Journal of Occupational and Environmental Hygiene, 12: 145–155 ISSN: 1545-9624 print / 1545-9632 online c 2015 JOEH, LLC Copyright  DOI: 10.1080/15459624.2014.957830

Modeling Flight Attendants’ Exposure to Secondhand Smoke in Commercial Aircraft: Historical Trends from 1955 to 1989 Ruiling Liu, Linda Dix-Cooper, and S. Katharine Hammond Department of Environmental Health Sciences, School of Public Health, University of California, Berkeley, California

Flight attendants were exposed to elevated levels of secondhand smoke (SHS) in commercial aircraft when smoking was allowed on planes. During flight attendants’ working years, their occupational SHS exposure was influenced by various factors, including the prevalence of active smokers on planes, fliers’ smoking behaviors, airplane flight load factors, and ventilation systems. These factors have likely changed over the past six decades and would affect SHS concentrations in commercial aircraft. However, changes in flight attendants’ exposure to SHS have not been examined in the literature. This study estimates the magnitude of the changes and the historic trends of flight attendants’ SHS exposure in U.S. domestic commercial aircraft by integrating historical changes of contributing factors. Mass balance models were developed and evaluated to estimate flight attendants’ exposure to SHS in passenger cabins, as indicated by two commonly used tracers (airborne nicotine and particulate matter (PM)). Monte Carlo simulations integrating historical trends and distributions of influence factors were used to simulate 10,000 flight attendants’ exposure to SHS on commercial flights from 1955 to 1989. These models indicate that annual mean SHS PM concentrations to which flight attendants were exposed in passenger cabins steadily decreased from approximately 265 μg/m3 in 1955 and 1960 to 93 μg/m3 by 1989, and airborne nicotine exposure among flight attendants also decreased from 11.1 μg/m3 in 1955 to 6.5 μg/m3 in 1989. Using duration of employment as an indicator of flight attendants’ cumulative occupational exposure to SHS in epidemiological studies would inaccurately assess their lifetime exposures and thus bias the relationship between the exposure and health effects. This historical trend should be considered in future epidemiological studies. Keywords

commercial aircraft, flight attendants, historical trends, modeling and simulation, secondhand smoke

Ruiling Liu is currently affiliated with the Cancer Prevention Institute of California, Berkeley, California. Linda Dix-Cooper is currently affiliated with the British Columbia Center for Disease Control, Environmental Health Services, Vancouver, British Columbia, Canada. Address correspondence to: S. Katharine Hammond, Department of Environmental Health Sciences, School of Public Health, University of California, Berkeley, CA 94720; e-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/uoeh.

INTRODUCTION

T

he aircraft cabin is a unique confined environment because of the high occupant density, its occupants’ inability to leave or open windows, and the need for cabin pressurization.(1) In aircraft cabins that allow smoking, secondhand smoke exposure (SHSe) has been reported as a main cause of eye irritation by flight attendants.(2) However, smoking had not been restricted in any commercial airlines in the United States until 1971, when United Airlines became the first carrier to offer separate smoking and nonsmoking sections.(3) In 1973, the U.S. Civil Aeronautics Board passed regulations to segregate smokers from nonsmokers by setting smoking sections on airplanes in response to passenger complaints regarding in-flight SHSe.(4) In 1988, the Federal Aviation Act was signed, making domestic flights of two hours or less smoke-free for a trial of two years. In 1990, the Act was extended to all domestic flights of six hours or less, affecting all but 28 of the 16,000 domestic flights in the United States.(3) By 1985 more than 40,000 flight attendants in the United States worked an average of 900 hours each year(4) in small, confined, and smoky aircraft cabins and experienced elevated SHSe. Investigation of SHSe and health conditions, especially chronic diseases and cancers, among flight attendants is especially important both intrinsically and as the results to inform our understanding of the health effects of SHS. Short-term health effects reported among flight attendants due to occupational SHSe include respiratory irritation, ocular symptoms, decreased tearfilm stability, and alterations in nasal patency(5,6); and chronic effects include pulmonary function abnormalities,(7,8) respiratory diseases,(9,10) increased rates of hypertension,(11) breast cancer, and malignant melanoma.(12) Interestingly, increased lung cancer incidence or mortality has not been observed among flight attendants comparing to the general population.(12–14) However, previous occupational epidemiology studies on long-term effects of SHSe in aircraft usually used cumulative working hours or years as the metric for lifetime exposure

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in smoky cabins without considering temporal trends in exposure intensity (i.e., concentrations).(9–11) During a flight attendant’s work life, several changes occurred in the United States, affecting SHS concentrations in airline cabins, and thus the magnitude of flight attendants’ SHSe at work and the attendant health risks. These changes include prevalence of current smokers, number of cigarettes consumed per hour by a smoker, the fraction of cabin seats occupied, and air recirculation systems used. Using only exposure duration to represent flight attendants’ lifetime SHSe in smoky cabins is inaccurate and could bias the association between the exposure and the health effect of interest. Thus, understanding the magnitude of these changes and historic trends is very important for epidemiological studies examining the association between SHSe and chronic health conditions and for related risk assessments. However, these changes and historical trends have not been examined in literature. SHS is a complex aerosol consisting of thousands of gases and volatile chemicals in which fine particulate matter (PM2.5 ) is suspended,(15) and its environmental concentrations have been commonly quantified using two tracers: airborne nicotine and respirable suspended particles (RSP) or PM2.5 .(16) Unfortunately, nicotine concentrations in commercial aircraft cabins were not measured in planes prior to the mid-1980s in the United States(6,17,18) and the earliest RSP measurements were conducted in 1970, with results available for military flights only.(19) This study aims to estimate the magnitude and historic trends of flight attendants’ SHSe in aircraft cabins by integrating historical changes in passenger smoking behaviors and aircraft characteristics, so that future epidemiological studies and risk assessment can better quantify flight attendants’ lifetime SHSe at work. Because limited information is available on these factors on international flights and before 1955, this study estimates flight attendants’ SHSe on U.S. domestic flights from 1955 until 1989. METHODS

M

ass balance models were developed to estimate SHS concentrations (nicotine and RSP) in passenger cabins experienced by flight attendants. Monte Carlo simulations were used to integrate the uncertainties of each parameter in the models and to estimate the distributions of flight attendants’ SHSe from 1955 to 1989 in order to allow more accurate estimates of the risk of chronic health effects due to the SHSe among flight attendants. Mass Balance Modeling of Time-Weighted-Average (TWA) SHS Concentrations SHS concentrations in passenger cabins depend on the emission strength, volume of the cabin, air exchange rate, and other removal mechanisms if applicable. Since manufacturers recommend air exchange rates of airline cabins be greater than 10 exchanges/ hour,(20) the passenger cabin was assumed to be well mixed during flights. 146

FIGURE 1. One-compartment model (upper panel) and twocompartment model (lower panel) for emission and removal of SHS in smoking passenger cabins

Smoking was generally unrestricted in passenger cabins before 1973, thus a one-compartment model was used to estimate in-cabin SHS concentrations (Figure 1). At steady state the mass of SHS entering the system equals that leaving the system in any specific time period. Cigarette smoking is the source (S), which is removed by ventilation and other mechanisms such as deposition. Equation 1 presents the mass balance equation, with the left side indicating the mass of SHS entering the compartment per hour, and the right side indicating the mass of SHS removed per hour: S = C × V × (λair + κ)

(1)

where C is the SHS concentration, μg/ m3: S is the source strength, μg/hr V is the volume of the passenger cabin, m3 λair is the effective air exchange rate, hr−1 κ is the combined removal rate of other mechanisms like deposition, absorption, hr−1. The compartment was assumed to be well mixed over time, thus the TWA concentration experienced by a flight attendant was expected to be the same as the area concentration (C) in the cabin, which can be estimated by Eq. (2), derived from Eq. (1): CTWA = C =

S V (λair + κ)

(2)

After 1973, all the U.S. domestic flights were required to restrict smoking to designated sections. To predict the concentrations in the smoking and the nonsmoking sections, a twocompartment model (Figure 1) was assumed, each well mixed, and in steady state. Since “The air supply and air exhaust flow rates are matched along the length of the cabin as much as possible to minimize net flows along the length of the aircraft,”(1) the air supply and air exhaust flow rates in the smoking and nonsmoking sections were assumed the same, and the net air exchange rate between the two sections was assumed to be zero. That is, the air exchange between the two sections is only due to turbulent diffusion Q. Eqs. 3 and 4 present the

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mass balance equation for the smoking compartment and the nonsmoking compartment, respectively. The left side of each equation presents the SHS mass entering the compartment per hour, and the right side presents the mass removed per hour: Smoking section: Q × Cs S + Q × Cns = (λair + κ) × Cs + Vs Vs

(3)

Nonsmoking section: Q × Cs Q × Cns = (λair + κ) × Cns + Vns Vns

(4)

where S is the source strength of SHS emission, μg/hr Cs is the SHS concentration in the smoking section, μg/m3 Cns is the SHS concentration in the nonsmoking section, μg/m3 Q is the air flow rate due to turbulent diffusion between the two sections, m3/hr Vs is the volume of the smoking section, m3 Vns is the volume of the nonsmoking section, m3 λair is the effective air exchange rate, hr−1 κ is the combined removal rate of other mechanisms like deposition, absorption, hr−1. SHS concentration in the smoking and the nonsmoking sections can thus be estimated by Eqs. (5) and (6), respectively: S Vs × (λair + κ) + Vns × (λair + κ) × Q/[Vns × (λair + κ) + Q] (5) Q = (6) × Cs Vns × (λair + κ) + Q

Cs =

Cns

To estimate the TWA concentration experienced by flight attendants, their time spent exposed in each section was assumed to be proportional to their respective volumes. Galley areas were assumed to be proportionally distributed. Many planes have galleys behind the smoking sections for each class of service, thus, this assumption might lead to underestimates of flight attendants’ SHSe because SHS concentrations in galleys adjacent to smoking sections are expected to be similar to those in the smoking sections; also flight attendants might spend more time than assumed in the smoky galleys. With these assumptions, the TWA concentration is estimated by Eq. (7): CTWA =

Cs × Vs + Cns × Vns Vs + Vns

(7)

Combine Eqs. (5), (6), and (7) to estimate the TWA SHS concentration: S CTWA = (8) V (λair + κ) Eq. (8) is the same as Eq. (2), which estimates the TWA concentration if smoking is unrestricted in the cabin. The source strength of SHS emissions, S, is a function of number of cigarettes smoked per hour and the emission rate of SHS per cigarette smoked, which can be estimated by Eq. (9): S = lf × Nseat × Psmk × Practive × Rsmk × EFSHS

(9)

where S is the source strength of SHS emission, μg/hr lf is the proportion of passenger seats occupied (load factor) Nseat is the total number of passenger seats in the cabin Psmk is the prevalence of current smokers among the flight passengers Practive is the proportion of current smokers who actually smoked in the flight or approximately the proportion of passengers who sit in the smoking section Rsmk is the smoking rate of each smoker or smoking-section passenger, cigarettes/hr EFSHS is the SHS emission factor, indicated by RSP/PM2.5 or nicotine, μg/cigarette. The TWA concentration can be estimated by combining Eqs. (8) and (9) to yield Eq. (10): lf × Nseat × Psmk × Practive × Rsmk × EFSHS V (λair + κ) lf × Psmk × Practive ×Rsmk × EFSHS = (10) V Nseat × (λair + κ)

CTWA =

where all the parameters are defined previously. Evaluating the Models with Field Measurement Data Measurements of SHS concentrations in U.S. domestic airlines are limited, and systematic investigations of flight attendants’ SHSe in a variety of aircraft are rare in the literature. Repace summarized environmental measurements of particulate matter (PM) and airborne nicotine in commercial aircraft.(21) Only two studies reported both measured cabin SHS concentrations and key information which could be used to predict the average concentrations during the flying time by mass balance models. Eatough et al.(22,23) reported measured PM2.5 and nicotine concentrations, number of cigarette butts at the conclusion of each flight, flight durations, default ventilation rates, and number of rows in smoking and nonsmoking sections in four DC-10 flights in 1989. The sampling was conducted by four nonsmoking volunteer passengers in smoking sections, nonsmoking sections bordering the smoking section, and the middle of nonsmoking sections. Mattson et al.(6) reported measured nicotine concentrations in two 5-hr Boeing 727 flights (without recirculation) and two 4-hr Boeing 767 flights (with 50% of the air recirculated), number of seats in the smoking section and nonsmoking sections, and the average number of active smokers per count (five to seven counts in total) in 1988; two types of sampling were conducted, one was area sampling by five nonsmoking passengers seated in the smoking section or in the nonsmoking section bordering the smoking section, and the other was personal sampling from four nonsmoking flight attendants assigned to work in the smoking or nonsmoking sections. These two studies were used to evaluate the mass balance models presented in this article (Eqs. (5)– (8)). The air exchange rate between smoking and nonsmoking sections due to turbulent diffusion was estimated by multiplying the root-mean-square fluctuating velocity (i.e., 0.008 m/s) measured in Boeing 747-400 cabins(24) with the average crosssection area (in m2) for each flight, which was obtained from

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online documents on airplane characteristics released by the Boeing Company.(25–27) All the parameters reported by Mattson et al.(6) for predicting the average SHS concentrations in smoking sections or nonsmoking sections were almost the same for the two 5-hr Boeing 727 flights and for the two 4-hr Boeing 767 flights, respectively. Thus the average concentrations in the two sections and flight attendants’ TWA concentrations during the flights were predicted the same for the two 727 flights and for the two 767 flights, respectively. A geometric standard deviation (GSD) of 2.5, which corresponds to a moderate variability,(28) was assumed. The 95% confidence intervals (CIs) were estimated based on predicted average concentrations, a GSD of 2.5, and log normal distributions. The 95%CIs were compared with the range of reported point field measurements to evaluate the performance of the mass balance models. SHS concentrations in nonsmoking sections decreased rapidly as the distance of the sampling location to the smoking sections increased,(22) and they were usually very low in the middle or remote end of the nonsmoking sections.(17,22) Because the overall average concentrations in nonsmoking sections were estimated by the mass balance models, while they could not be estimated according to field measurements reported in the two papers, the performance of the mass balance models in estimating nonsmoking-section concentrations was not evaluated in this article. Determining the Parameters for Monte Carlo Simulation The definition of each parameter and source of information are briefly described in Table I; the associated values for the arithmetic means, coefficients of variation (CV), and ranges are also listed in Table I. Most of the parameter information was based on literature values; if information was not available from literature, parameter values were estimated. When the standard deviation (SD) of a parameter was not reported, the CV was assumed to be 20% to 80% depending on the expected magnitude of variance. Because little information was available for the distributions of these parameters, a typical normal distribution was assumed for each parameter; for parameters with low and upper ends, double truncated normal distributions were assumed. The mean and CV of the double truncated normal distributions were slightly different with those of the un-truncated ones. Load factors (proportion of passenger seats occupied) in each year from 1955 to 1989 were estimated basing on the average of load factors of passenger flights reported by the Air Transport Association of American in its annual reports of the years 1961, 1971, 1977, 1979, 1981, 1983, 1985, 1987, and 1989. The standard deviations and ranges of this parameter were estimated from a national survey of 82 U.S. departed flights (23 nonsmoking flights and 69 smoking flights) in 1989 by the Department of Transportation (DOT) (referred to as the 1989 DOT Survey hereafter).(29) The demographics of U.S. airline passengers in early years were different from the general population due to the relatively higher cost of airline 148

travel as opposed to other travel approaches, and the former were primarily Caucasians ≥25 years old with at least a high school education, and there were more male fliers than female fliers.(30–33) The prevalence of current smokers among fliers in the years from 1955 to 1989 was hence estimated by adjusting the current smoking prevalence of the general population with the race/ethnicity, age, education and gender of commercial flight passengers. Proportion of current smokers who actually smoked in flights or proportion of smoking-section fliers were estimated basing on a survey of in-flight smoking behaviors in 66 flights of two-hours durations before 1961,(34) a 1971 survey among 14 U.S. domestic civilian flights,(31) and the 1989 DOT survey.(29) Smoking rates of smoking-section passengers were estimated from the cigarette consumption and the current smoking prevalence among U.S. adults in each select year from 1955 to 1989, and the active smoking rate of 1.5 cigarettes/hr observed in the 1989 DOT survey.(35,36) Emission factors (μg/cigarette) of RSP and airborne nicotine from cigarette smoking were based on emission factors of these two markers by type of cigarettes (full flavor, with mainstream tar ≥15 mg/cigarette; full flavor, low tar, with mainstream tar 6–15 mg/cigarette; and ultra-low tar, with mainstream tar ≤6 mg/cigarette)(37) from the 1960s to 1991 as reported in the literature,(38–40) and the market shares of those types of cigarettes from 1967 to 1989 reported by the Federal Trade Commission (FTC) for the years from 1967 to 1989.(41) Cabin volumes and number of passenger seats of the major models of aircraft from 1964 to 1995 were estimated from a review by Hocking,(20) and were integrated into one parameter, volumn/Nseat to represent volume per passenger seat. The effective removal rates by ventilation were estimated for RSP and nicotine separately basing on whether recirculation systems were used or not. For RSP, the effective removal rate by ventilation was the total air exchange rate in hour−1, no matter if the aircraft was built with recirculation systems or not, because aircraft with recirculation systems usually used filters, which were believed to remove most of RSPs. For airborne nicotine, the effective removal rate by ventilation was the fresh air exchange rate, because the filters were ineffective in removing gas phase chemicals. The air exchange rate of total air or fresh air by different aircraft models in different time periods were obtained from the reports by Hocking and by the National Research Council (NRC).(1,20) Removal rate of RSP by deposition was ignored because it was much smaller than the removal rate by ventilation, and removal rate of nicotine by deposition and absorption was estimated according to the paper by Van Loy et al.(42) Details on how all these parameters were obtained or estimated are presented in the supplemental. Monte Carlo Simulation of Flight Attendants’ Exposures to RSP and Nicotine in Cabins Monte Carlo simulations were used to estimate the annual SHS concentrations to which flight attendants were exposed in cabins for select years from 1955 to 1989 when the

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emission factor, μg/cigarette

air exchange rate/hr

EF

λ

volume per seat, m3/seat

V seat

1.3

25–50% depending on years 2%–38% depending on years 16

30%

50%

30%

20%

50%

20%

70%

20%

20%

35%

CV

1

8

1/3

20%

1/3 of mean

0.2

1/2 of mean

1/2 of mean

1/3 of mean

lower bound

Notes: CV, coefficient of variation; ND: normal distribution; DTND: double truncated normal distribution.

κ

f2

percentage of air that was recirculated percentage of seating hours with recirculation nicotine removal rate by surface absorption/hr

f1

SHS PM: 14307-22000; nicotine: 1600–1693 depending on years no recirculation :21, with recirculation: 24

1.17–1.66 depending on years

60%–90% depending on years

proportion of current smoking fliers actually smoking in flights or proportion of fliers seated in the smoking section smoking rate of active smoking fliers or smoking-section fliers, cigarettes/hr

Practive

Rsmk

23%–42% depending on years

current smoking prevalence of the fliers

Psmk

0.48–0.64, depending on years

arithmetic mean

load factor, fraction of seats filled with passengers

definition, unit

Description of Parameters Used in the Monte Carlo Simulations

LF

symbol

TABLE I.

2

32

2 times of mean

60%

2 times of mean

6

100%

100%

1.0

upper bound

DTND

DTND

DTND

DTND

DTND

ND

DTND

DTND

DTND

DTND

distribution

Based on the study by Van Loy et al.(42) and an assumption of a 1 to 5 height to surface ratio Based on averages of manufacturer specified volume and number of seats(1,20)

Based on the NRS 1989 report(4)

The CV and the range were based on the report by the 1989 DOT Survey,(29); the average was adjusted with the number of cigarettes consumed among U.S. current smoking adults each year. Estimated by weighting the EFs for different types of cigarettes with their market shares Based on averages of manufacturer-specified ventilation rates(1,20). Based on the NRC 2002 report(1)

Annual reports by the Air Transport Association of America,(47) and the 1989 DOT Survey(29) Adjusted for gender, age (≥25 years), race (Caucasian), and education ( ≥high school) Based on three reports,(29,31,34)

Sources/comments

FIGURE 2. Comparison of monitored and predicted SHS levels for two B727 flights and two B767 flights in 1988, and for four DC10 flights in 1989 Notes: Predicted levels were based on parameters of the flights during the flying time when field samples were conducted; error bars represent ±2standard deviations of predicted values, with geometric standard deviations assumed to be 2.5 and the distributions to be log normal. Results for the two B727 flights and for the two B767 flights were combined, respectively, since the smoking rates and the operational status were quite similar.(6) Monitored and predicted SHS levels for the two B727 and the two B767 flights in 1988 were based on Mattson’s paper.(6) Monitored SHS PM concentrations were background-level corrected for the four DC 10 flights; no data were available on monitored PM levels in the smoking section of DC10 plane c.(22) Results for the four DC10 flights in 1989 were based on Eatough’s paper.(22) FA, flight attendant.

FIGURE 3. Distribution of simulated SHS PM and air nicotine concentrations (μg/m3) during 200 flight shifts (daily exposure) for one flight attendant (left panel) and distribution of the annual average concentrations for 10,000 flight attendants (right panel) in 1979

150

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TABLE II. Summary of Predicted Annual Exposure Concentrations of SHS PM2.5 and Airborne Nicotine in Select Years from 1955 to 1989, µg/m3 annual average, SHS PM2.5

annual median, SHS PM2.5

annual average, nicotine

annual median, nicotine

year

mean

min

max

mean

min

max

mean

min

max

mean

min

max

1955 1960 1964 1965 1966 1970 1974 1978 1979 1980 1983 1985 1987 1988 1989

265 267 235 225 240 187 196 176 167 153 126 114 103 98 93

207 210 184 180 190 147 152 137 132 118 96 89 80 76 72

350 342 286 273 301 236 247 222 216 203 166 154 130 124 120

197 196 182 173 187 140 142 130 124 113 94 85 77 74 70

142 145 137 120 132 104 109 95 95 84 67 64 59 55 52

270 257 239 225 245 189 191 174 168 153 127 112 103 96 90

11.1 11.2 10.6 10.1 10.9 8.3 8.6 8.4 8.3 7.8 7.2 6.9 6.8 6.7 6.5

8.8 8.9 8.1 8.0 8.8 6.6 6.7 6.4 6.4 6.2 5.7 5.4 5.4 5.2 5.0

14.3 14.6 13.5 12.7 13.8 10.7 10.9 10.3 10.5 10.0 9.1 8.7 8.7 8.4 8.3

8.5 8.5 8.1 7.6 8.3 6.2 6.4 6.3 6.3 5.9 5.4 5.3 5.1 5.0 4.9

6.5 6.4 6.1 5.8 6.1 4.7 4.8 4.5 4.7 4.4 3.9 3.8 3.6 3.8 3.5

11.4 11.0 10.5 9.9 11.0 8.6 8.5 8.2 8.1 7.8 7.0 7.0 7.0 6.4 6.4

Note: Predicted values were based on Monte Carlo simulation of 10,000 sets of 200 exposure scenarios for each of the selected years.

parameters could be estimated. A flight attendant has approximately 156 days off each year,(43) thus, an average of one flight shift per working day and 200 in total each year were assumed for a flight attendant working in U.S. domestic airlines. For each of the select years, 10,000 sets of 200 exposure scenarios were simulated, with each set of 200 exposure scenarios representing one individual flight attendant’s daily exposure during that year, the mean and median of one set of 200 exposure scenarios representing one flight attendant’s annual exposure, and the 10,000 sets of means and medians representing 10,000 flight attendants’ annual exposure in that year. To examine SHSe among the whole flight attendant population, the distribution of the means, medians, and the 5th and 95th percentiles of the 10,000 individual flight attendants’ daily exposure in each year were analyzed. Sensitivity Analysis To examine the sensitivity of simulated results to the assumed distribution of each parameter, one parameter was varied within its distribution for each set of simulations and all others were set as their means as assumed in Table I. Then 10,000 exposure scenarios were simulated, and the simulated RSP and airborne nicotine concentrations were plotted with the varying parameters. The Kernel density of the varying parameters was also plotted on the same graph to show the a priori distribution of the variable. These graphs were used to examine how the largest changes of simulated RSP and airborne nicotine concentrations were related to where the uncertainty in the parameter estimations lies. Sensitively analyses were also conducted to see how the simulation results

would be changed if the aircraft ventilation rate and nicotine surface absorption rate were assumed to be lognormal rather than normal distribution (with the same lower bound and upper bound). Because the same modeling equation was used for the simulation in different years, relationships of the simulated results to each of the parameters were expected to be the same for different years, and the simulation in 1979 was used to conduct the sensitivity analysis. RESULTS Evaluating the Models with Field Measurement Data Figure 2 compares the field monitored SHS concentrations for different flight attendants and at different seats in the smoking sections with the average SHS concentrations predicted by the mass balance models in this article, using parameters for the specific flights with the field monitoring, as reported by Mattson(6) and Eatough. (22,23) Most of the monitored concentrations fell into the range of the predicted average PM2.5 or nicotine concentrations ±2 standard deviations, indicating that the predicted average SHS concentrations for both flight attendants and smoking sections were quite reasonable when compared to the field monitoring results for the same flights. Flight Attendants’ Exposure to SHS in Commercial Aircraft from 1955 to 1989 The log normal distribution of 200 daily exposure scenarios represents an individual flight attendant’s SHSe in each select year from 1955 to 1989. The mean, median, CV, and the 5th and 95th percentiles of each set of 200 exposure scenarios were calculated to describe the annual exposure of an individual

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FIGURE 4. Changes of modeled flight attendants’ exposure to SHS PM and airborne nicotine over time from 1955 to 1989 (mean, minimum, and maximum of annual average concentrations, μg/m3)

flight attendant. The average and range of these 10,000 sets of means and medians for each of the select years from 1955 to 1989 are presented in Table II. The distributions of the annual average SHS concentrations were normal for the flight attendant population. Figure 3 presents the simulation results of an individual flight attendant’s daily SHS exposure and 10,000 flight attendants’ annual average exposure for the year 1979 as an example. Using data from Table II, Figure 4 shows the trends of the annual average of the flight attendant population’s TWA PM2.5 and nicotine concentrations from 1955 to 1989. The mean of annual average exposure to PM2.5 among the flight attendant population was approximately 265 μg/m3 from 1955 to 1960. Concentrations steadily decreased to about one-third (93 μg/m3) in 1989. The annual average nicotine also decreased over time, but to a less extent, with the mean of the statistic of 11.1 μg/m3 in 1955 halved (down to 6.5 μg/m3) in 1989 (Figure 4). SHSe of individual flight attendants varies greatly across different flight shifts in a year. The CV of the daily exposure averaged about 90% in each year for both PM2.5 and nicotine. In 1955, the mean of the 5th percentiles of the simulated flight attendant’s daily exposure to SHS PM2.5 was 43 μg/m3, and the mean of the 95th percentiles was 829 μg/m3; the mean of the 5th and 95th percentiles of the simulated daily nicotine exposure was 1.9 and 29.1 μg/m3, respectively. In 1989, the year before smoking was banned on all U.S. domestic flights and smoking was permitted in designated smoking sections only, the mean of the 5th and 95th percentiles of simulated individual flight attendant’s daily exposure was 15 and 251 μg/m3, respectively, for SHS PM2.5 , and 1.0 and 17.5 μg/m3, respectively, for nicotine. Sensitivity Analysis Both simulated SHS PM concentrations and simulated airborne nicotine concentrations increased linearly with the increase of parameter values of load factors, current smoking 152

prevalence of fliers, percentage of current smoking fliers who actually smoked in cabins, number of cigarettes smoked per hour by each smoker, and emission rate per cigarette; they were inversely proportional to values of air exchange rate and volume per seat (Figure S4 and S5 in the supplemental document). Simulated SHS PM concentrations were most sensitive to the parameters of smoking rate and air exchange rate, and were least sensitive to percentage of recirculated air and percentage of seat-hours with recirculation systems. Simulated airborne nicotine concentrations were also most sensitive to the parameter of smoking rate and least sensitive to air recirculation rate and percentage of seat-hours with recirculation air, but they were not as sensitive to the air exchange rate as SHS PM concentrations because of the other removal rate of surface absorption. No significant changes for SHS PM or airborne nicotine occurred in the range of each parameter. Assuming that the air exchange rate and the nicotine surface absorption rate to be log normal (with lower and upper limits) did not change the mean and distribution significantly (data not shown). DISCUSSION

E

nvironmental characteristics of aircraft cabins are clearly defined, and the demographics of airline passengers are well recorded, which makes chronic SHSe potentially more amenable to estimation in this environment compared to other occupational settings, such as restaurants, offices, and manufacturing facilities. Due to the impact of point sources (smokers), SHS concentrations measured at different locations in the smoking sections vary greatly, as shown by Mattson et al.(6) The mass balance models presented in this article predict the average concentrations in the smoking sections or experienced by flight attendants reasonably well assuming a moderate variability. This supports the use of two-compartment mass-balance models to predict average SHS concentrations in smoking sections and to predict flight attendants’ TWA exposure concentrations. Unfortunately, no information is available on SHS concentrations measured in commercial aircraft unrestricting smoking, as was the situation in the United States prior to 1971; thus the one compartment model presented in this article could not be compared to observed concentrations, but that model is expected to have better performance since it is less complex and suffers from fewer assumptions than the two-compartment model. Both the concentrations measured in the literature and those simulated in this study confirm that flight attendants were exposed to very high concentrations of SHS in commercial aircraft when smoking was allowed. A flight attendant could be exposed to SHS concentrations greater than 800 μg/m3 of RSP and 29 μg/m3 of nicotine in passenger cabins in 1955, and to greater than 250 μg/m3 of RSP and 17 μg/m3 of nicotine in passenger cabins in 1989. This study is the first of its kind to examine the historical changes in magnitude and trends of flight attendants’ SHSe

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in commercial aircraft by using information available on both airline cabin environmental and occupant characteristics from 1955 to 1989. Flight attendants’ average SHSe in passenger cabins, as indicated by PM and nicotine concentrations, varied little between 1955 and 1960, and then steadily decreased until 1989, the year before smoking was prohibited on all U.S. domestic flights. The historical changes of flight attendants’ SHSe are ultimately the result of a combination of historical changes in the parameters that were used in model. Flight load factors generally decreased from 1955 to 1972 and increased from 1972 to 1989. Current smoking prevalence was lower among fliers than in the general population and remained slightly above 40% until 1966, after which time it began to steadily decline to 25% in 1988, which was likely due to the release of the Surgeon General’s Report on Smoking and Health in 1964 and subsequent early tobacco control efforts. Number of cigarettes smoked per hour by each active smoking passenger or smoking-section passenger increased slightly from 1955 to 1989. The increase in the early years from 1955 to 1965 was probably explained by the changes in the tobacco industry’s advertising strategies,(44) and the increase in the later years might be due to the fact that current smokers in the later years who continued to smoke tended to be more addicted to nicotine and thus more likely to be heavier smokers. The tobacco industry first introduced and marketed the light and ultra-light cigarettes in the 1950s and 1960s(45); as a result, light and ultra-light cigarettes accounted for 2% of the U.S. market share in 1967 and increased to 55% by 1989.(41) Because of the different PM emission rates among cigarettes with different designs, SHS PM emission rates decreased over time as the market share of light flavor cigarettes increased. Total air exchange rates (including outside air and recirculated air) and volume per seat of the common aircraft models, as designed by the manufacturers, did not change dramatically from 1964 to 1989 based on data reported by Hocking and the NRC.(1,20) However, over time, as the use of recirculated air in the aircraft ventilation system increased, the outside air supply per passenger decreased.(46) Flight attendants’ exposure to SHS PM decreased more rapidly than their exposure to nicotine, especially after 1974. Two main factors may contribute to these different magnitudes in decline. First, changes in cigarette design led to decreases in the emission rate of SHS PM per cigarette (more than one-third from 1955 to 1989), with little change in nicotine emission rate. Second-, in later years there was an increased use of recirculated air ventilation systems and the filters used in recirculation systems usually have high efficiency for fine particle removal but do not remove gas phase contaminants such as nicotine(20) and many other toxic chemicals in SHS (e.g., N-Nitrosodimethylamine, N-Nitrosopyrrolidine, acetaldehyde, acrylonitrile, benzene, and 1,3-butadiene). This shift to recirculation systems with particle filters tended to remove more SHS PM but not airborne nicotine, and thus the historical trends of flight attendants’ exposure to SHS PM and airborne nicotine are different. The average concentration

of SHS PM in 1955 was estimated to be 2.8 times the average concentration in 1989, and of nicotine the ratio was 1.7. Epidemiological studies which use years or hours working in smoky aircraft cabins to represent flight attendants’ lifetime exposure to SHSe at work would misclassify their cumulative lifetime exposure, and thus may bias the relationship between the exposure and health effects. Future epidemiological studies would benefit from considering the historical trends found in the current analysis (e.g., exposure to SHS PM for one year in 1955 would be comparable to approximately three years worth of exposure in 1989). Using the distribution rather than a point estimate of each parameter and the Monte Carlo simulation, this study takes into account the variation and uncertainty of each parameter. The model provides valuable exposure assessment information to improve SHSe quantification for epidemiological studies on the health effects of flight attendants’ lifetime SHSe in flight. There are some limitations of the study. First, ventilation rates depend on the model of aircraft, how much recirculated air is used, and how much the cockpit crew can control the ventilation system. Therefore air exchange rates are highly variable in actual flight practice. Due to the lack of information collected through systematic measurements of ventilation rates in different types of aircraft, ventilation rates recommended or specified by manufacturers were used in the modeling, which may have introduced some inaccuracies to the simulation and underestimates of SHS concentrations if lower ventilation rates were used, which is likely to conserve fuel costs. Second, nicotine concentrations in commercial aircraft cabins were not measured in flights prior to the mid-1980s, and only one study, in 1988, conducted personal sampling of flight attendants for nicotine.(6) Though the earliest PM measures were performed in military personal transport flights in 1970, no information was available for civilian flights.(19) Lack of systematic measurement of SHS concentrations in a variety of commercial aircraft limits the ability to evaluate the models in the study. Another limitation is that the mass balance model for flights with designated smoking sections simplifies the cabin environment by dividing the passenger cabin into two compartments: a smoking section and a nonsmoking section, which might introduce inaccuracies due to the existing of different classes, galleys, and lavatories. However, the mass balance model predictions of the average SHS concentrations in smoking sections and of flight attendants’ time-weighted exposure to SHS were comparable to field monitoring results, adding to the likelihood that the models are reasonably accurate. CONCLUSION

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light attendants’ exposure to SHS in aircraft cabins, as indicated by PM and airborne nicotine, has declined from 1955 until 1989, the year before smoking was banned in airlines. Using working durations to represent cumulative exposure to SHS would inaccurately assess flight attendants’ lifetime exposure, and thus bias the relationship between

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exposure and chronic health conditions. The historical changes in magnitude and trends of flight attendants’ exposure to SHS identified in this article will inform future related epidemiological investigations in the United States. FUNDING

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his study is funded by the Dr. William Cahan Distinguished Professor Award to S.K.H. from the Flight Attendants Medical Research Institute. SUPPLEMENTAL MATERIAL

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upplemental data for this article can be accessed at tandfonline.com/uoeh. AIHA and ACGIH members may also access supplementary material at http://oeh.tandfonline.com/. REFERENCES 1. National Research Council (NRC): The Airliner Cabin Environment and the Health of Passengers and Crew. Washington, DC: National Academies Press, 2002. 2. Eng, W.G.: Survey on eye comfort in aircraft: I. Flight attendants. Aviat. Space Environ. Med. 50(4):401–404 (1979). 3. Americans for Nonsmokers’ Rights: “Smokefree Transportation Chronology,” 2005. Available at http://no-smoke.org/pdf/transportationchronology.pdf (accessed May 1, 2013). 4. National Research Council (NRC): The Airliner Cabin Environment: Air Quality and Safety. Washington, DC: National Academies Press, 1986. 5. Wieslander, G., T. Lindgren, D. Norback, et al.: Changes in the ocular and nasal signs and symptoms of aircrews in relation to the ban on smoking on intercontinental flights. Scand. J. Work Environ. Health 26(6):514–522 (2000). 6. Mattson, M.E., G. Boyd, D. Byar, et al.: Passive smoking on commercial airline flights. JAMA 261(6):867–872 (1989). 7. Arjomandi, M., T. Haight, R. Redberg, et al.: Pulmonary function abnormalities in never-smoking flight attendants exposed to secondhand tobacco smoke in the aircraft cabin. J. Occup. Environ. Med. 51(6):639–646 (2009). 8. Arjomandi, M., T. Haight, N. Sadeghi, et al.: Reduced exercise tolerance and pulmonary capillary recruitment with remote secondhand smoke exposure. PLoS ONE 7(4) (2012). 9. Beatty, A.L., T.J. Haight, and R.F. Redberg: Associations between respiratory illnesses and secondhand smoke exposure in flight attendants: A cross-sectional analysis of the Flight Attendant Medical Research Institute Survey. Environ. Health 10(1):81 (2011). 10. Ebbert, J.O., I.T. Croghan, D.R. Schroeder, et al.: Association between respiratory tract diseases and secondhand smoke exposure among never smoking flight attendants: A cross-sectional survey. Environ. Health 6:1–8 (2007). 11. Ren, X., P.Y. Hsu, F.L. Dulbecco, et al.: Remote second-hand tobacco exposure in flight attendants is associated with systemic but not pulmonary hypertension. Cardiol. J. 15(4):338–343 (2008). 12. Tokumaru, O., K. Haruki, K. Bacal, et al.: Incidence of cancer among female flight attendants: A meta-analysis. J. Travel Med. 13(3):127–132 (2006). 13. Buja, A., G. Mastrangelo, E. Perissinotto, et al.: Cancer incidence among female flight attendants: A meta-analysis of published data. J. Women’s Health (Larchmt) 15(1):98–105 (2006). 14. Reynolds, P., J. Cone, M. Layefsky, et al.: Cancer incidence in California flight attendants (United States). Cancer Causes Control 13(4):317–324 (2002).

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