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6. GENERAL APPROACH TO RISK ASSESSMENT
The general approach to risk assessment in this investigation was that
described by the National Research Council (1983) of the National Academy of
Sciences. This report defines risk assessment as a systematic, multi-step
process of data evaluation designed to characterize the nature and magnitude
of health damage posed by an environmental agent under various conditions of
A comprehensive risk assessment contains four major steps:
Hazard identification is the determination of whether exposure to a
particular chemical is or is not causally linked to a particular health
Dose-response assessment is the determination of the relation between
the magnitude of exposure and the probability of occurrence of the health
effect(s) in question
Exposure assessment is the determination of the extent of human
exposure before or after application of regulatory controls
Risk characterization is a description of the nature and often the
magnitude of human risk, including attendant uncertainty.
The process of conducting a risk assessment involves integrating the
information in each of these areas in a systematic fashion, first by
identifying the health hazards, then deriving a quantitative expression of the
dose-response relationship based on the identified health hazards of greatest
concern, and then combining the derived dose-response algorithm with an
independent quantitative exposure assessment to produce a characterization of
risk. Prior to the collection and analysis of data for the quantitative
estimation of risk, underlying decisions must be made about the population(s),
pollutant(s), and health effect(s) of interest, so that the ensuing expression
of risk targets those areas.
6.1 POLLUTANTS AND HEALTH EFFECTS OF INTEREST
The pollutants of concern in the airliner cabin environment and their
attendant health effects (hazard identification) have previously been
identified (National Research Council 1986), so that exposure assessment and
dose-response assessment were the critical elements requiring definition for
risk characterization. In this investigation, multiple procedures were
required to characterize risk, depending on the health endpoint of interest,
the chemical entity of interest, its mode of action, and the degree of
scientific understanding about the chemical:
Environmental tobacco smoke (ETS) was of interest as a chemical mixture
because of its carcinogenic potential, and respiratory and cardiovascular
effects. For carcinogenicity, it was necessary to select the most
appropriate dose-response model(s) that correlate expected individual risk
with degree of exposure to RSP as a surrogate for the ETS mixture.
Nicotine, as a constituent of ETS, is an appropriate indicator for its
acute respiratory effects. Human inhalation dose-response data exist for
the irritant properties of ETS, using nicotine as a surrogate.
Carbon monoxide, like nicotine, can be used as an ETS surrogate for
acute respiratory effects.
Universally applicable procedures for risk assessment of bioaerosols
(both fungi and bacteria) hare not been established. As a result,
conventional expressions of risk assessment cannot be used. For fungi, the
20 Genera that occur most frequently in highest concentrations on growth
plates were identified. Their relative clinical significance was then
ascertained using their ability to cause allergies and infections as
benchmark clinical weight-of-evidence criteria. This relative significance
is reported for the 20 identified genera. A similar procedure was used for
bacteria to determine prevalence.
Ozone presented a unique problem because the scientific community is
divided on the lowest ambient air concentration causing an increase in
lung infectivity. Concentrations aboard aircraft were compared with the
current FAA regulatory 3-hour standard of 0.10 ppm.
The risks from exposure to cosmic radiation were based on dose-response
data provided by the United Nations Scientific Committee on the Effects of
Atomic Radiation (1986, 1988) and the Federal Aviation Administration
(1989). Combining these data with plausible exposure levels and durations,
risks were determined for cancer, fetal retardation, and birth defects.
6.2 POPULATIONS OF INTEREST AND FREQUENCY OF FLYING
In order to establish meaningful estimates of risk, it was necessary to
subdivide the entire population of flyers according to frequency of flying
(which would influence the amount of exposure to cabin air) and health and
maturational status (which would influence the dose-response relationship
between specific pollutants and their health effects).
The populations of interest in this investigation included cabin
crewmembers, who are representative of occupational exposure, and all
passengers. Children, fetuses, asthmatics, and individuals with preexisting
cardiovascular disease constituted four passenger sub-populations of special
interest. Flight crewmembers, whose environment on the flight deck is
different from the aircraft cabin, were not considered in this investigation.
The specific pollutants and associated health effects of concern varied among
these populations and sub-populations:
ETS was considered for cancer in all passenger populations without
preexisting illness and cabin crew members, for chronic respiratory
Illness in children, for acute respiratory effects in all individuals
without preexisting illness and asthmatics, and for cardiovascular disease
in cabin crew numbers and individuals with this preexisting illness.
Bioaerosols (fungi and bacteria) were considered in all populations for
their clinical significance as allergens and infectious agents.
Ozone was considered in all passengers without preexisting illness and
in cabin crew members, in accordance with the basis of the FAA ozone
standard in aircraft.
Cosmic radiation was considered for cancer in all passengers and cabin
crewmembers, and for birth defects and retardation in fetuses.
The relationship among pollutants, populations, and health effects is
presented in Figure 6-1.
AVERAGE NUMBER OF HOURS FLOWN BY
MEMBERS OF THE ASSOCIATION OF FLIGHT
ATTENDANTS (AFA). FIGURES REPRESENT
COMBINED DOMESTIC AND
|Percentage of Number
of Hours Flown
||64 or fewer
||90 or more
Source: 1985 AFA Survey
Frequency of flying is important where exposure over a protracted time
period (e.g., years) affects health, such as in case of development of cancer.
Among passengers, frequency of flying was not distinguishable into apparent
and justifiable categories since there were no universally applicable criteria
for what constituted a frequent and non-frequent flyer. Accordingly, for this
investigation classifications of frequency were set aside. Instead, in the
case of cancer, frequency-variable risk tomograms were developed for ETS and
cancer so that frequency-specific cancer risks can be developed.
Exposure to cosmic radiation is also dependent on frequency, as well as on
altitude and latitude of flight. Greatest radiation occurs at high altitude
over the earth's poles, gradually diminishing in intensity toward the equator.
Exposure can be determined by adding individual doses received during
individual flights. The cumulative dose is then applied to a dose-response
curve for the health effect of interest.
Frequency of flying was not relevant for other health effects that were
considered since they were a result of short-term episodic exposure.
Cabin crewmembers were estimated to log approximately 80 hours of flight
time per month (Association of Flight Attendants 1988). This is based on the
distribution of cabin crew flight frequencies contained in Table 6-1.
Association of Flight Attendants. 1988. Letter to Robert R. McMeekin.
Federal Aviation Administration. October 3, 1988. Washington, D.C.
Federal Aviation Administration. 1989. Radiation Exposure of Air Carrier
Crewmembers. AAM-624. Federal Aviation Administration, U.S. Department of
National Research Council. 1983. Risk Assessment in the Federal
Government. National Academy Press. Washington, D.C. .
National Research Council. 1986. The Airliner Cabin Environment: Air
Quality and Safety. National Academy Press. Washington, D .C .
United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR).
1988. Sources Effects and Risks of Ionizing Radiation. Report to the
General Assembly. Annex F: Radiation Carcinogen in Man. UN Publication
Sales No. E.88.IX.9 United Nations, New York, NY
United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR).
1986. Genetic and Somatic Effects of Ionizing Radiation. Report to the
General Assembly. Annex C: Biological Effects of Pre-natal Irradiation. UN
publications Sales No. E.86.IX.9 United Nations. New York, NY.