By John Briley
Special to The Washington Post
Tuesday, June 3, 2003; Page HE01

The proven transmission of SARS on aircraft adds new urgency to a question: are airliner cabins hot zones for disease

Let's start with the frightening part: There is considerable scientific evidence suggesting that airliner passenger cabins are places where infectious disease and respiratory illness spread among people more often, and more easily, than in other environments.

The comforting news, if it can be called that, is that the cabin's air circulation system, long targeted by frequent flyers as the source of their ills, is probably not to blame.

On May 22, the World Health Organization (WHO) raised to 27 its estimate of the number of people worldwide who became infected with severe acute respiratory syndrome, or SARS, during an airline flight. Earlier estimates had put that number at 16. Twenty-two of those 27 were infected during one flight, Air China's Flight 112 from Hong Kong to Beijing on March 15, the WHO said.

Contrary to earlier reports that SARS infection was a risk only within two rows of an infected person, the WHO said passengers sitting seven rows in front of and five rows behind the carrier were infected. While WHO officials declined to speculate on the mechanism for the wider distribution of the virus, independent experts believe that transmission may have occurred as infected people moved around the cabin, or as flight attendants (four of whom are among the 27 infected in flight) unwittingly passed the germ among passengers.

No in-flight SARS transmission has occurred since March 23, due largely to preflight passenger health checks instituted by airports and airlines serving SARS-affected areas, according to aviation officials.

But the transmission of those 27 cases raises two nagging questions: Should -- or can -- the airlines do anything to prevent the onboard spread of the many other conditions that, like SARS, are transmitted between passengers by sneezes, coughs, touches and other unsavory germ-launching mechanisms? And does anyone really know whether pathogens are spread via airliner cabin ventilation systems?

The National Research Council, an arm of the National Academy of Sciences, looked into the aircraft cabin air quality issue two years ago, and in January 2002 issued a report urging the Federal Aviation Administration to impose stricter controls. The House Aviation Subcommittee has scheduled a hearing on cabin air quality for June 5.

The National Research Council offered no conclusive link between airborne pathogens and passenger health, but wrote, "Available exposure information suggests that environmental factors, including air contaminants, can be responsible for some of the numerous complaints of acute and chronic health effects in cabin crew and passengers."

Clean Air Acts

 

The cabin air in most commercial aircraft is a 50/50 mix of air taken from outside the aircraft during flight and recirculated air from within the cabin.

The outside air is first pressurized in the aircraft engine compressors, explains David Space, an air cabin quality research scientist at Boeing. That air is then mixed with recirculated air from within the cabin. The recirculated air normally passes through a high-efficiency particulate air (HEPA) filter, the same filters used in hospital operating rooms, before it is mixed with outside air, Space says. HEPA filters were designed in the 1950s for use in nuclear reactor facilities and "are basically 100 percent efficient," says William Nazaroff, an environmental engineering professor at the University of California, Berkeley, and an author of the NRC cabin air quality report.

The filters contain very fine glass fibers that snag particles as small as 0.3 micrometers in diameter. (A strand of human hair, by comparison, is 75 to 100 micrometers in diameter.) Nazaroff says the coronavirus that is linked to SARS is likely smaller than one micrometer, but it is usually carried by a particle of mucus between one and 5 micrometers -- a size easily trapped by a HEPA filter, should the particle make it to the filter through the plane's circulation system.

While the use of HEPA filters is not required, nearly all planes used for commercial passenger traffic are outfitted with them, says Jon Jordan, the federal air surgeon for the Federal Aviation Administration (FAA).

The outdoor air at a plane's maximum cruising altitude (roughly 35,000 feet) lacks sufficient oxygen for passengers to breathe, meaning the air has to be compressed before it can be used in the plane. At high altitudes, cabin air is pressurized to a density equivalent to air found 6,000 to 8,000 feet above sea level, meaning the air in the cabin is about as "thin" as the air in Aspen, Colo.

That means the cabin air contains about 75 percent of the oxygen available at sea level, which, according to the National Research Council, could cause "[s]erious health effects" in infants and persons with cardiopulmonary disease, due to the reduction in oxygen reaching bodily tissues.

The oxygen content of in-flight air was one of only two issues that the National Research Council cited as "high concern" in its report. "The FAA should rigorously demonstrate in public reports the adequacy of" the FAA's cabin pressure requirements, the report reads, and "should provide quantitative evidence and rationales to support it."

The other issue was the presence of ozone in the cabin. Ozone can enter planes along with outside air, but only on the rare occasions when the ozone layer dips into flying altitudes. This is more of a threat near the North and South poles, where the ozone layer sits closer to the earth, but it also occurs elsewhere around the globe.

Ozone, as anyone who listens to summer air-quality warnings knows, can cause respiratory irritation and reduced lung function. "Lots of flight attendants complained about these symptoms when airlines started frequent transcontinental flights [which are long, high-altitude routes] in the 1970s," Nazaroff says. In response, airlines installed ozone converters on many long-haul aircraft, but these converters, which turn ozone into oxygen, are not required. The research council's report recommended that they be made mandatory.

A person would have to fly at least one long-haul flight per week for ozone to be even a potential hazard, Nazaroff adds. The FAA's Jordan concedes that while research shows no negative health effects from ozone exposure during flights, "those studies perhaps have not been as extensive as they should be."

Other concerns cited by National Research Council include the occasional contamination of cabin air by leaked oils or hydraulic fluid in the engine, exposure to pesticides, airborne allergens and carbon monoxide, and the relatively low ventilation rate on aircraft compared with buildings. The report calls for better monitoring and reporting of in-cabin pollutants.

The recirculated air on planes is "dumped" and replaced with outside air 10 to 15 times per hour. Office air typically undergoes one to three exchanges per hour. But according to Martha Waters, senior research industrial hygienist at the National Institute of Occupational Safety and Health (NIOSH), office buildings actually clear their air two to three times more frequently than planes when the number of people per cubic foot typically occupying those spaces is considered. "No one really knows if the ventilation rate [on planes] is adequate," Waters concedes.

A NIOSH study of cabin air on 37 flights between 1995 and 1999 showed high levels of carbon dioxide, which itself is not a contaminant but in high levels indicates poor air circulation that could be unhealthy. Carbon dioxide on the flights ranged from 900 to 2,400 parts per million (ppm). The industry maximum for commercial buildings is 1,000 ppm. "In non-aircraft environments, studies have shown respiratory symptoms in people exposed to carbon dioxide levels above 1,500 ppm," Waters said. "So 2,400 ppm is a bit of a red flag."

Air Sickness

So airplane cabin air may be thin, occasionally contain ozone and other pollutants and have a higher level of carbon dioxide than other environments. Do any of these factors make it easier for passengers to get sick on a flight than, say, in a theater or restaurant?

The National Research Council report found no data either to support or debunk the common belief that flying raises one's chance of contracting infectious illnesses. The council says that the most important factors in the transmission of infectious agents on aircraft "appear to be high occupant density, and the proximity of passengers."

But that conclusion is not universally shared. Judith Murawski, an industrial hygienist with the Association of Flight Attendants (AFA), says bad cabin air itself does contribute to higher illness transmission on planes. She cites a recently completed survey conducted by NIOSH that found that female flight attendants had notably higher incidence of respiratory symptoms compared with the general population of working women; that study is scheduled to be published by the end of the year in the journal Occupational and Environmental Medicine.

In that survey, flight attendants reported an incidence of irritated eyes, runny nose and dry throat that was two to four times higher than the general population. Flight attendants also reported five times more cold and flu episodes and four times more incidence of chest illness.

At least one other study has suggested increased infections onboard. A survey of 1,100 passengers published in the July 2002 Journal of the American Medical Association (JAMA) showed that 20 percent of them reported upper respiratory tract infections within a week of flying from San Francisco to Denver in the winter of 1999. "That percentage is four times higher than what the general population experiences in the winter months," Murawski says.

The main purpose of the JAMA study was to determine whether passengers preferred the 50/50 mix of outside and recirculated air or a feed of 100 percent outside air, which Murawski says is healthier for passengers and crew, provided the air is free of fumes from leaking oils or fluids from the engine. She claims that the airline industry switched from 100 percent outside air in the 1970s to save money. Compressing and distributing outside air into the cabin costs more than recirculating inside air.

The survey showed no passenger preference for either type of air.

Surface Problems

If cabin air is filtered and unlikely to circulate pathogens, why would people be more likely to get sick while on aircraft?

The recent onboard SARS transmission examples may be instructive. SARS, like many viruses and bacteria, can survive on hard surfaces such as plastic. But even the Centers for Disease Control and Prevention (CDC) can't say for certain how long the germs stay alive. "We think for SARS it may be around two to three hours, but we're not sure," a CDC spokeswoman says.

Brad Connor, president of the International Society of Travel Medicine, says for the most contagious diseases, like tuberculosis and meningitis, "the risk is limited to people within two rows" of the infected person.

But Connor added that the pathogens that cause the common cold can live for up to 18 hours on surfaces like armrests and seat cushions. "Theoretically, if you are waiting to use the restroom and you touch the wall or a seat where a sick person has coughed, and then touch your face, you could get sick."

Many bacteria and viruses -- including SARS -- can survive longer in feces, highlighting the importance of cleaning restrooms between flights. But nobody cleans them between uses on the same flight, perhaps offering another mechanism for disease transmission on aircraft. While conventional spraying and cleaning kills most pathogens, Murawski says cabin-cleaning crews lack standardized procedures and oversight that would ensure that surfaces are germ-free. The FAA's Jordan acknowledged that airlines "set their own standards" for cleaning aircraft.

So apparently more could be done to sanitize aircraft cabin surfaces and air. But it is doubtful that any industry practice will eliminate the risk of disease transmission on aircraft, where 300 people may occupy for an extended period a space more appropriate for 50. Ultimately, it's this crowding that may affect disease transmission more than anything else about the physics of flying or of air circulation.

"I'm speculating, but I would expect infectious disease transmission risk to be higher today . . . than in the past," says Nazaroff. "The two main reasons are closer spacing of seats in coach and higher load factors," or higher percentage of seats filled.

Which could result in an odd situation: If fewer people travel by air due to fear of SARS, the risk of contracting it or other infectious diseases in the air will fall. The greater the fear, the safer it will be.•

John Briley, a frequent contributor to The Post's Health and Travel sections, is an editor for iJet Travel Intelligence. Brad Connor, who is mentioned in this article, is an advisor to iJet Travel Intelligence.