A recent post of mine seemed to briefly address the issue of the spread of infectious disease during air travel, without giving too much “meat” behind it. I thought there might be some interest in this subject and I would like to refer to an outstanding article, from the Lancet 2005; 365: 989-96, written by Mangili and Gendreau.
Clearly, the number of people who travel via air, is a staggering number with annual estimates well over 1,000,000,000 worldwide and over 600,000,000 in America, alone. Air travel is exciting and can be the most enjoyable part of the trip for some, but it also places one at risk for infectious disease.
The Cabin Environment
The spread of infection in commercial aircraft cabins is subject to several factors. The environment of the the cabin itself is perhaps the most important. The cabin, offering a pressurized environment and low humidity, requires that passengers and crew are in close proximity of each other. When parked, the fresh air is supplied via an auxilary power unit but during flight, fresh air is sent into the cabin from the engines where it is has been heated and compressed; outside air at typical cruising altitude is presumed to be sterile. Air enters the cabin from overhead, circulating downward and exiting from the floor. This is designed to help limit the spread of airborne particulate through the cabin.
Most commercial airliners use 50% re-circulated air for improved fuel performance and humidity control. This re-circulated air is filtered with a HEPA (high efficiency particulate air filter) system. Several studies have shown that standard cabin air is exchanged 15-20 times per hour compared to 12 per hour in a typical office. A survey demonstrated that most airlines use HEPA filters, although it is not a federal requirement.
How Disease is Spread
Before we look at specific diseases, transmission of disease should be understood. Mangili and Gendreau offer 4 “modes of disease transmission”: Contact, Aerosolized, Common Vechicle (food and water) and Vector. “Contact” is just what you’d think, direct contact with a person or their body fluids. This also includes direct contact with a contaminated intermediate host, like a fomite. Large droplets (>5 microns) also fall into this category and can be transmitted when a person coughs, talks and/or sneezes. These particles travel a short distance (less than 1 meter) and land on a mucous membrane like the eyes or nasal passages or fall quickly to land on a lower surface. Aerosolized droplets are considered <5 microns and can disperse widely, remaining suspended in the air for indefinite periods of time. Contact and Aerosolized are key transmission routes for airborne illnesses, such as TB, SARS and Influenza. Remember to stay hydrated, which will help keep those mucous membranes (nasal passages, lips and eyes) moist and not dried or cracked.
Tuberculosis is a major global health concern and subject of recent media attention related to travel. An excellent reference for learning about this can be found in the World Health Organization publication called: Tuberculosis and Air Travel: Guidelines for Prevention and Control. TB has offered the most information and is perhaps the most studied of all infectious disease on aircraft. Two of the seven studies, cited by Mangili et. al, demonstrated a probable link with onboard transmission. One specific study found the index (first) case to be a flight attendant and of the almost 300 exposures on the flights, there were only two documented skin test reactions. A second case study involved a passenger infected with TB who flew from Baltimore though Chicago to Hawaii. Four of fifteen passengers, all seated within 2 rows of the index case subsequently had positive TB skin tests. While the transmission of TB on an airliner is obvious, it is of interest that there has never been an active case of clinical tuberculosis acquired from air travel.
The association of influenza and airline travel extends far beyond the infection of passengers. Airlines are a major contributor towards the rapid migration of infected people throughout the world. Influenza can spread, with the assistance of commercial airline travel, around the globe in a few hours. One particular study of interest, was the case of Influenza A/Texas Strain outbreak on a commercial airliner in 1979. The authors of the paper found that 72% of the passengers were infected with influenza within 72 hours of flight. They went on to state that the most likely reason for this high attack rate was the 3 hour ground delay with a non-functioning ventilation system, during repair work. The passengers were kept on the plane the entire time.
SARS (sudden acute respiratory syndrome) has also contributed to the study of inflight disease transmission. This atypical pneumonia-like illness is caused by a virus. There are obvious connections with the spread of SARS and in-flight infections. The case of interest, featuring SARS spread within aircraft, is of a flight from Hong Kong to Bejing. This flight was only a 3 hour duration and there were passengers infected as far away as 5 rows infront and 7 rows behind the index case. Luckily, there have been no reported cases of SARS on-board transmission, since March 2003. This is largely due to the WHO guidelines on SARS containment aboard aircraft.
Meningitis and Measles also deserve to be mentioned. Measles caused a recent scare aboard a flight from San Diego to Hawaii and there were 21 suspected cases of air travel associated meningococcal disease from 1999-2001.
Let’s not forget the food-borne illnesses, such as those spread by the “fecal-oral” route. A very well-written article titled Hygienic Quality of Food Served on Aircraft presents a wealth of information on the topic. For example, a total of 41 food-borne outbreaks aboard aircraft were documented from 1941 to 1999. These outbreaks infected almost 4000 people and caused 7 deaths. There have even been several cases of Cholera transmission in-flight. Most cases involve cold food dishes served in-flight and prepared by local suppliers. Thankfully, the last 5 years have seen a decrease in food-related illnesses, likely due to more common use of frozen and pre-packaged meals.
The authors of “Transmission of infectious diseases during commercial air travel” have noted several factors that place one at risk for airline acquired infectious disease. First, as with real-estate, location is key. Sitting within 2 rows of an infected person for a duration of 8 hours is the key. Second, ventilation of the cabin has an obvious role. One air exchange of the cabin removes 63% of the airborne organisms. Remember there are normally 15-20 air exchanges per hour in a commercial aircraft, versus an office with 12. One investigation involving TB modeled that doubling the ventilation rate within the cabin decreased infection risk by half. Lastly, handwashing works.
As more specialties begin to collaborate, new methods of data gathering are becoming available. There is a lack of sufficient data at present for a meta-analysis (study method to infer disease risk) because many studies are limited by incomplete passenger manifests. In fact, one study found that contact information for passengers with a tuberculosis exposure was inaccurate 15% of the time, when health officials were attempting follow-up.
While this is in no means a comprehensive review, it has hopefully given a good survey of infectious control in commercial airlines and suggestions of further reading. As with anything, there is no subsititute for common sense and you should always consult with your personal health care provider for further details and individual needs.