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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.
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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
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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
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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.
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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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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
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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
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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
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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.
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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.
Representative terms from entire chapter:
airliner cabin
