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 exposure.
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 effect(s)
- 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.
Table 6-1. AVERAGE NUMBER OF HOURS FLOWN BY MEMBERS OF
THE ASSOCIATION OF FLIGHT ATTENDANTS (AFA). FIGURES REPRESENT
COMBINED DOMESTIC AND INTERNATIONAL FLIGHTS.
|AFA Membership Per Month||Percentage of Number of Hours Flown
|3||64 or fewer
|4||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 Transportation.
- 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.