In February 1987, the U.S. Department of Transportation received recommendations from the National Academy of Sciences related to airliner cabin air quality. In response to their recommendation that smoking be banned on all commercial domestic flights, the Department indicated its intention to conduct a study to Quantify pollutant levels in airliner cabins and to assess the associated health risks. The study was conducted during the period when smoking was banned on scheduled commercial flights having durations of two hours or less, pursuant to Public Law 100-202.* This report presents methodological aspects and results of that study.
The study addressed the broader topic of airliner cabin air quality rather than the single issue of environmental tobacco smoke (ETS). The purpose of this work was to develop information to be used for determining health risks from exposures to ETS for nonsmoking airliner occupants as well as risks from other pollutants of concern for all airliner occupants. To meet this primary objective, secondary objectives were established to (1) identify air contaminants and other parameters requiring measurement, (2) select appropriate instrumentation, (3) develop measurement protocols for collection of data that are representative of in-flight conditions, (4) develop a statistical sampling frame that enables representation of commercial flights departing from major U.S. airports, (5) collect data on flights chosen for monitoring, (6) analyze data to characterize concentration patterns in different types of aircraft under different conditions, (7) identify health effects of the chosen contaminants and select populations of interest for developing a risk assessment framework, (8) apply the framework for risk assessment, and (9) develop and evaluate options for mitigation of contaminants as required.
Pollutants were selected for monitoring that had known or suspected sources in the aircraft and could be monitored or sampled in airliner cabins with small, unobtrusive instrumentation. The monitoring package configured for the study consisted of instruments and sensors for measurement of time-varying concentrations of contaminants in addition to samplers for collection of time-integrated samples. It also included a data acquisition system for recording outputs from the continuous monitors. The instrument was packaged in a single, compact carry-on bag typical of that carried by airline passengers. Electromagnetic compatibility tests of all monitoring devices were performed by the Federal Aviation Administration (FAA) to ensure that they did not interfere with aircraft navigation or communication systems.
The ETS contaminants monitored during the study were nicotine, respirable suspended particles (RSP), and carbon monoxide (CO). Nicotine was measured through collection of time-integrated samples and CO was measured with portable continuous monitors; RSP was measured both by integrated and continuous methods. The other pollutants that were monitored were ozone and microbial aerosols. In addition, carbon dioxide (C02) was monitored. C02 and ozone were measured with time-integrated samples whereas short-term samples were collected for microbial aerosols (bacteria and fungi) near the end of each flight, prior to descent.
Temperature, relative humidity, and cabin air pressure were monitored continuously with portable sensors; these measurements were used to further characterize the cabin environment and to provide appropriate correction factors for the flow rates of pumps used for sampling. Air exchange rates were measured using constant release and integrated sampling of perfluoro- carbon tracers. All aspects of the measurement protocol were pre-tested on four commercial flights that were monitored over a three-day period in March 1989.
Monitoring was to be performed by each technician at an assigned seat. Based on pretest monitoring at a variety of locations, the following four locations were chosen for monitoring on smoking flights: (1) coach smoking section; (2) boundary region of the no-smoking section within three nonsmoking rows near the coach smoking section; (3) middle of the no-smoking section; and (4) remote no-smoking section (i.e., as far as possible from coach smoking, usually near the first-class smoking and nonsmoking sections). Because less substantial variations were expected on nonsmoking flights, two locations (middle and rear of the plane) were chosen for those flights. ETS contaminants were monitored at all seat locations and other pollutants were monitored at half of the locations. The instrument package was typically placed on the technician’s lap or lap tray to obtain measurements of contaminants most representative of passenger breathing levels.
The target sample size for the study was 60 to 120 smoking flights on jet aircraft, including some international flights. A smaller set of 20 to 40 nonsmoking flights was targeted to provide a baseline for comparison. The target sample size for nonsmoking flights was smaller because flight-to-flight variations in ETS contaminant levels were expected to be lower than for smoking flights.
A total of 70 airports that collectively accounted for 90 percent of U.S. enplanements during 1987 was used as the sampling frame for selection of flights to be monitored. Airports of departure were selected for study flights to provide proportional representation of airports associated with all smoking and nonsmoking flights scheduled for departure during January 1989, based on computer data files supplied by DOT. The specific flights to be monitored were chosen by randomly chaining together the selected airports of departure, subject to constraints relating to the smoking/nonsmoking status of flights. For a typical chain of flights, four technicians monitored six smoking flights and then split into two teams to monitor five nonsmoking flights. In total 92 flights were monitored between April and June 1989; 23 nonsmoking flights and 69 smoking flights which included eight international flights were monitored.
The monitored smoking flights proved to be representative with respect to airlines, types of aircraft, flight durations, and times of day for departures. A wide range of smoking rates was observed, ranging from as little as one cigarette per hour to as much as one cigarette per minute. Comparative analyses indicated that smoking rates based on technician observations agreed very well with rates based on collected cigarette butts. An average of 20 cigarettes per hour, or 68 cigarettes per flight, was smoked by passengers in the coach smoking section on smoking flights that were monitored.
ETS contaminants occur in both the gaseous and particulate phases; measurements were made for both phases. Levels of ETS contaminants that were measured on smoking and nonsmoking flights are summarized in Exhibit 1. Based on both Gravimetric and optical measurements, RSP concentrations were highest in the smoking section, averaging near 175 micrograms per cubic meter (ug/m3) compared to a background level of 35 to 40 ug/m3 on nonsmoking flights. Differences across the no-smoking sections of the aircraft for smoking flights, and differences between these no-smoking sections and nonsmoking flights, were less pronounced. The optical measurement method indicated some migration of ETS contaminants into the no-smoking sections on smoking flights in terms of one-minute peak RSP concentrations.EXHIBIT 1. AVERAGE CONCENTRATIONS OF ETS CONTAMINANTS ON SMOKING AND NONSMOKING FLIGHTS
|Smoking Section||No-smoking Section|
|Boundary Rows||Middle Rows||Remote Rows||Rear Rows||Middle Rows|
|Average RSP*, ug/m3||175.8||53.6||30.7||35.0||34.8||40.0|
|Peak RSP+ (1 minute),|
|Average Nicotine, ug/m3||13.43||0.26||0.04||0.05||0.00||0.08|
|Percent Nicotine Samples|
Below Minimum Detection
|Average C0, ppm'||1.4||0.6||0.7||0.8||0.6||0.5|
|Peak CO (1 minute), ppm||3.4||1.4||1.7||1.6||1.3||0.9|
Observed effects of tobacco smoking, based on gas-phase measurements, were more discernible for nicotine than for C0. Beyond the marked increase in nicotine in the smoking section, the boundary region of the no-smoking section was most affected. Differences between nicotine levels for the remaining no-smoking locations and levels on nonsmoking flights were within the range of measurement uncertainty, but nicotine levels were more often above detection limits in the no-smoking locations of smoking flights than on nonsmoking flights. The only discernible effect for CO was in the smoking section itself. CO levels were generally highest before aircraft were airborne, both for smoking and nonsmoking flights, due to intrusion of ground-level emissions.
Measured RSP levels in the boundary region were strongly related to observed smoking rates (i.e., higher levels when smoking rates were ‘ ppm: parts per million higher) and to the distance from the coach smoking section (i.e., higher levels at shorter distances). Measured levels of nicotine and CO in the boundary region did not correlate with smoking rates or distance from the smoking section, but measured levels of all ETS contaminants in the smoking section were strongly related to smoking rates.EXHIBIT 2. AVERAGE CONCENTRATIONS OF SELECTED POLLUTANTS ON
SMOKING AND NONSMOKING FLIGHTS
|Smoking Flights||Nonsmoking Flights|
|Parameter||Smoking Rows||Middle Rows|
|Average C02, ppm*||1562||1568||1756|
|Percent C02 Samples |
z 1,000 ppm
|Average Ozone, ppm||0.01||0.01||0.02|
|Percent Ozone Samples|
> 0.1 ppm
|Average Bacteria', CFU/m3||162.7||131.2||131.1|
|Average Fungi, CFU/m3||5.9||5.0||9.0|
*ppm: parts per million
Relatively high C02 levels were measured, averaging over 1,500 parts per million (ppm) across all monitored flights (Exhibit 2). Measured C02 concentrations exceeded 1,000 ppm, the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) level associated with satisfaction of comfort (odor) criteria, on 87 percent of the monitored flights. Depending on assumed C02 exhalation rates, measured levels were as much as twice those predicted by a cabin air quality model. Even if the measured levels were to be lowered by half, however, C02 concentrations would still exceed 1,000 ppm on 24 percent of the study flights.
Monitored ozone levels were relatively low, averaging an order of magnitude below the FAA three-hour standard of 0.10 ppm and never exceeding this level. Bacteria levels were higher than fungi levels and somewhat higher in smoking than nonsmoking sections, but the measured bacteria and fungi levels in all cases were low, relative to those that have been measured in other indoor environments.
Some difficulties were encountered in measuring air exchange rates, particularly for aircraft without recirculation, due to (1) the limited number of tracer sources and samplers that could be deployed within the constraints of remaining unobtrusive and (2) the lower extent of lateral air movement within the airliner cabin. Based on measurement results for aircraft with recirculation, there were some indications that air exchange rates were higher on smoking than nonsmoking flights, but the number of measurements was too limited to allow firm conclusions.
Relative humidity levels measured during the study were quite low, below 25 percent for about 90 percent of the monitored
Humidity levels were lower on smoking flights (average of 15.5 percent) than on nonsmoking flights (average of 21.5 percent). Temperatures averaged near 24 C (75 F) for both smoking and nonsmoking flights.
Estimates of lifetime lung cancer risk for nonsmoking cabin crew members (flight attendants) and nonsmoking passengers were developed by combining data on measured RSP concentrations with assumptions concerning relative amounts of time spent in different sections of the cabin, respiratory rates for each group, and models expressing dose-response relationships for cancer. Two dose-response models were used, one with risk linearly related to dose (phenomenological model) and one based on the multistage theory of carcinogenesis, which takes into account the age at which exposure begins (multistage model). Resultant estimates of lifetime lung cancer risk (i.e., premature deaths per 100,000 persons at risk) for nonsmokers exposed to ETS are summarized in Exhibit 3 for crewmembers, business passengers (frequent flyers), and casual passengers. The estimated risks were highest for cabin crew members; it was assumed that cabin crew members sustain higher exposures due to larger amounts of time flying, higher respiratory rates and more time spent in the smoking section of aircraft cabins. Estimates from the two dose-response models were quite consistent except in the case of business passengers; for this group, the assumption that frequent flying begins at a later age resulted in lower estimates with the multistage model.EXHIBIT 3. ESTIMATED LIFETIME RISKS OF PREMATURE LUNG CANCER DEATH-ASCRIBABLE
TO ETS ON SMOKING FLIGHTS PER 100,000 NONSMOKING CABIN OCCUPANTS
|Type of Flight/ Model||Cabin Crew Member*||Business Passenger`||Casual Passenger'|
*ppm: parts per million
Applying the risk estimates in Exhibit 3 to the entire U.S. cabin crew population results in an estimated O.18 premature lung cancer deaths per year for domestic flights (that is, approximately 4 premature deaths can be expected every 20 years) and 0.16 premature deaths per year for international flights. Corresponding estimates for the U.S. flying population are 0.24 premature lung cancer deaths per year for domestic flights and 0.18 premature deaths per year for international flights.
Acute upper respiratory and ocular irritation effects of ETS exposure were estimated using CO concentrations as a proxy for ETS levels.
Measured 30-minute peak CO concentrations were compared with empirical data provided by human chamber studies on the numbers of individuals experiencing irritation by various levels of CO as an ETS surrogate.
Based on this comparison, it was estimated that on one-third of smoking flights about one in eight persons — smokers and nonsmokers — seated in the smoking section would experience irritation due to ETS exposure. A similar type of analysis, using nicotine as a surrogate for eye and nose irritant effects of ETS, indicated that on about one-third of smoking flights ETS levels in the smoking section would be sufficiently high to evoke a marked sensory response in the eye and nose of an airliner cabin occupant.EXHIBIT 4. ESTIMATED LIFETIME RISKS OF PREMATURE CANCER
DEATH ASCRIBABLE TO IN-FLIGHT COSMIC RADIATION EXPOSURE
PER 100,000 FLYING CABIN OCCUPANTS
|Cabin Crew Members||Passengers|
|Type of Flight/Path||Flying 960 Hours|
|Flying 480 Hours
Per Year *
|East-West (<_2 hours)||299 to 714||149 to 357|
|East-West (>3 hours)||988 to 1,026||494 to 513|
|North-South (<_2 hours)||90 to 526||45 to 263|
|North-South (>3 hours)||830||415|
|Long, circumpolar (13 hours)||512||256|
(7 - 9 hours)
|387 to 484||194 to 242|
|220 to 291||110 to 146|
*Assuming 20 years of flying.
Cosmic radiation levels were not monitored because an assessment performed at the outset of the study indicated that extensive existing data provided a sufficient basis for risk assessment. Cancer risk estimates, dependent primarily on flight altitude and latitude, were developed for a number of different flight paths using dose-response data developed by the United Nations Scientific Committee on the Effects of Atomic Radiation. As indicated in Exhibit 4, the highest risks are associated with longer domestic and international flights, primarily due to higher altitudes. Because the risks scale linearly with dose, the estimates for cabin crewmembers assumed to fly 960 hours per year are double those of passengers assumed to fly 480 hours per year (Exhibit 4).
Mitigation options were not explored for ozone or biological aerosols because of the low levels that were measured in this study. For ETS, procedural options such as restriction of smoking and technological options such as increased ventilation were assessed. Of these options, a total ban on smoking was estimated to provide the greatest benefit at least cost. Estimated benefits were based on reduced lung-cancer mortality risks. Costs for procedural options associated with smokers’ inconvenience and discomfort, or displacement of smokers to other modes of transportation, could not be estimated due to data limitations.
Relative to the case of unrestricted smoking, the two-hour ban in effect during the past two years would reduce risks ascribable to ETS exposure on domestic flights by about 45 percent. A four-hour ban would reduce risks by about 86 percent, and a six-hour ban would reduce risks by approximately 98 percent. A different type of strategy to curtail smoking, such as allowing smoking during a 10-minute period every two hours, could reduce average exposures to ETS by as much as 70 percent. However, such a strategy could substantially increase the risks of respiratory and other irritant effects from acute exposure to ETS during the brief periods when smoking would be allowed.
Increasing ventilation rates could lower ETS exposures by as much as 33 percent, but associated fuel penalties would result in costs estimated to be greater than the benefits. Improved filter efficiency was estimated to provide only a marginal reduction (about 5 percent) in ETS exposures.
Exposure management was considered to be the only viable option for reducing exposures of cabin crewmembers and passengers to cosmic radiation. In the case of cabin crewmembers, this strategy would involve careful scheduling of personnel to avoid persistent exposure to higher cosmic radiation levels generally associated with high-altitude flights and flight paths toward extreme northern or southern latitudes.
For removal of C02, sorption on solid adsorbent beds whose absorbent capacity for C02 can be regenerated by heating was considered to be a method with potential benefits for aircraft with recirculation. Cost or reliability data were not available for comparison with costs of additional ventilation, which could also be used to bring C02 levels closer to the guidelines specified by ASHRAE.in other confined environments (e.g., residential, office, public access buildings) and ambient outdoor environments began to shed light on previously unstudied phenomena, such as bioaerosols, and began to illustrate previously unrecognized chemical complexity. Continuing studies of exposure to ETS, for example, cast some doubt on the utility of the much earlier FAA/PHS study because more effective marker constituents had been identified, and, of at least equal importance, improved measurement capabilities allowed more precise monitoring of a wider range of field environments.
In that light, it came as no surprise that a series of Congressional hearings held in 1983 and 1984 concluded that the available data on the airliner cabin environment were contradictory and that present standards and practices could be questioned. As a result of the hearings, Congress, through Public Law 98-466, directed the Secretary of Transportation to commission an independent study by the National Academy of Sciences to examine the adequacy of industry practices and FAA rules and regulations as they affect the health and safety aspects of the airliner cabin environment aboard civil commercial aircraft.
This mandate served as a major collection point to review previous work directed specifically to the environmental quality aboard aircraft and to examine other pollutants and sources that, based on emerging concerns from other fields, could be responsible for health
problems in the long or short run. The Academy was directed to recommend remedies for problems discovered and to outline safety precautions to protect passengers from smoke and fumes produced by in-flight fires.
To maintain the independence of the study, FAA did not participate or take any actions that could affect findings, conclusions, or recommendations of the study. At the request of the Academy, however, FAA provided data and rendered assistance to the committee established in the National Research Councils Commission on Life Sciences that was assembled to conduct the study. In the course of the study, the Committee on Airliner Cabin Air Quality reviewed the available technical literature including characteristics of various models of modern aircraft. The Committee also held a series of technical meetings and briefings with experts in relevant fields and made a number of site visits to evaluate specific issues.
The Committee’s report (NRC 1986a), issued in August of 1986, identified several potential sources of environmental quality problems on aircraft including tobacco smoke, ozone, cosmic radiation, humidity, and microbial aerosols. The Committee noted, however, that available empirical evidence was of insufficient quality and quantity for a scientific evaluation. Unique aspects of the airliner cabin environment precluded drawing valid conclusions on the basis of data from other environments. Consequently, recommendations from the study focused largely on defining areas of data collection necessary to more fully understand potential exposures.
The Committee recommended that smoking be banned on all commercial flights to lessen irritation and discomfort and to reduce potential health hazards associated with ETS by bringing that aspect of cabin air quality into line with established standards for other closed environments. The smoking ban was also cited as a means to eliminate the possibility of fires caused by cigarettes.
There has been a growing concern that exposure to ETS may be associated with adverse health and comfort effects among nonsmokers. This concern is further enhanced by the growing interest in indoor air quality, the recognition that ETS is a major indoor contaminant source, and the fact that a large number of people are exposed to ETS. The health and comfort effects of involuntary smoking have been extensively reviewed by the Committee on Passive Smoking of the National Research Council (NRC 1986b) and by the U.S. Surgeon General (DHHS 1986). Both reviews concluded that exposure of nonsmokers results in:Acute irritation of the eyes, nose, and throat along with perception of odor.
Upper airway problems in children including increased prevalence of respiratory symptoms (cough, sputum production, wheezing), decreased lung function, increased lower respiratory illness, and increased rates of chronic ear infections
Increased risk of lung cancer. The reviews also noted other outcomes related to the growth and health of children, including lower birth weight.
After completing a review of the Academy report on the airliner cabin environment, DOT assembled a report to summarize its responses (DOT 1987) to accompany submittal of the Academy report to Congress in February 1987. DOT accepted in full or in part most of the recommendations made in the Academy report. While recognizing that exposure to ETS could be viewed as a problem by some crewmembers and passengers, DOT suggested that further study was needed to better define health effects, concentrations and possible technical solutions before proposing a definitive response to a smoking ban on all commercial aircraft.
In December of 1987, Public Law 100-202 was enacted, prohibiting smoking by passengers on any scheduled commercial flight of two hours or shorter duration. This limited smoking ban is effective for 24 months beginning April 23, 1988. At the same time, DOT also received Congressional approval to conduct a study to resolve technical questions that must be answered before continuing or broadening the prohibitions contained in PL 100-202.
REVIEW OF AVAILABLE DATA
The information incorporated into the Committee on Airliner Cabin Air Quality report constitutes a comprehensive survey of the published literature to about 1985 (NRC 1986a). This section briefly summarizes the results of relevant studies identified by the Committee together with research results that have been published since that time.
Environmental tobacco smoke is a complex mixture of gas- and particulate-phase contaminants. More than 3,800 compounds have been identified in ETS. Field monitoring studies, however, seek to quantitate a relatively small number of marker constituents. The aircraft environment has not been systematically investigated for ETS contaminant levels. Early studies conducted by FAA and PHS (1971) measured cabin levels of C0, hydrocarbon vapors, TSP, and PAH on twenty Military Airlift Command flights and fourteen domestic flights over an 18-month period. Environmental sampling revealed very low levels of each contaminant measured, well below occupational and environmental air quality standards, and these contaminants were not judged to represent a hazard to non-smoking passengers. Analysis of subjective questionnaires, however, also revealed that a significant proportion of nonsmoking passengers were bothered by tobacco smoke, leading to regulations to segregate smoking passengers.
Other ETS studies of the airliner cabin environment identified by the committee utilized measures of CO and RSP. Anecdotal measurements carried out by Committee members during the Academy study included very limited measurements of N02, RSP, and C02 using portable instruments on commercial flights. Although suggesting the possible range of concentrations of ETS-based contaminants, none of these earlier data provide definitive results.
More recent sampling studies aboard commercial airliners have been published by Oldaker and Conrad (1987) and by Mattson et al. (1989). Oldaker and Conrad measured vapor-phase nicotine in no smoking and smoking sections of three types of commercial aircraft (Boeing 727-200, 737-200 and 737-300). Forty-nine measurements were conducted in no-smoking sections, out of which 40 measurements were conducted in the boundary region (i.e., two rows in no-smoking sections adjacent to smoking sections).
Additionally, 26 measurements were conducted in smoking sections. Average nicotine concentrations (+_standard deviations) were 22.4 t 28.4 Ug/m3 in smoking sections, 10.6 +_ 9.7 Ug/m3 in the boundary region of no-smoking sections, and 3.3 +_3.6 Ug/m3 in the remainder of the no-smoking sections. They did not find any significant correlation between nicotine concentrations and the number of smokers; however, smoking rates were not measured but assumed to be 2 cigarettes per hour per passenger seated in the smoking section.
Data on nicotine exposures, cotinine (a major metaoblite of nicotine) excretion levels, and acute symptoms from a subsequent study of passive smoking on commercial airliner flights showed that a total separation of smoking and nonsmoking sections was not achieved (Mattson et al. 1989). The study was conducted with 9 subjects on tour flights lasting approximately 4 hours each. Two of the four flights were on aircraft with 100 percent outside air ventilation (Boeing 727) and the other two were on aircraft with 50 percent recirculation (Boeing 767). The observed nicotine levels were similar to those measured in the Oldnker and Conrad study: 13.6 +_ 23.0 ug/m3 in the boundary region of no-smoking sections and 16.5 +_ 7.1 ug/m3 in smoking sections. Aircraft with no recirculation had significantly lower nicotine concentrations than those with recirculation. Urinary cotinine levels were related to nicotine exposure for the subjects — those with the highest nicotine exposures had the highest levels of cotinine excretion. Eye and nose symptoms indicative of acute symptoms were related to nicotine and cotinine levels.
Although these studies have been useful in suggesting ranges of concentrations of ETS tracers encountered in the general airliner cabin environment, the samples were not randomly selected and the number of observations was generally small, precluding any generalization of the results. Similarly, determining factors (e.g., ventilation systems, eating patterns) of ETS concentrations for the general airliner cabin environment have not been systematically investigated.
Although ETS is of obvious importance in the context of PL 100-202, additional pollutants and factors identified by the Committee warrant attention. Essentially no published measurement data exist with regard to ventilation rates (i.e., fresh-air dilution rates in the passenger breathing zone), carbon dioxide levels, or microbial aerosols. As cited in the Academy report (NRC 1986a), some data exist to confirm expectations of low relative humidity. Similarly, the committee identified fairly abundant data to confirm intrusions of stratospheric ozone into the flight cabin, but also cited the need for additional data to establish compliance with FAA standards. Issues surrounding potential exposures to cosmic radiation (particularly at high altitudes) were also raised.
- U.S. Department of Health and Human Services. 1986. The Health Consequences of Involuntary Smoking . Report of the Surgeon General. Rockville, MD.
- U.S. Department of Transportation. 1987. Report to Congress: Airline Cabin Air Quality . Prepared Pursuant to Public Law 98-466 by the U .S . Department of Transportation, Washington, DC.
- Federal Aviation Administration Public Health Service. 1971. Health Aspects of Smoking in Transport Aircraft. U.S. Federal Aviation Administration and U. S. National Institute for Occupational Safety and Health. Available from the National Technical Information Service (Report No. AD736097).
- Federal Aviation Administration. 1980. Results of FAA Cabin Ozone Monitoring Program in Commercial Aircraft in 1978 and 1979. Report No. FAA-EE-80-10 , Office of Energy and Environment, U .S . Federal Aviation Administration, Washington, DC.
- Federal Aviation Administration. 1985. Air Carrier Operation Pollution. Consolidated Reprint, U.S. Federal Aviation Administration, Washington, DC.
- Federal Aviation Administration. 1989. Draft Advisory Circular on Radiation Exposure of Air Carrier Crew Members.U.S. Federal Aviation Administration, Washington, DC.
- Mattson, M.E., et al. 1989. “Passive Smoking on Commercial Airlines.” J. Am. Med. Assoc., Vol. 261, No. 6, pp. 867-872.
- National Research Council. 1986a. The Airliner Cabin Environment. National Academy Press, Washington, DC.
- National Research Council. 1986b. Environmental Tobacco Smoke: Measuring Exposures and Assessing Health Effects.
- National Academy Press. Washington, Oldaker, G.B., and F.C. Conrad. 1987. “Estimation of Effects of Environmental Tobacco Smoke on Air Quality Within Passenger Cabins of Commercial Aircraft.” Environ. Sci. Technol., Vol. 21, No. 10, pp. 994-999.