How do Infectious Diseases spread on Airplanes and How are they prevented?

What happens when you spend six or eight hours in an airplane with 300 other people, breathing recirculated air?

People sometimes believe they’ve been made sick by flying in an airplane, but in fact it’s difficult to prove that anyone is any more likely to contract an infectious disease spending three hours in an airplane than spending the same three hours in any indoor space with other people.

Passengers, flight attendants, and pilots often complain about the effects the air in an airplane has on them, but those non-specific symptoms are very rarely caused by germs. In fact, the Department of Transportation and Consolidated Safety Services did several studies that found that the only organisms detected on 35 flights of eight different U.S. airlines were normal skin flora, with no respiratory pathogens at all. They even found that bacterial counts were somewhat lower than they were in public buildings.

Large commercial airliners are required by government regulation to keep pressure at the equivalent of an outside air pressure of 8,000 feet. This is a comfortable air pressure for almost everyone to breathe normally. Some people believe that when you’re in an airplane you’re “breathing the same air over and over,” but this isn’t so.

In older airliners, such as DC-9 and the 727, air is drawn from the outside of the plane, compressed to cabin pressure, cooled, circulated, and exhausted. In new airplanes, half of the cabin air is recirculated. Cold air from the outside enters air compressors controlled by the plane’s jet engines. Compressing air heats it up, so it must be cooled before it can be used.

The air is cooled first by heat exchangers and then, after flowing through ducts in the wings, cooled further by air conditioning units located underneath the cabin floor. The cooled air is mixed half and half with cabin air and then pumped through filters to overhead outlets.

Meanwhile, air is drawn off by grilles located in the cabin’s walls or ceiling. Half of this air goes into the air mixer and the other half is exhausted outside the airplane, through a valve that also controls air pressure inside the plane. Mixing outside air and cabin air is more economical for the airline, it saves fuel. Some complain that changing only half the air and then mixing it with cabin air is less healthful than changing all of it, but there is little scientific evidence to support this view.

The filters through which the air travels, high-efficiency particulate air (or HEPA) filters, are not much different from those used in hospitals, and they can trap particles as small as bacteria and even viruses. Odors dissipate very quickly because the air is exhausted and replaced 20 to 30 times an hour, depending on the airplane, with a combination of outside air and filtered air.

This is somewhat more often than air is recirculated in large office buildings, but of course the population density is much higher and the volume of air available per person is much smaller in the confined space of an airplane cabin. Bringing in a constant supply of outside air replenishes oxygen, while the exhausted air dissipates carbon dioxide outside the plane. The recirculated portion of the air is constantly diluted with outside air. On the whole, the system works very well and keeps almost everyone comfortable and healthy.

Still, there have been some unpleasant incidents of infectious disease transmission aboard airplanes, even though an airplane’s air circulation system has never been implicated in any of them. Several instances of exposure to tuberculosis on airplanes are illustrative.

Between January 1993 and February 1995, the CDC investigated six instances in which passengers or crew flying on commercial airliners traveled while infected with TB. All six were symptomatic with the pulmonary form of the illness and were highly infectious at the time of the flights. Two of them had a strain that was resistant to both rifampin and isoniazid, the most serious kind of drug-resistant TB. Two of the patients were aware of their condition, and in fact were on their way to the United States for treatment.

In one case, a flight attendant had had a positive skin test for TB but continued to fly anyway. She developed an increasingly severe cough and was diagnosed with pulmonary TB in November 1992. The investigation revealed that the flight attendant had transmitted TB to a number of members of the flight crew, but probably not to passengers. The longer the flight crew flew with her, the more likely they were to be infected. The type of airplane they flew in didn’t matter, but if they spent more than 12 hours with her as a crew member, they were much more likely to have positive TB tests.

In another case, a passenger with TB traveled in the first-class section on a flight from London to Minneapolis that lasted nine hours, but he didn’t transmit anything to anyone. In a third case, a TB patient flew from Mexico to San Francisco, and when the San Francisco health department investigated, they found that 10 people had positive TB tests of the 22 they were able to contact and test. But even in this case, it wasn’t clear that they had contracted the disease during the flight: nine of them were born outside the United States, and the tenth was a U.S. citizen who may have been infected outside the country. The health department found was no conclusive evidence that the illness had been transmitted during the flight.

In only one of the six cases was there clear evidence that the disease had been transmitted during the flight. This was a flight in April 1994 after which six people appeared to become infected. Four of these were seated near a foreign-born passenger with pulmonary TB on a nine hour flight from Chicago to Honolulu.

In all cases where transmission of tuberculosis occurred in an airplane, the infection was passed person to person to nearby passengers during long flights. Essentially, this means that your chances of contracting TB on an airplane are no different from what they are in any confined space, you have to be exposed directly to an infected person for a significant length of time. M. tuberculosis, even though it is an airborne bacterium, is not carried to other people through the plane’s air circulation system.

The bacterium N. meningitidis is the leading cause of bacterial meningitis in the United States, and it can be deadly. Of 2,600 people who get it every year, 13 percent die. The CDC believes that meningococcal disease might theoretically be transmitted on airplanes, even though there have been no confirmed instances of this happening.

Still, anyone who sits near someone who is infected is at greater risk for infection. Antibiotic prophylaxis is recommended only for those who have direct contact with the infected person’s respiratory secretions or who sat next to the person for eight hours or more. Prophylaxis is also recommended for household members traveling with the person and other close contacts, such as roommates or members of the same sports team, but not for any other passengers. Again, the bacteria are not transmitted through the air circulation system of the airplane.

An outbreak of flu on an airplane in 1979 suggests that an airplane’s ventilation system, far from passing on infections, actually prevents them from being passed on by filtering the air. The flight was delayed on the ground because of an engine failure during a takeoff attempt. While repairs were being made the engines and the ventilation system were inoperative. Most passengers stayed on the plane for three hours. Within four days of the flight, 72 percent of the passengers had come down with cough, fever, fatigue, headache, sore throat, and myalgia. In other words, the flu, in this case identified as A/Texas/1/77(H3N2).

Some countries require planes to be sprayed with insecticide while the passengers are aboard. These countries are located in Latin America and the Caribbean as well as in Australia and the South Pacific region. There is good reason for this, because killing insects, particularly mosquitoes, before they are imported to another country is an important health measure. Some people may be adversely affected by the spraying, those with allergies, for example.

There is a high likelihood that mosquitoes brought in by airplane have introduced malaria into regions where the disease was previously unknown. This happened in Brazil in 1996. Three people were diagnosed with malaria after taking a plane flight from Lebanon to São Paulo on August 16.

The trip involved a half-hour stopover in Abidjan, Ivory Coast, Africa, which is apparently where some nonhuman passengers came on board. During an epidemiological examination of the airplane, four mosquitoes were found, one in the luggage compartment and three in the first-class cabin. All three patients had been traveling first class. The species was Anopheles gambiae, predominant in Africa. The three recovered, but not before two of them had become seriously ill.

Cholera is a very rare disease in the United States, but the few cases that occur are almost all attributable to travelers coming into the country, either by airplane or ship, from countries where the disease is endemic. The cholera now being brought into the country, even though it is in very small numbers, is often resistant to antibiotics, but antibiotics are, fortunately, not essential in treating the illness. The only cholera vaccine available in the United States has been discontinued because it was not highly effective. There are, however, two cholera vaccines available in other countries that seem a little better, but they are not recommended for travelers.

Cholera is transmitted by the fecaloral route, not through the air. Most infected people have no or very mild symptoms. Travelers to Africa, Asia, and Latin America should be aware of the risk and avoid consuming suspect water or food.