Aircraft bacteria levels and traveller health

Popular wisdom says that aircraft provide the perfect environment for spreading disease, but few studies exist to confirm or deny this suspicion. Now, a team of researchers from the University of Massachusetts Amherst, a leading federal agency and Harvard University has measured concentrations of bacteria in the cabin air of 12 commercial passenger aircraft, and found that flying may be safer than we think.

“In general, bacterial concentrations and types found during the study should not pose a risk to travellers,” says Christine Rogers, a Professor of public health at UMass Amherst. “While we did find elevated levels of bacteria at several intervals during the flight, they were common residents of human skin and mucus membranes, dust and outdoor air, including Pseudomonas, Bacillus and Staphylococcus.” Rogers adds that passengers infected with diseases such as tuberculosis are a special case that could pose a risk to fellow travellers.

The study was headed by Lauralynn Taylor McKernan of the National Institute for Occupational Safety and Health. Additional members of the research team include Harriet Burge and Robert Herrick of the Harvard School of Public Health and Kenneth Wallingford and Misty Hein of the National Institute for Occupational Safety and Health. Results were published in the March 2008 issue of the Annals of Industrial Hygiene.

The team sampled cabin air on 12 randomly selected flights using Boeing 767 aircraft, with flight times lasting from 4.5 to 6.5 hours. Samples were taken in the front and rear of the coach-class cabin at six times during the flight, including boarding, mid-climb, early cruise, mid-cruise, late cruise and deplaning. Additional air samples were taken from the outside and inside of airline terminals at the cities of departure and landing. Flights were sampled during the summer to eliminate the effect of seasonality.

An analysis of the data showed some interesting trends that could be used to predict how disease organisms would move through an aircraft in the event of an emergency. The highest concentrations of bacteria were measured during boarding and deplaning. “Human activity during boarding and deplaning is greatest, which stirs up a lot of dust and causes increased shedding of bacteria from skin,” says Rogers. “This activity may also stir up microbes hidden in the seats of the plane.”

Bacteria levels dropped during the middle of the flight when compared to boarding and deplaning. The drop in mid-flight bacteria may be the result of less human activity, and the settling of bacteria from outdoor air brought into the plane before takeoff.

Since samples were taken from different locations in the plane at several time intervals, the researchers were able to study the pattern of air movement during the flight, and notice how human activity and bacterial shedding changed over time.

“Concentrations of bacteria were higher in the front of the plane during boarding, which makes sense since the planes were boarded back to front, with standing lines common at the front of the plane,” says Rogers. “This pattern shifted during the flight, with slightly higher bacterial concentrations in the rear of the plane. This could be the result of passengers shedding bacteria as they moved to the restrooms in the back of the plane.”

Levels of bacteria detected in the aircraft were compared to a study by the US Environmental Protection Agency documenting bacteria in the air of indoor offices and other non-manufacturing workplaces. Concentrations of total bacteria were higher in the aircraft during boarding, cruise and deplaning, probably due to the number of occupants in a given space and higher levels of human activity.

“Workers and passengers in commercial airliners are exposed to higher levels of common bacteria than people in office buildings,” says Rogers. “This points to the need for additional research to evaluate disease transmission on commercial aircraft.”