on September 11, 2001, also prompted many governments to consider using chemical sensors in public places. Some U.S. subways, including stations in Boston and Washington, DC, now have such sensors installed. But Chen says that these systems are intended to identify the presence of a contaminant and not pinpoint the source. (For security reasons, officials for the subway systems declined to say how the sensors work or what agents they can detect.)

If accurate systems were installed on commercial airlines to identify the source of biological and chemical agents, planes might be a less likely target for such attacks, Chen says.

To develop his air-flow system, Chen and mechanical-engineering doctoral student Tengfei Zhang used a full-scale reproduction of a plane cabin with four rows of seats in three different configurations. Special heaters on seats recreated the effect of body temperature on air flow, and passenger exhalation was simulated using tubes that emit gas. Instead of a harmful chemical agent or virus, Chen used a harmless tracer gas to act as the contaminant. By studying the way the tracer gas moved around the cabin, Chen’s team was able to develop a computer model of the fluid dynamics involved. (A detailed account of Chen’s research will be published in the June issue of the scientific journal Indoor Air.)

Chen says his studies suggest that the number of passengers on board the plane doesn’t significantly affect air flow. But the seating arrangement and ventilation system can change the way a toxin travels. Consequently, a new model would need to be created for each type of plane and all possible seating arrangements. Sensors for various biological and chemical agents would also need to be installed throughout the cabin. But Chen says that for any single contaminant, only one sensor is needed for every nine rows of seats.

Once a sensor identifies a contaminant, Chen’s software finds the source using the air-flow model. Essentially, the system tracks the contaminant backward as it travels from sensor to source in a process known as inverse simulation. Chen says that in the event that a contagion or toxin is detected while the plane is in the air, the flight attendant could ask all passengers to put on a mask for their protection. Once the plane has landed, the passenger thought to be responsible for the release of the contaminant could be questioned by the appropriate authorities and the plane decontaminated.

Chen’s inverse-simulation system could be ready for real planes in two years, he says. But first, he has to make it identify sources faster. “At present, the inverse modeling takes a few days for a twin-aisle cabin. Thus, the technology is only good for aftermath analyses,” he says, adding that his system is currently limited by computer processing power and its own design.

Chen says that he’s currently working on perfecting the algorithm. He also plans to switch from a central processing unit to a graphics processing unit, as the latter is far faster when handling this kind of simulation. Ultimately, Chen says, his system will be able to detect a contaminant in one minute, and he calls that estimate “rather conservative.”

Still, to really safeguard public spaces like commercial airplanes, sensor technology will also have to improve. “Most chemical agents can be detected by sensors in real time, but not biological agents,” Chen says. “However, the development on biological sensors has momentum. As soon as we have biological sensors that can give us real-time information, we can use it for our system and protect passengers and crew.”