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Airborne infections on commercial flights

Note: This report represents information on this subject as of June 1998.  

Full text

In June 1997, the International College of Surgeons, United States Section, introduced Resolution 421 (A-97), which asked that: "The American Medical Association (AMA) encourage well-designed studies to discover the major causes of airborne infections on commercial flights, as well as reasonable solutions for minimizing this health hazard to travelers on such flights; and further, discuss the types of airborne diseases, methods of spread of these diseases and possible methods of prevention with the airline industry, the Federal Aviation Administration and other appropriate interested groups and agencies, and strongly encourage voluntary measures to minimize such health hazards."

This resolution was referred to the Board of Trustees because testimony at the Reference Committee indicated that existing data do not suggest airborne infections on commercial flights are a significant problem. Speakers stressed that airborne infection is primarily an issue of confined spaces and less an issue of airline conditions. This report summarizes what is known about the risk of contracting airborne infections on commercial airflights.

Methods

Information for this report was derived from searches of the MEDLINE database for articles published from 1980 to 1997 using the key terms, aircraft and respiratory tract Infectious diseases, environmental exposure, and tobacco smoke pollution, and through discussions with members of the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) and the National Institute of Occupational Safety and Health (NIOSH).

Findings

In 1997, the Aerospace Medical Association (AsMA) prepared a position statement on aircraft cabin air quality.1 This report indicates that most of the concerns about cabin air quality, voiced by cabin crews and passengers, are nonspecific symptoms not attributable to biological contaminants. Several studies conducted by Consolidated Safety Services and the Department of Transportation (DOT) found bacterial or fungal counts lower than those found in public buildings.1,2 In a DOT study of 35 flights on 8 large US airlines, the organisms detected were normal skin flora with no respiratory pathogens, although the technology was not available to identify airborne viruses in flight.2 Based on these studies and others, it could be concluded that the level of microbial contaminants found in airline cabins was very low and unlikely to cause adverse health effects. The data also indicate that the measured air quality easily exceeded the requirements for maintaining a healthy, indoor air quality environment.2

Despite these findings, cases of airborne transmission of infectious diseases have been reported among airline passengers. Although concerns have been expressed that individuals who fly frequently are more likely to contract an airborne infectious disease, it is difficult to study this issue. The incubation time of most airborne infections is much longer than the time spent on the aircraft, so illnesses become clinically apparent several days after air travel has been completed. As a result, the source of the infection and the association with air travel are not identified.  3

Known infections transmitted as a result of airflights are measles, smallpox, influenza, and most recently, tuberculosis (TB).3-9 Epidemiological mapping of recent influenza epidemics indicates that although the aircraft is a significant and efficient "vector" of influenza, only on rare occasions has the cabin environment been the focal environment of a disease outbreak.3 A strain of influenza (A/Texas) in 1977 was likely transmitted to 72% of interviewed passengers and crew members who subsequently became ill from a passenger who developed symptoms shortly after boarding the flight. Because of mechanical problems, the aircraft was kept on the ground for more than 3 hours, passengers were not allowed to deplane, and the ventilation system was turned off. The high attack rate was attributed to being in an enclosed space with an index case for a prolonged period. One week after the incident, a 20% secondary influenza rate was found among household contacts of the passengers.3,8

Several investigations have discovered that TB has been transmitted to passengers or flight crew members from an infected flight attendant or passenger.  3,5,6,10,11,12 Risk factors for transmission include the duration of flight, proximity to the person with infectious TB, and infectiousness of the index case.  3,5,6,11 In one study, the lack of passengers with skin test conversion in other cabin sections of the aircraft provided evidence that TB was not transmitted through the aircraft's air-recirculation system.5

Questions remain concerning the transmissibility of airborne infections, particularly viruses, and whether the existing ventilation and filtering systems protect adequately against airborne viruses. Data from epidemiological studies and microbial assays indicate that the risk of airborne infections currently appears to be very low. However, different commercial aircraft vary in ventilation systems as well as the amount of airflow or air packs used for a particular flight.13 

Ventilation systems in today's commercial airplanes use compressed air, with approximately 50% outside air and 50% recirculated in most aircraft.6 Although airplane cabin air is exchanged every 4 to 6 minutes compared to every 5 to 12 minutes in offices and homes, airflow patterns in commercial aircraft differ from those in standard indoor ventilation systems. Airplanes also usually have a greater density of people per unit floor space or per unit air volume. In aircraft, air comes in from overhead and circulates across the aircraft, with limited flow from the front to the back.6  Filtration of air is highly variable. Some small airplanes have no filtration whereas most large commercial aircraft circulate air through high-efficiency particulate air (HEPA) filters, which remove more than 99% of particles greater than 0.3m. Most airborne particles as well as bacteria and fungi are >1m and are effectively filtered, resulting in low counts for airborne bacteria and fungi. Airborne viruses range in size from 0.01m to 0.1m although they often clump into particles larger than 3m and are similarly filtered.2  Researchers are studying ways to sample and analyze cabin air for viruses.2

Research

To examine factors associated with air cabin air quality and take into account the unique ventilation design and requirements of commercial aircraft, an Aviation subcommittee of ASHRAE has been convened to develop information and conduct research on aircraft indoor air quality. In addition, a standards project committee (SPC161-Air Quality in Commercial Aircraft) is developing a standard for indoor air quality in commercial aircraft. This committee will review and expand on existing Federal Aviation and Administration regulations for ozone (0.2ppm for < 3 hours of flying), carbon dioxide (5000 ppm), carbon monoxide (9 ppm), and a ventilation rate of 10 ft3 /minute/person of outside air.13 The subcommittee consists of 19 panel members representing the AsMA, airline industry, manufacturers, NASA, flight attendants, consultants, and researchers. A preliminary draft report should be available by June 1999, pending research findings from NIOSH and ASHRAE.13

Recommendations

The following statements, recommended by the Council on Scientific Affairs, were adopted by the AMA House of Delegates as AMA policy at the 1998 AMA Annual Meeting.

  1. That under usual aircraft operation procedures, cabin air quality does not present a significant risk for transmission of airborne infections.    
  2. That the AMA supports efforts of the Aerospace Medicine Association and other groups to educate physicians and the public about the public health risks associated with flying with airborne transmissible diseases.    
  3. That the AMA supports the ongoing research of organizations such as the American Society of Heating, Refrigeration and Air Conditioning Engineers and the National Institute of Occupational Safety and Health to determine standards for cabin air quality.

References

  1. Thibeault C. Cabin air quality. Passenger Health Subcommittee, Air Transport Medicine Committee, Aerospace Medical Association. Aviation Space Environ Med. 1997;68(1):80-82.    
  2. Rayman RB. Passenger safety, health, and comfort: a review. Aviation Space Environ Med. 1997;68(5):432-440.    
  3. Roberts MA. Role of aviation in the transmission of disease. In: DeHart RL, ed. Fundamentals of Aerospace Medicine. 2nd ed. Baltimore, MD: Williams & Wilkins; 1996:1091p.    
  4. Amler RW, Bloch AB, Orenstein WA, Bark KJ, Turner PM Jr., Hinman AR. Imported measles in the United States. JAMA. 1982;248:2129-2133.    
  5. Kenyon TA, Valway SE, Ihle WW, Onorato IM, Kenneth GC. Transmission of multidrug-resistant mycobacterium tuberculosis during a long airplane flight. N Engl J Med. 1996;334(15):933-938.    
  6. Wenzel R. Airline travel and infection N Engl J Med. 1996;334(15):981-982.    
  7. Kendal AP, et al. Laboratory-based surveillance of influenza viruses in the United States during the winter of 1977-1978. Am J Epidemiol. 1979;110:462-468.    
  8. Moser MR, Bender TR, Margolis HS, Noble GR, Kendal AP, Ritter DG. An outbreak of influenza aboard a commercial airliner. Am J Epidemiol. 1979;110:1-6.    
  9. Ritzinger FR. Disease transmission by aircraft. Aeromed Rev. 1965;4:1-10.    
  10. Centers for Disease Control. Exposure of passengers and flight crew to Mycobacterium tuberculosis  on commercial aircraft, 1992-1995. MMWR. 1995;44:137-40.    
  11. Driver CR, Balway SE, Morgan WM, Onorato IM, Castro KG. Transmission of Mycobacterium tuberculosis associated with air travel. JAMA. 1994;272:1031-1035.    
  12. Miller MA, Valway S, Onorato IM. Tuberculosis risk after exposure on airplanes. In: National Immunization Program, Division of Tuberculosis Elimination, National Center for HIV, STD and TB Prevention, Centers for Disease Control and Prevention. 1996;414-419.    
  13. Larry Hocum, Chair, ASHRAE Committee on Air Quality Within Commercial Aircraft. Personal communication, January 1998 

Also see the AMA Infectious diseases Web site.
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Last updated: Sep 24, 2007
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