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2C H A P T E R 1 Commercial air travel has seen a steady increase in passengers over the last 20 years, including higher passenger densities per aircraft. Perhaps even more importantly, transformational change to the nationâs air traffic control systems, embodied by the Federal Aviation Administrationâs (FAA) Next Generation Air Transportation System (NextGen) initiative, is planned to accom- modate a 2.3 times increase in aviation growth by 2025 over the baseline year of 2005. As such, millions of people currently pass through regional, national, and international airports every day, and it is clear that an even larger volume will need to be accommodated in the future. During these travels, passengers, visitors, and airport and airline employees may be exposed to viruses and bacteria shed by fellow passengers and airline and airport employees who harbor infectious diseases. Therefore, infectious disease transmission is a significant and growing concern dur- ing air travel and at airports for flight crew members, airport employees, the flying public, and airport guests. This document is designed to provide aircraft operators and airline operators with guidance for strategies that can be implemented in the air travel industry to mitigate the risk of disease transmission in airports and aircraft. The document begins with a brief introduction to the risk of acquiring a disease and the various means by which diseases can be transmitted. That back- ground information is followed by an explanation of how these mechanisms influence possible exposures with an emphasis on the key environments that are relevant to the airline industry. The document then provides specific guidance for strategies that can be implemented in the air travel industry. Infectious Disease Risk The spread of infectious diseases is dependent upon many factors, perhaps the most impor- tant and obvious of which is the close contact of a contagious individual with susceptible indi- viduals. Disease transmission is reliant on sustained transmission to new hosts. In the absence of new hosts to become infected, the disease will be self-limiting. While there are other factors that play a role in disease transmission, including host susceptibility to infection and vaccina- tion status as well as duration of exposure and conditions of the environment, a key factor is the mixing of an infectious individual or population with susceptible individuals. Air travel has long been identified as an environment of interest for disease transmission. The risk of disease transmission in airports and on aircraft is, in many ways, similar to other settings where people congregate in high-density, high-usage and confined space environ- ments and pass through the same choke points (e.g., schools, malls, movie theatres). How- ever, the airport environment is also unique in that there is an interaction of a large number Introduction
Introduction 3 of individuals from geographically diverse regions with differing population immunity and endemic diseases, who all interact with airline and airport operation staff, as well as with each other. Routes of Transmission An understanding of how infectious diseases are transmitted from an infected individual to an uninfected individual is needed to develop strategies to prevent transmission. While infec- tious organisms can be spread through many routes, including via insects and sexual contact, the focus of this project is on infectious organisms that are spread by three general routes of transmission: 1. Aerosols that remain airborne and can be inhaled. 2. Large droplets that settle on surfaces. 3. Direct contact with secretions, bodily fluids, or contaminated surfaces. Infectious diseases spread by the aerosol route are transmitted by particles most often gener- ated by coughing and sneezing. However, these particles may also be generated by other common activities, such as talking or breathing. These particles are very small (around 10 micrometers); can remain airborne for hours at a time; and can even be transported to other areas of a building by heating, ventilation, and air conditioning (HVAC) systems. Tuberculosis represents the proto- typical airborne transmission disease, as the organism, Mycobacterium tuberculosis, is small enough to remain suspended in air for long periods of time (Mycobacterium tuberculosis must not only be inhaled, but reach deep into the lung to start an infection). For other diseases, like influenza, aerosols play a role in transmission, but other routes can contribute to the spread of disease as well. The physical acts of sneezing and coughing can generate large droplets in addition to the aerosols described herein. These large droplets cannot remain airborne for more than a minute or so, and fall to surfaces and the ground within several feet of their release location. These large droplets can be transmitted directly to susceptible individuals that were near the infectious individual during the act of sneezing or coughing or can con- taminate inanimate objects that can then be contacted by sus- ceptible individuals. Many infectious diseases (e.g., influenza) that can be transmitted by aerosols can also be transmitted by large droplets. Infectious diseases transmitted by direct contact can be spread when a person comes in contact with contaminated sur- faces or bodily fluids (e.g., vomit, blood, feces). For these infec- tious organisms, surfaces become contaminated through the spread of contaminated large droplets, nasal secretions, feces, vomit, or other means. These organisms, if they survive and remain infectious, may then be picked up by susceptible indi- viduals, through contact with these surfaces. Following con- tact, the susceptible individuals typically expose themselves by contacting their contaminated hands to their mouth, eyes or nose. Studies have shown that individuals whose hands are contaminated with a live virus may contaminate up to
4 Infectious Disease Mitigation in Airports and on Aircraft seven additional clean surfaces. Studies in which surfaces are evaluated have shown that the majority of commonly touched surfaces, such as faucets, ATM screens, and escalator railings are contaminated with microorganisms. Surfaces can remain contaminated for a long period of time if adequate disinfec- tion is not performed, as evidenced by a norovirus outbreak on an airplane where flight crew from different shifts became ill up to five days after an infectious passenger vomited on the airplane. Transmission by direct contact can be mitigated with barrier precautions, such as gloving, thorough washing of the hands, and effective cleaning of contaminated surfaces. Examples of microorganisms that can be spread through direct contact include the common cold virus (rhinovirus) and influenza. Research Directions for Infectious Disease Transmission in the Air Travel Industry For many infectious organisms, the route of transmission is known. For example, Mycobacte- rium tuberculosis is transmitted via aerosol generation from a contagious individual followed by inhalation of the aerosol by a susceptible host. However, for other infectious agents, the route of transmission or the relative importance of the various routes of transmission is not known with a high degree of certainty. By including mitigation measures that target all routes of transmission that are generally accepted by the scientific community, the findings and approach detailed in this document are not limited by the fact that knowledge about the details of transmission routes continues to evolve for many infectious agents. Furthermore, as new infectious agents enter the realm of possible meaningful exposure, the basic principles of exposure described herein will remain relevant. It is well understood that much research remains to be conducted to fully understand the dis- ease transmission process for many infectious agents. For example, at a recent symposium titled âResearch on the Transmission of Disease in Airports and on Aircraft,â 18 areas of foundational research were discussed as needing additional investigation. The research areas identified ranged from improvements of quantification of infectious particles and droplets for human exhalation to identifying environmental and personal factors that make individuals more or less susceptible to infection. More broadly, additional research is needed to determine the most important pathways for disease transmission for many important infectious agents. Although the areas of additional research need to identify the uncertainty surrounding elements of infectious disease transmission, it is clear that the fundamental, broad-based approaches presented in this document will be effective in helping minimize risk to the traveling public as well as workers at airport facilities and on aircraft. Transportation Hubs and Disease Transmission: The Airport and Airplane Environments In order to identify areas where interventions should be targeted, it is necessary to have an understanding of the systems currently in place, both mechanical and operational, that influ- ence disease transmission in the airport and airplane environments. The following two sections provide a general description of relevant systems found in these two environments, but are not necessarily representative of all situations.
Introduction 5 The Aircraft Cabin The aircraft cabin environment is presumed to be relevant to disease transmission due to: close proximity of passengers, long duration of close contact during flight, confined space, mixing of passengers from disparate geographical regions, and large numbers of travelers that use the space with only limited cleaning/disinfection between uses throughout a given day. As described above, disease transmission can occur through inhalation of aerosols or droplets, or through direct con- tact with contaminated surfaces. Opportunities for disease transmission may occur while directly adjacent to an infectious person during flight, but can also occur during boarding or disembark- ing as the passenger traverses a contaminated area or touches a contaminated surface. Beyond personal behavior and hygiene, control of biological agents in the cabin environment is primarily accomplished by two means: the environmental control system (ECS) and surface cleaning. Aircraft are equipped with ECSs to maintain suitable temperature, humidity, pressure, ven- tilation and ozone concentrations in the cabin. Ventilation specifications for aircraft require a minimum of 0.55 lbs/minute/person in the aircraft, which provides a high air exchange rate in the cabin (10â15 air changes per hour). The ECS generally provides a 50:50 mix of outdoor and recirculated air. The outdoor air, sterile and particle-free at cruising altitudes, enters the ECS from the engines after undergoing compression and conditioning. This air is mixed with recirculated air from the cabin. The recirculated air passes through a high-efficiency particulate air (HEPA) filter capable of removing a minimum of 99.97% of 0.3 micron particles. (Note: The removal effi- ciency is generally greater for particles both larger and smaller.) Particles generated by sneezing or coughing and that contain bacteria or viruses that enter the recirculation mode of the ventilation system are effectively removed by HEPA filters. Air delivery diffusers in aircraft are located in the ceilings with return air collection systems located at floor level on the cabin walls of the aircraft. The air flow in the cabin is designed to move from ceiling center-to-side which should act to limit the transport of particles along the length of an aircraft (Figure 1). However, perturbations to this air flow pattern can occur during normal cabin activities (e.g., passenger and flight Figure 1. Cross-section of airflow in an air- plane cabin (adapted from the World Health Organization. Tuberculosis and Air Travel: Guidelines for Prevention and Control. WHO/ TB98.256. Geneva, Switzerland: World Health Organization, 1998).
6 Infectious Disease Mitigation in Airports and on Aircraft attendant movement through the cabin). On the ground, aircraft ventilation is provided by auxil- iary power units. In some instances, ventilation systems may not be operational or only minimally operational while aircraft are parked at the gate. These periods of low ventilation and high front- to-back movement in the aircraft during boarding/disembarking may be an important window of exposure for disease transmission. The aircraft ECS, when operating at cruise altitude and according to specifications, provides a high ventilation rate of cleaned air. For example, in a study that evaluated the temporal variation of airborne bacteria and fungi on aircraft, higher levels were seen during boarding and deplan- ing when the ECS was not likely to be operational. The airborne microbial levels dropped during flight due to the HEPA filtration. This is important and relevant for minimizing the risk of dis- ease transmission during flight, especially for diseases transmitted by aerosols (e.g., tuberculosis), which will be removed by the HEPA filtration during recirculation of air from the cabin. The risk of exposure to diseases that are primarily spread via large droplet or inanimate objects will not be mitigated effectively by the ventilation system. Mitigation for infectious agents with these expo- sure pathways is primarily achieved through cleaning and personal hygiene (e.g., washing hands, covering a cough). A general cleaning of the aircraft typically occurs after each flight, with a more thorough, detailed cleaning protocol followed during overnight servicing. Ineffective cleaning, either due to technique or choice of disinfectant, may not only fail to remove the infectious agent, but may also aid in its spread to other surfaces. The Airport Terminal Airport terminals and other transportation hubs are relevant for disease transmission for many of the same reasons that airplanes are a relevant environment: mixing of passengers from diverse regions; large numbers of unique visitors; close contact; sharing of communal spaces (e.g., rest- rooms, waiting areas, dining tables); and high number of commonly touched surfaces (e.g., kiosks, handrails). Unlike airplanes, the terminal has many different micro-environments, each of which has its own exposure/risk profile (e.g., security screening v. waiting area). Mitigation of risk from biological agents within the terminal is achieved in a similar man- ner as on airplanesâthrough the ventilation/filtration and cleaning of surfaces. Building HVAC systems provide a mechanism for diluting and filtering airborne contaminants in a building. While most buildings are ventilated at lower rates compared to inside an aircraft, the volume per person in buildings is generally much larger. As most biological agents are in a liquid aerosol form, and much of the aerosol will quickly evaporate in buildings that have their thermal environment maintained to provide occupant comfort, the much larger volume of space per person, when contrasted with that of an aircraft, will generally lessen the expo- sure potential from a biological contaminant released by another person in close proximity to their physical space. Airport terminal buildings generally contain a variety of occupancy classifications that include Business, Assembly, and Mercantile. These classifications, while all having similar per person ventilation requirements to meet code and provide comfort conditions, may have large differ- ences in the number of occupants per volume of space, with Assembly areas having occupancy densities as high as 120 persons/1000 ft2 versus 5 to 7 persons/1000 ft2 for Business occupancies. Many areas of the airport terminal have high-density occupancy, particularly in Assembly areas. These areas include departure/arrival gates, waiting areas, and corridors. Generally these areas are also characterized as having highly transient occupancy profiles. Other areas of the build- ing, such as the âjet-waysâ and inter-terminal transport trolleys will have micro-environments more similar to buses and aircraft, while Business areas will be similar to more typical office environments.
Introduction 7 The HVAC systems in the airport terminal have the ability to minimize transmission of airborne infectious agents by two primary mechanisms: dilution and filtration. HVAC sys- tems dilute point source pollutants, such as aerosols released by infectious individuals, both by the introduction of outdoor air, and by spreading point source pollutants over a much larger volume (i.e., dilution). Air filtration in buildings for infectious agents is generally achieved by passing air through filters that rely on particle diffusion, impaction, and interception to remove aerosols. Filters that rely on other mechanisms, such as electrostatic charge, are also available, but are not in widespread use in U.S. buildings. In recent years, particulate filtration in U.S. commercial buildings has generally been improved by the growing use of guidance from the U.S. Green Building Councilâs Leadership in Energy and Environmental Design (LEEDÂ©) requirements in the selection of equipment and supplies. The introduction of LEEDÂ© ratings for building construction and operations has led to more buildings equipped with filters with higher minimum efficiency report value (MERV) ratings. Filters with higher MERV ratings can provide better filtration for all particle sizes, including the size range in which infectious disease-bearing particles are typically found. Airport terminals can be complicated structures, involving an array of complex building systems. The quality assurance process of commissioning increases the likelihood that a newly constructed building or space will function as designed. Commissioning spans the entire design and construc- tion process, and includes inspecting the building systems during construction and when the project is near completion, to ensure they are performing as expected. Among the benefits of commission- ing, there is an expectation that commissioned spaces will be more energy efficient; have lower operation costs (due to properly sized and functioning equipment); and be less likely to have HVAC system issues. Since the September 11, 2001 terrorism event, numerous security changes have been made to the nationâs airports for the purposes of providing safer air transportation. These changes have included enhancing areas for security screening of passengers before entry to departure and arrival gates. These security screening areas are often high-density occupancy areas that previ- ously were not used, or designed, for high-density occupancy. In some airports, security areas may have been created in areas that do not permit full consideration of the factors that would typically be taken into account in the design of a densely occupied space. As a result, the dilution and filtration mechanisms traditionally offered by a properly sized and designed HVAC system may not be provided in these improvised, high-density areas. Surface cleaning can be a means of limiting the spread of infectious agents that are transmitted through contact with contaminated surfaces. While surfaces are never sterile and are populated with a wide variety of microorganisms, surface contamination can lead to the spread of infec- tious diseases that are transmitted via direct contact (e.g., norovirus, influenza, MRSA). However, the frequency of cleaning is important as heavily touched surfaces are quickly recontaminated. Several significant issues related to cleaning are that cleaning of surfaces is typically performed to a visual standard and is generally not based on bacterial or viral loading, and cleaning protocols and strategies are not standardized across airports. Further complicating the issue is the fact that even within airports, several different groups are often responsible for maintaining different areas and coordination may be limited, or non-existent. For example, the airport operator is responsible for the terminal, while the food service operators are responsible for cleaning dining areas, and airlines are responsible for airplanes and check-in areas. Other Travel-Related Environments Air travel, by its nature, is not limited to the time spent in the airport terminal or time spent on airplanes. The air travel experience includes time spent in many other micro-environments
8 Infectious Disease Mitigation in Airports and on Aircraft that are relevant to disease transmission. A typical traveler may take a bus, train, or taxi to the airportâmicro-environments that represent spaces shared by many people, some of whom may be infectious and may contaminate these spaces. Even prior to the actual ride to the airport, the traveler is likely to spend time in transportation hubs that would have similar exposure profiles to an airport terminal (e.g., train station, bus depot). After arrival in the destination city, the traveler is again exposed to potentially crowded, communal envi- ronments as they leave the airport by taxi, bus, or train, and then spend time in a hotel or motel. These micro-environments are beyond the control of airport administrators and air- line operators. However, the risks of disease transmission attributable to time spent in the airport and airplane cannot be fully disentangled from these other travel-related environ- ments. The goal of this report, however, is to provide mitigation strategies for airports and aircraft, without consideration of other travel-related exposures or comparison of airport risks to other settings (e.g., hotels, hospitals, buses, trains). Process for Selection of Mitigation Measures The recommended mitigation measures developed by the Expert Committee were divided into the following three categories as a simplifying scheme to aid in the implementation of the measures: â¢ Buildings, â¢ Airplanes, and â¢ People. Final selection of the recommended mitigation measures was a result of a six-phase process that culminated in an expert workshop, all of which is described in a separate report available on the Project website. The main objective of holding the workshop was to draw upon the knowledge of the members of the Expert Committee and identify specific mitigation mea- sures that target the highest risk exposure opportunities for each of the three transmission pathways, leveraging the knowledge gained in the initial phases of this research project. An initial and broad list of mitigation measures drafted prior to the meeting was evaluated in order to screen and prioritize the selections with the goal of developing a consensus list of recommendations. The final list of specific mitigation measures was developed by having the Expert Committee select measures that were evidence-based (or were able to be evaluated by applying knowledge from other environments, such as hospitals) and could realistically be implemented in the airport and aircraft environment. An overview of the selected mitiga- tion measures is presented in Table 1. Details of each measure are presented in the following sections. Each recommendation in the following sections is listed with information on the area of the airport or air travel experience targeted, the population targeted, and the route of transmission targeted. Further, each recommendation is categorized on the basis of existing scientific data, rationale, applicability, and feasibility. The recommendations are evidence-based wherever pos- sible. However, certain recommendations are derived from empirical infection-control or engi- neering principles, theoretic rationale, or from anecdotal evidence. Each recommendation was rated according to the following categories: â¢ Highly Recommended. Highly recommended for implementation and supported by experi- mental, clinical, or epidemiological studies. â¢ Recommended. Recommended for implementation and supported by suggestive clinical or epidemiological studies, or a theoretical rationale. â¢ Suggested. Suggested for implementation and supported by indirectly relevant studies or anecdotal evidence.
Introduction 9 Additional Information The recommendations that are described in this report are based for the most part on primary scientific literature. This literature is cited in ACRP Project 02-20Aâs Final Report available at http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=3028 and in an online ref- erence repository maintained by the John A. Volpe National Transportation System Center at http://volpedb.volpe.dot.gov/outside/owa/vntsc_outside.emrdtaa_lib.display_lib#search:â Table 1. Summary of recommended mitigation measures.