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INTRODUCTION Air travel has become an essential form of transportation in modern society. It has also been one of the safest, even though aviation accidents have received considerable attention in recent years and have caused public concern about personal safety. Concern for the quality of air in the passenger cabins of commercial airliners has been publicized, but it has focused on occupational exposures of cabin crews. Rising fuel costs might have prompted airlines to reduce the amount of outside air in the ventilation of passenger cabins to conserve fuel, consequently adversely affecting the air quality. Finally, new models of aircraft use recirculation of cabin air to a greater degree then older models in the fleet, so the general tendency is toward the use of less outside air. The specific concerns regarding the quality of cabin air include not only the amount of outside air, but also the adverse effects that might result from exposure to this unique confined environment. In aircraft, people are exposed to a particular combination of low relative humidity, reduced air pressure, presence of ozone and other pollutants (some of which have been demonstrated to be harmful to human health), and increased cosmic radiation. In the last year, 28% of the general public took at least one trip by air, and 5X of those who flew took 10 or more trips. In addition, more than 40,000 flight attendants are exposed to the cabin environment for an average of approximately 900 h each year. Yet, in the face of the knowledge of these acute and chronic exposures to pollutants with proven health effects, very little research has been done to characterize either the quality of the air in airliner cabins or the potential health effects of exposure to that environment. 13
14 Many airline travelers complain about cabin air quality. The nature of and reasons for their complaints are important clues to the problem. Complaints from airline passengers about catching colds or experiencing other health problems as a result of air travel are not uncommon. Although airliner cabins are divided into smoking and nonsmoking sections, many travelers still insist on being seated as far an possible from the smoking section; complaints by some groups have led to the suggestion of eliminating smoking in aircraft. Concern has also been registered about the possible relation of this environment to acute exacerbations of underlying chronic diseases, such as allergic rhinitis or asthmatic attacks, and about the adequacy of onboard medical equipment and the availability on every flight of trained personnel to handle emergency situations. For years, flight attendants have reported various health problems--from chronic bronchitis to difficulties in pregnancy--that they have attributed to their occupational exposures. Flight attendants' careers have become longer, and female flight attendants are permitted to work until late in pregnancy. Furthermore, a larger portion of the general public, some with health conditions that might make them more susceptible to the airliner cabin environment, are now flying. It in therefore important to understand the potential for adverse health effects of chronic exposure to airliner cabin air. Onboard fires are a special condition of cabin air quality that is the basis of additional concern because they produce large quantities of smoke and toxic fumes. There is concern about the adequacy of emergency fire procedures and equipment, including both firefighting and protective breathing devices. The scientific community and more recently the general public have become aware of the effects of pollution in confined spaces. Unlike other modes of transportation or other public spaces, airliners do not offer users the freedom to open a window, move away, or step outside if the cabin air is not suitable. Throughout the course of its work, the Committee on Airliner Cabin Air Quality had to confront the problem of answering questions on which almost no scientifically
15 valid data were available. The study began with a public meeting, whose purpose was to collect information for the Committee to review. In addition, several experts made presentations to the Committee on various relevant issues throughout its work. The Committee also inspected the environmental control system of an MD-80 airplane and visited research facilities--such as the Federal Aviation Administration (FAA) Technical Center, the United Airlines flight attendant training center, and the Boeing Commercial Airplane Company--to gain a better understanding of the state of the science related to the issues at hand. Information was obtained from the airline industry and flight attendants' unions, and computerized FAA data banks were used for accident and incident data. STRUCTURE OF REPORT This report is the product of extensive Committee deliberations on the issues associated with the potential health effects of exposure to airliner cabin air. Chapter 1 describes the magnitude of the population exposed to cabin air. Chapter 2 discusses the current environmental control systems on commercial passenger aircraft, and Chapter 3 describes airliner safety procedures, equipment, and passenger instructions. Chapter 4 discussed the effects of cabin fires and depressurization on air quality. Chapter 5 identifies the sources and exposures of cabin air pollutants, and Chapter 6 discusses the reported health effects associated with cabin air. Chapter 7 considers the desirability and feasibility of collecting additional data. PROBLEMS IN STUDYING AIRLINER CABIN AIR QUALITY The Committee faced two fundamental and related problems in its attempt to assess exposure to pollutants in airliner cabins: although their presence is known or suspected, very few data are available on the concentrations of pollutants of interest; and National Research Council committees do not generally conduct basic research and gather their own primary data, but rather rely on the available, published, peer-reviewed literature. Thin Committee explored the idea of
16 collecting primary data, but was unable to complete a sampling program within its schedule and budget constraints. The Committee therefore identified and reviewed other relevant studies and models of indoor air pollution that could be extrapolated to the airliner environment and, with the assistance of consultants from Harvard University, developed a computer model of pollution in the airliner cabin to simulate typical exposures under various operating conditions. Although most of the concerns raised about airliner air quality have been related to the cabin, the only existing aircraft regulations that specify ventilation rates apply to the flight deck (cockpit), not to the cabin. That is the case because of safety considerations, which dictate that the cockpit be adequately ventilated--both to provide a safe working environment for pilots and to cool sensitive equipment. Air-exchange rates in the cockpit are typically more than 10 times those in the passenger compartment. The Committee chose not to address the issue of cockpit air quality specifically, however, because the conditions and issues are different. The Committee focused its attention on cabin air quality and chose not to expand the scope of its study to include cockpit conditions. Airliner cabin air consists primarily of air drawn from compressors in the engine (bleed air) and often contains recirculated air from within the cabin. On the ground, some aircraft with vapor-cycle cooling can use primarily recirculated air. There are no federal standards regulating ventilation, relative humidity, and mixing efficiency, all of which greatly affect the quantity, distribution, and overall quality of air in the cabin. Instead, individual manufacturers set performance requirements for airliner environmental control systems; as a result, design and performance can vary among different models of aircraft. If a system fails, emergency "standard operating procedures"--some net by government regulation and some by individual airlines--govern the operation of backup systems and in-flight procedures for ventilation and air quality. Given human nature, however, and the fact that these procedures are often carried out under stressful conditions (i.e., when normal systems have failed), there might be variation in the actual performance of these procedures and operation of emergency equipment.
17 Moreover, experts disagree as to whether aircraft cabins are adequately ventilated even during normal operation. The Committee examined standards developed by federal agencies and other organizations for relevant pollutants in other environments, both indoor and outdoor, ranging from offices and homes to spaceships and submarines--the last two of which share some properties with the airliner cabin environment (high-density occupancy in confined space, air recirculation, and problems with humidity)--and compared these standards with the concentrations commonly found in aircraft cabins (see Table I-1. The documents developed in support of federal standards for other environments also provided useful information for making inferences about the applicability of standards to the aircraft cabin. However, those standards are not directly applicable to airliner cabins. The three substances in aircraft cabin air that FAA regulates are ozone, carbon monoxide, and carbon dioxide. Ozone may not exceed 0.25 ppm at any time and may not exceed 0.1 ppm for periods longer than 3 h.3 Carbon monoxide in excess of 1 part in 20,000 parts of air (50 ppm) is considered hazardous.7 Carbon dioxide in excess of 3% by volume (sea-level equivalent), or 30,000 ppm, in considered hazardous in the case of crew members, 7 but may be allowed in crew compartments if appropriate protective breathing equipment is available. As to relative humidity and low pressure, the Committee relied on reports from the toxicologic, clinical, and epidemiologic literature and estimated health effects associated with combinations of humidity, pressurization, and pollutant exposures. The Committee could find no published data on biologic contamination in aircraft cabins, although instruments are available for measuring many biologic contaminants and measurements have been made in other confined ~paces. Therefore, the Committee felt that it would be especially worth while to determine the feasibility of detecting and measuring there contaminants.
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19 The Committee also considered the general issue of exposure. Exposure to cosmic radiation is difficult to predict, because it depends on the variable solar flux and the amount of radiation reaching our atmosphere. Clearly, an occupant's location in the aircraft affects his or her exposure to air contaminants that have point sources, such as cigarette-smoking, food-based materials, specially applied cleaning agents, and specific infectious agents carried by passengers. Other possible pollutants, such as ozone and hydrocarbons, might be distributed more uniformly. The most commonly noticeable differences in passenger air quality are associated with locations in or near designated smoking zones and with the density of such smoking zones in a given area. Because of aircraft cabin airflow patterns, there can be significant differences in the exposure of passengers seated toward the tail, in the middle, near the lavatories and galleys, and in the forward compartments of large aircraft. Flight galleys and lavatories, which have local vents and fans, have patterns of ventilation and sources of air contamination different from those in the rest of the passenger compartments. There are differences in exposures to pollutants during boarding (e.g., in the waiting areas) and during the various stages of flight. During boarding--with cabin doors open, no smoking allowed, and little lavatory use or food preparation--air quality is different from that during flight. The increase in smoking frequency after the "no smoking" sign is turned off leads to a worsening of air quality in the smoking area and contiguous zones. Extended periods of holding at the dock, taxiing to the runway, or awaiting clearance to take off or land can adversely affect cabin air quality. Unusual events can also have dramatic effects on cabin air quality--e."., mechanical problems; sudden changes in heating, ventilation, and air- conditioning controls (HVAC) or in cabin pressure; spills or fluid leakage; and undetected, smoldering electric or cigarette fires. The presence of so many complex variables that affect cabin air quality led the Committee to commission the preparation of a mathematical model that could be used to calculate concentrations of substances in the air of different parts of the cabin with different
20 passenger load factors and operating modes. The model described in Appendix A is based on sound physical principles, but has not yet been verified in practice. PROBLEMS IN STUDYING THE HEALTH EFFECTS ASSOCIATED WITH AIRLINER CABIN AIR QUALITY Data relevant to the health effects of airliner cabin air quality can be considered in several ways, for example, by types of persons exposed, by acute and chronic health outcomes of concern, and by source and type of data. Each implies tradeoffs. Health effects, even those of great concern, can be hard to detect, measure, and attribute to specific causes (such as a component of cabin air). The reasons, which are numerous, include the lack of baseline observations on most persons who fly and who have been adversely affected, ethical constraints on and practical Unfeasibility of follownp of persons who fly and might be adversely affected by the various features of cabin air, the imprecise nature of many relevant symptoms of acute effects (such as tightness of the chest), the rarity of chronic health problems directly attributable to cabin air, and the self-selection characteristics of both crew and passengers. These difficulties are mitigated in some types of research studies on biologic correlates of illness, for example, respiratory function studies of crew members before, during, and after flight. However, although acute changes in FEV1 (forced expiratory volume--the maximal amount of air that can be expelled in 1 s) are well correlated with acute respiratory symptoms and disease, they are subject to a potentially misleading measurement bias. It is not at all clear that a small decrease in FEV1 in the average healthy person has any health (or regulatory) meaning. Thus, there is a three-way tradeoff that involves feasibility of detecting effects through epidemiologic studies, precision of measurement, and relevance to public health. Several problems complicate assessments of the effects of cabin air quality on the health of passengers and crew. Biochemical measurements, such as those of the absorption and excretion of toxic products, can be made with precision and reliability, but the responses
21 of people exposed to those substances are most difficult to ascertain and to assess. The prevalence and incidence of abnormal symptoms among exposed people can be determined through questionnaires and direct questioning, but questions in both approaches must be carefully worded to avoid suggesting answers and to avoid unintentional inherently biased responses. Some questions not apparently related to the effects of the substances at issue must be included, and people in control groups must be selected with care and treated in exactly the same way as the people in the test groups. When selecting controls, one must keep in mind the likely differences between the types of people who might apply for jobs in the air and jobs on the ground and the possible impacts of those differences on the end points being measured. Biases resulting from the expectation of compensation for occupation-related illness must also be considered. None of the studies of health effects found by the Committee satisfied these criteria of reliability. The Committee discovered that, with the exception of pilots, few routine health data are collected by the airlines on either the flying public or airline personnel. In the case of the public, only acute episodes that occur on planes are noted. As to chronic effects associated with intermittent or continuous small exposures, no routine monitoring is available. The health of pilots has been studied several times from the standpoint of the safety of the other occupants of the plane, who depend on them, but little information is available on the impact of flying on the pilots' health. Even less is available on the health of cabin attendants. The scientific process involves the collection of reproducible facts, and scientific evidence is considered valid only if it is reproducible under similar circumstances or if lack of reproducibility can be explained. In medical surveys, findings tend not to be reproducible when the observer's subjective error is large and thus permits ample opportunity for erroneous or distorted conclusions to be drawn. Few researchers check for systematic internal errors; almost without exception, the authors of the studies reviewed for this report did not measure observer error either by duplicating observations or by attempting to control
22 observer bias through the use of double-blind procedures, regardless of whether the tests were laboratory or clinical examinations. Demonstrating that a specific health effect is due to a specific cause is usually difficult. There might be limited evidence that exposure to particular characteristics of aircraft cabin air causes acute symptoms or measurable changes in physiologic function, but even this relatively easily found evidence has not been systematically recorded or assembled. Furthermore, very few studies of air pollutants under normal flight conditions and of the reactions of airline personnel to the pollutants have been carried out, and none qualify as exemplary scientific efforts. Some health-related topics were considered by the Committee to be outside its province to investigate, e.g., the availability of medical kits and training of flight attendants in emergency medical procedures. However, the Committee noted that the recent regulation that such kits be used6 contains a requirement that all medical emergencies resulting in the use of such kits be reported annually during the first 2 yearn after implementation of the regulation. This reporting will provide a unique aid to epidemiologic surveys of events possibly related to cabin air quality (see Chapter 7~. The seemingly unrelated problem of evacuation time and instructions in the case of fire was considered by the Committee to be within its charge, because death from smoke inhalation occurs almost every year in airplanes and is a most poignant example of the effects of airliner cabin air pollution (see Chapter 4~. REFERENCES 1. American Conference of Governmental Industrial Hygienists. In Documentation of the Threshold Limit Values. 4th ed. Cincinnati, Ohio: American Conference of Governmental Industrial Hygienists, 1984.
23 2. American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. ASHRAE Standard: Ventilation for Acceptable Air Quality. ASHRAE 62-1981. Atlanta, Gal: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., 1981. 3. Cabin ozone concentration. Code of Federal Regulations, Title 14, Pt. 2S.832. Washington, D.C.: U.S. Government Printing Office, 1985. 4. National primary and secondary ambient air quality standards. Code of Federal Regulations, Title 40, Pt. 50. Washington, D.C.: U.S. Government Printing Office, 1985. Toxic and hazardous substances. Code of Federal Regulations, Title 29, Pt. 1910.1000. Washington, D.C.: U.S. Government Printing Office, 1985. 6. U.S. Federal Aviation Administration. Emergency medical equipment: Final rule. 51~9 Jan.~:1218-1223, 1986. Federal Register 7. Ventilation. Code of Federal Regulations, Title 14, Pt. 25.831. Washington, D.C.: U.S. Government Printing Office, 1985.