1 Program in Occupational Health and Environmental Medicine, McMaster University, Hamilton, Ontario, Canada.
2 Institute of Environment and Health, McMaster University, Hamilton, Ontario, Canada.
3 School of Geography and Geology, Health Studies Program, McMaster University, Hamilton, Ontario, Canada.
4 Department of Medicine, Faculty of Health Sciences, McMaster University, Hamilton, Ontario, Canada.
Received for publication September 20, 2002; accepted for publication February 5, 2004.
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ABSTRACT |
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air pollution; automobiles; mortality; urbanization; vehicles
Abbreviations: Abbreviations: CI, confidence interval; FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity; ICD-9, International Classification of Diseases, Ninth Revision.
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INTRODUCTION |
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Although these studies and the earlier Six Cities Study (3) demonstrated a relation between air pollution and mortality rates, the impact of air pollution relative to other causes of death in a population has not been well established. This relation is of public health importance. Pope et al. (1) compared the mortality effect of fine particulate air pollution with that of cigarette smoking and obesity. They concluded that the risk associated with exposure to particulate air pollution was much smaller than the risk of cigarette smoking but was comparable to the estimated effect of being moderately overweight.
One measure of the impact of a risk factor on mortality is the "rate advancement period" developed by Brenner et al. (4). The rate advancement period estimates the impact of a risk factor on the timing of death; that is, the risk is phrased in terms of a premature mortality rate among exposed persons. In this study, we estimated the rate advancement period associated with intraurban traffic pollution exposures in a cohort of subjects in the city of Hamilton, Ontario, Canada.
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MATERIALS AND METHODS |
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The province of Ontario provides universal health insurance for all residents. A family-based Medicare number was replaced with a personal Health Insurance Number in 1992. This Health Insurance Number was used to link subjects to Ontario Health Insurance Plan physician billing files for 19921999.
Subjects entered mortality follow-up on January 1, 1992, or the date of their first clinic visit, whichever was later. To ensure that subjects without a visit after January 1, 1992, were still alive at the start of follow-up, we excluded subjects without an Ontario Health Insurance Plan transaction after 1991. Mortality was ascertained by linkage to the Ontario Mortality Registry for the period 19922001. Because there were only 27 deaths among younger subjects, this study was restricted to subjects aged 40 years or older. Subjects diagnosed with cancer, either before or within 45 days of the initial visit, were excluded. This left 5,228 subjects (950 total deaths, 923 nonaccidental) available for analysis. This study was approved by the Research Ethics Board at St. Josephs Healthcare, Hamilton.
Underlying medical diagnoses
We used the Ontario Health Insurance Plan linkage to classify underlying respiratory disease status, utilizing diagnostic codes submitted to the Ontario Health Insurance Plan by specialist physicians. Diagnoses included chronic obstructive pulmonary disease (International Classification of Diseases, Ninth Revision (ICD-9), codes 491, 492, and 496: 2,035 subjects, 431 deaths), asthma (ICD-9 code 493: 604 subjects, 28 deaths), bronchiectasis (ICD-9 code 494: 141 subjects, 31 deaths), asbestosis (ICD-9 code 501: 39 subjects, four deaths), silicosis (ICD-9 code 502: 24 subjects, three deaths), and postinflammatory pulmonary fibrosis (ICD-9 code 515: 240 subjects, 75 deaths). Subjects with diagnoses of both asthma and chronic obstructive pulmonary disease were classified as having chronic obstructive pulmonary disease.
The Ontario Health Insurance Plan billing file and the Ontario hospital discharge database were also searched for diagnoses of diabetes mellitus (ICD-9 code 250) and chronic ischemic heart disease (ICD-9 codes 412414). Subjects were classified as having these disorders if the diagnosis had been made in two or more claims submissions by a general practitioner, in one claim submission by a specialist, or in any hospitalization. This algorithm has been validated for diagnosis of diabetes in Ontario (6).
Income and road traffic indicators
Postal codes were used to estimate household income on the scale of the enumeration area, the smallest unit in the Canadian census. They were also used to assign residential addresses to road traffic buffers. Canadian six-character postal codes are localized to small areas in urban regions, and the domain included may be as small as a single apartment building or city block. The ArcView 3.2 geographic information system (ESRI, Inc., Redlands, California) was used with the Statistics Canada 1999 postal code conversion file and the Census of 1996 to estimate mean household income in the subjects neighborhoods by overlay of enumeration area data. The geographic information system was also used to map subjects residences with respect to major sources of road traffic pollution. We used the same indicator of traffic-related pollution as Hoek et al. (2). Subjects postal codes were mapped with respect to buffers of 50 m around major urban roads and 100 m from highways (road/highway buffers).
Modeling of mortality relative risk
The Cox proportional hazards model (7) was used to model the risk of mortality from all natural causes. We tested for deviations from the proportional hazards assumption by introducing interactions of time with all covariates. To account for possible geographic "clustering" of mortality, we grouped subjects among census tracts (n = 120 tracts; mean number of subjects per census tract = 59 (range, 1224)), and we adjusted estimates of variance for clustering in census tracts. Calculations were performed with Stata statistical software (8).
Rate advancement periods
The effect of traffic-related pollution with respect to the time dimension of premature mortality was assessed using Brenner et al.s (4) "rate advancement period," which estimates how much older a reference population would have to be to experience the same "attrition rate" as subjects residing within a road buffer. The rate advancement period is derived from a linear model of the form log(hazard) = b1 x exposure + b2 x age + covariates. From this model, a point estimate of the rate advancement period is b1/b2 per unit increase in exposure. A formula for the variance is given in the paper by Brenner et al. (4).
Additional deaths per year
Rates of mortality from all natural causes are dependent on age. To compute the "additional deaths per year" attributable to traffic pollutant exposure, we added the rate advancement period to chronologic age and performed interpolation in Ontario life tables. We subtracted the mortality rate at a given reference age from the rate at the "rate-advanced" age to obtain the number of additional all-natural-cause deaths per 1,000 subjects per year, adjusted for clinical and disease variables.
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RESULTS |
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Rate advancement periods
We used the formulae in the paper by Brenner et al. (4) to compute the rate advancement periods and their variances. The formula for the rate advancement period requires a correct statement of the dependency between log(hazard) and age. The hypothesis of linearity between log(hazard) and age was not rejected after examination of the martingale residuals (7). Table 3 shows the mortality rate advancement period attributable to residence within a traffic pollution buffer. Rate advancement periods attributable to diagnoses of chronic disease are presented for comparison.
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By interpolation in Ontario life tables, we estimated the number of deaths attributable to an "aging effect" of 2.5 years. The estimates were 0.4 additional deaths/1,000/year among subjects aged 40 years, 1.6 deaths/1,000/year among subjects aged 50 years, 4.4 deaths/1,000/year among subjects aged 60 years, and 10.9 deaths/1,000/year among subjects aged 70 years.
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DISCUSSION |
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Use of the rate advancement period provides one view of the impact of increased mortality rates. Since interpretation of the rate advancement period is conditional on the absence of competing causes of death (4), we restricted our analysis to mortality from all natural causes and did not consider specific underlying causes, such as cardiopulmonary diseases, which are most causally linked to air pollutant exposures (1). The use of all-natural-cause mortality provides a global assessment of mortality impact. We found that the "aging effect" attributable to traffic pollution exposure was 2.5 years, which is similar to the rate advancement period attributable to common chronic diseases. We emphasize that the rate advancement period does not equate directly with "years of life lost" but rather indicates the advancement of mortality rates. The extent of life shortening will depend on the ages of the subjects.
A weakness of our study is that we have, as yet, no measurements of pollutant levels within the road buffers. In North American cities, pollution is generated by gasoline-powered cars and diesel-fueled buses and trucks. The burning of fossil fuels creates primary pollutants, including carbon dioxide, carbon monoxide, oxides of nitrogen, sulfur dioxide, hydrocarbons, and airborne particulates. It has been estimated that particulate emission from diesel engines, per distance traveled, is over 100 times higher than that from gasoline engines fitted with catalytic converters (14). Dutch investigators (15) reported that daytime concentrations of particulate matter less than 10 µm in aerodynamic diameter were, on average, 30 percent higher near busy roads than at background locations, differing by 713 µg/m3. Researchers in another Dutch study reported that indoor and outdoor particulate concentrations were higher at homes located on high-traffic-intensity streets than at homes on low-traffic streets (16). The mean indoor concentration of particulate matter less than 2.5 µm in aerodynamic diameter was 27 µg/m3 in high-traffic homes and 12 µg/m3 in low-traffic homes. In Roxbury, Massachusetts (17), concentrations of fine particulate matter and particle-bound polycyclic aromatic hydrocarbons were higher on roads reported to have heavy bus traffic. In the South Bronx (New York City), sidewalk measures of diesel exhaust particulates were correlated with the count of large trucks (18).
As in the other studies of air pollution cohorts, we did not have a measure of residential mobility during the follow-up period, so misclassification of traffic exposure was possible. Similar to several of the other air pollution cohorts, our cohort was not a random sample of the population. The cohort in the Six Cities Study (3) was a random sample of 8,111 White adults. Participants in the American Cancer Society study (1) were friends, neighbors, or acquaintances of American Cancer Society volunteers. In the Dutch study by Hoek et al. (2), only 36 percent of potential subjects completed the baseline questionnaire.
Our subjects underwent lung function testing at an academic clinic. Although only half of the subjects were diagnosed with respiratory disease, they presumably had respiratory symptoms such as cough and shortness of breath prior to referral. Although our cohort contained a greater proportion of persons with respiratory illness than is found in the general population, we believe that our results can be generalized because of the availability of clinical diagnoses. To our knowledge, this was the first cohort study with access to physicians diagnoses of underlying disease status. Thus, we were able to adjust for diagnoses of chronic respiratory and pulmonary diseases and diabetes. We found no effect modification by underlying respiratory disease status; and our adjusted estimate of the all-natural-cause mortality risk associated with residential exposure to traffic was 1.18 (95 percent CI: 1.02, 1.38), similar to the Dutch estimate of 1.35 (95 percent CI: 0.93, 1.95) (2).
In conclusion, we found an association between residential proximity to traffic and mortality. The mortality rate advancement attributable to traffic pollution was similar to that associated with chronic respiratory and pulmonary diseases and diabetes. This suggests that decreasing pollutant exposures may have a substantial public health impact.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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