Feature

Detecting infectious agents aboard airliners

Grace M. Hwang, David Van Cleave, Michael Harkin, Elaine Mullen

In 2007, more than 80 million passengers flew to and within the continent of Africa on over 900 thousand flights. Unfortunately, many of those passengers carried more than just luggage—some of them brought or acquired illnesses that were spread during the journey. The health hazards associated with international disease transmission were highlighted in March 2003 when 22 out of 120 passengers were infected with Severe Acute Respiratory Syndrome (SARS) during a three-hour flight from Hong Kong to Beijing. The SARS outbreak of 2002-2003 affected five continents and 18 countries and resulted in 774 fatalities. The economic impact on Asia alone was $11 billion.

A lesser known but relevant incident took place in 2004 when a passenger with Lassa Fever traveled from West Africa to the U.S. and subsequently died a few days later. The U.S. Center for Disease Control and Prevention investigated the incident and identified 188 at-risk persons, 19 of whom were travelers seated close to the infected passenger. The Lassa Fever incident suggests that one cannot always rely on airline passengers to know about the status of their health—or to make the right decisions when they do.

Lassa Fever is not the only known disease prevalent in Africa that can be transmitted from animals to humans. Viruses such as monkey pox, yellow fever, dengue fever, rift valley fever and bacteria such as mycobacterium tuberculosis and bacterial meningitis are potentially transmissible on an airplane. Although there are a few other vector-borne diseases prevalent in Africa such as lymphatic filariasis —caused by a microscopic, thread-like worm that is transmitted by mosquitoes; leishmaniasis—caused by a parasite spread by phlebotomine sand flies; trypanosomiasis—caused by a parasite spread by tsetse fly; and malaria; these parasites have a very low probability of being transmitted on an airplane.

To protect the health of travelers, more research should be done on the development of an affordable, rapid, onboard biosensor that would alert passengers to the possibility that they may have a serious infection, especially when some symptoms take up to six months to develop.

In 2006, a study by the U.S. National Research Council evaluated two types of systems to counter bio-threats on airplanes: a continuous air decontamination system for the near-term and a detect-to-prevent system for the longer term. The MITRE Corporation, which manages three Federally Funded Research and Development Centers (FFRDCs), is conducting research on detect-to-prevent systems for infectious diseases onboard aircraft.

Detecting pathogens during flights

Although sensors and analysis techniques that can detect pathogens in air samples already exist, many systems give false alarms once every 100 tests. If the system we envision is to be deployable, the biosensor will have to operate at a near zero false alarm rate. We’re aiming for one false alarm per million flights. Furthermore, most biosensors cannot identify a threat agent in less than ten hours. This research is aimed at detecting onboard biothreats automatically in less than two hours during flight.

To bring together the many aspects of this biosensor design, MITRE formed a multidisciplinary team with the US National Center of Excellence for Research in Intermodal Transport Environments, and the University of California, San Diego. The team will be examining three areas:

1. Aerobiology.

Engineers, physicists and biologists working in the field of aerobiology develop methods to collect, concentrate, and separate air-borne particles. For this project we developed a fluid dynamics model that simulates the velocity and trajectory of particles exhaled by passengers. Modeling particle transport from human exhalation (e.g., coughing, and sneezing) under realistic flight airflow conditions in combination with a bio-collector allows us to compute the mean particle concentration per cubic meter of air. This is critical in establishing a minimum probability of detection of the biosensor (i.e., how many particles are needed for the biosensor to detect a pathogen in an airliner cabin).

Modeling also enables us to determine the optimum number and placement of aerosol collectors and biosensors in an aircraft. To get an idea of the sensor system’s sensitivity, an early-stage test will involve putting a small number of collectors and biosensors in a short wind tunnel that MITRE provided to the University of Massachusetts Lowell. An atomized mist of safe biological particles will be generated by a biological aerosolizer. This mist will be pulled through the wind tunnel by a fan while researchers measure the biosensor performance as a function of airflow velocity, humidity, temperature, and particle sizes.

2. Biochemistry and Microbiology.

Biochemists study the makeup of complex carbohydrates on human cells and the proteins that bind to them. Microbiologists study pathogens and mechanisms called “virulence factors” that pathogens use to cause disease. Virulence factors include adhesive proteins on viruses and bacterial cells that bind to human tissues. By combining our knowledge of biochemistry and microbiology, we will exploit certain adhesive proteins to capture infectious pathogens such as tuberculosis and influenza. Biological receptors suited for reuse in the onboard sensor will be identified. Receptors that use the least amount of consumable liquids will be selected so that flight attendants don’t have to become involved with the chemistry.

Isolating pathogens among the hundreds of naturally occurring microbes in the air is another part of the problem. The device selectively captures pathogens by taking advantage of a biological phenomenon where specific complex carbohydrates that coat our cells bind to certain bacteria and viruses. When a target pathogen comes in contact with its receptor carbohydrate, the binding causes a change in density in the liquid that is detected by the biosensor.

3. Optics

Optics are a central part of the biosensor which, in this device, is a complex system that focuses polarized light through the liquid onto a gold surface with a grid of thousands of nanoholes—holes a few billionth of a meter in diameter. The interaction between the gold nanohole grid and the laser light causes electrons in the metal to form a charge density wave at the interface of the gold and liquid. This charge density wave is also known as a surface plasmon polariton (SPP) wave and is very sensitive to tiny changes in the index of refraction of light. As the particles flow over receptor carbohydrates attached to the perforated gold surface, the sensor monitors changes in the SPP, which correspond to changes in the index of refraction of the fluid due to pathogen binding. The magnitude of the index of refraction change is translated to quantify the biothreat.

Future research

The MITRE team is continuing to study how air flows in an airplane cabin, how to collect and concentrate particles in the air, and how to identify pathogens among other particles within the air. Based on initial analysis, it’s likely that multiple biosensors with near-single particle sensitivity would need to be placed throughout an aircraft to optimize detection probabilities. Additionally, to be considered operationally acceptable by the airline industry, the sensor system must be reliable (no more than one false alarm per million flights), rapid, small, and relatively inexpensive. In the future, MITRE’s biosensor research will parallel systems engineering analyses, which will quantify the operational impact and the cost-benefits of medical intervention in international air travel.

Glenn Roberts, chief engineer for MITRE’s Center for Advanced Aviation System Development, which manages an FFRDC for the U.S. Federal Aviation Administration, says: “This research area has a lot of promise. When we prove that an accurate sensor can be built affordably, then we’ll want to see what sort of concept of operation would make sense and we’ll work on that with our government sponsors.”

More information:

 Grace M. Hwang, MITRE Corporation, Bedford, Massachusetts, USA, (+1) 781-271-2165, gmhwang@mitre.org 

http://acer-coe.org 

http://www.africaguide.com/health.htm 

http://www.africa.upenn.edu/health/diseases.htm 

Pang L, Hwang GM, Slutsky B, Fainman Y (2007), Spectral sensitivity of two-dimensional nanohole array surface plasmon polariton resonance sensor, Applied Physics Letters, 91(12):123112.

Hwang GM, Dicarlo A, Gene L (2008), Detecting infectious and biological contaminants on aircraft – Is it Feasible? Proceedings: Eighth annual IEEE International Conference on Technologies for Homeland Securities, May 2008.

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