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We sought to demonstrate the advantages of using individual-level survey data in quantitative environmental justice analyses and to provide new evidence regarding racial and socioeconomic disparities in the distribution of polluting industrial facilities.
Addresses of respondents in the baseline sample of the Americans’ Changing Lives Study and polluting industrial facilities in the Environmental Protection Agency’s Toxic Release Inventory were geocoded, allowing assessments of distances between respondents’ homes and polluting facilities. The associations between race and other sociodemographic characteristics and living within 1 mile (1.6 km) of a polluting facility were estimated via logistic regression.
Blacks and respondents at lower educational levels and, to a lesser degree, lower income levels were significantly more likely to live within a mile of a polluting facility. Racial disparities were especially pronounced in metropolitan areas of the Midwest and West and in suburban areas of the South.
Our results add to the historical record demonstrating significant disparities in exposures to environmental hazards in the US population and provide a paradigm for studying changes over time in links to health.
Concerns about the health effects of the disproportionate exposure to environmental burdens have been a major driving force in mobilizing minority communities into a national environmental justice movement.1,2 More and more research suggests racial and socioeconomic disparities in exposure to environmental hazards,3–6 but nearly all of the studies in this area have yielded only indirect evidence, describing the demographic composition of areas and their proximity to hazardous sites.7–19
In this “spatial coincidence”20 or “unit-hazard coincidence”21 methodology, predefined geographic units of analysis (such as census tracts or zip code areas) that do or do not contain a hazard of interest are selected and then the demographic characteristics of host and nonhost units are compared.21–23 One limitation of this approach is that it assumes that people living in host units containing the hazard under investigation live closer to it than do those living in nonhost units, which is not necessarily the case.21 Another limitation is the great variability in the size of the units. For example, the smallest tract containing a hazardous waste facility is less than 0.1 square mile (0.26 km2), whereas the largest is more than 7500 square miles (19500 km2).21
Distance-based methods overcome these limitations by assessing the precise distance between the location of environmental hazards and the individuals or places under study. In 1 study, the proportions of minority and poor individuals living in the units within the 1-, 2-, or 3-mile (1.6-, 3.2-, or 4.8-km) buffer zones around hazardous facilities were much greater than the proportions observed when only the host units were considered,21,23 and meta-analyses have shown that studies in which geographic information system (i.e., distance-based) methods are used reveal greater racial and socioeconomic disparities in proximity to environmental hazards than do studies in which the conventional (i.e., unit-hazard coincidence) method is used.6
Although distance-based studies are not as prevalent as conventional unit-hazard coincidence studies, they are increasing in frequency.12,20–26 However, researchers applying distance-based methods to census and other predefined geographic units still must address the problem that many units are only partially captured by buffers, and there is no single standard for determining whether or how to categorize a partially captured unit as within or outside a buffer.
An alternative to the research designs just discussed, both of which rely heavily on census geography to define analytic units, is to use survey data to examine individual-level racial/ethnic and socioeconomic disparities in residential proximity to hazardous sites. This strategy has several advantages. First, survey respondents can be represented as geographic points, leaving little ambiguity in determining whether they are located within specified distances of hazardous sites. Second, using individual-level data avoids the ecological fallacy of incorrectly assuming that relationships among geographic units translate to relationships at the micro- or individual level.
Third, survey data afford more-extensive and -detailed information about the life circumstances of people living near hazardous sites than are available from the short and long forms of the decennial census. This allows better adjustments for confounding and offers additional insight about the characteristics of those living near environmental hazards. Finally, longitudinal survey data provide unique opportunities to examine how living near hazardous sites is related over time to racial and socioeconomic differences in future health and mortality.
Only a few localized studies have used survey data to assess environmental inequalities, and they generally confirm the presence of social disparities in proximity to environmental hazards.5,27 In this study, in which we used survey data from a representative probability sample of the American population linked with national data from the US Environmental Protection Agency (EPA), we conducted the first national-level analysis, to our knowledge, of social inequalities in the distribution of polluting industrial facilities employing individual-level survey data. Our goals were to demonstrate the advantages of using survey data over traditional census data approaches in conducting environmental inequality analyses and, by contributing to the growing body of evidence pertaining to disparities in environmental exposures, to lay a foundation for future analyses of the role environmental factors play in racial and socioeconomic disparities in health and mortality.
The data for this study were derived from the 1986 baseline survey of the Americans’ Changing Lives Study (ACL),28 a nationally representative panel study of the US adult population. The ACL sample was recently geocoded to determine precise geographic locations of the respondents and linked to similar geocoding of point locations of sites in the EPA’s 1987 Toxic Release Inventory (TRI),29 a national database of 21894 industrial facilities reporting on-site and off-site disposal of almost 650 toxic chemicals.
In the 1986 ACL baseline, face-to-face interviews were conducted with a stratified, multistage sample of 3617 noninstitutionalized adults 25 years or older in the coterminous United States.28 Response rates were 70% for households and 68% for individual participants. Blacks and individuals older than 60 years were oversampled. In all analyses, weighting was used to adjust for different probabilities of selection and response rates.
Although ACL respondents were reinter-viewed in 1989, 1994, and 2001, we focused on the 1986 sample because it was representative of the US adult population when the study was initiated. As a result of immigration and attrition of the sample because of deaths and nonresponse, more recent waves cannot be considered representative of the full US population in 1989, 1994, or 2001. Furthermore, most national-level studies of racial and socioeconomic disparities in proximity to hazardous sites rely on the 1990 census, allowing an easy comparison of the 1986 ACL with previous studies. We used data from the 2001 sample to replicate our analyses, and the results were very similar to our analyses focusing on the original 1986 sample.
We used the first year (1987) of the TRI database because it provided information closest to the time of the 1986 ACL interviews. The 1987 TRI included 21894 industrial facilities and was created in response to the 1984 industrial disaster in Bhopal, India.29 The TRI seeks to provide citizens with information about the presence of toxic chemicals in their communities to help plan for and avert similar disasters.29 Facilities within certain industrial sectors are required to report to the TRI if they employ 10 or more full-time-equivalent employees and “manufacture or process” in a given year more than 25000 pounds (11250 kg) or “otherwise use” more than 10000 pounds (4500 kg) of any chemical listed in section 313 of the 1986 Emergency Planning and Community Right to Know Act,30 which established the TRI. In the case of persistent bioaccumulative toxic chemicals, the minimum thresholds are 0.1 gram for dioxin and dioxin-like compounds and 10 or 100 pounds (4.5 or 45 kg) for other chemicals in this category.
We determined point locations for both ACL respondents and TRI facilities by geocoding their address information, including street number, street name, city, state, and zip code area. Point locations for 3059 (84.6%) of the 3617 ACL 1986 respondent addresses were obtained. Most of the remaining 558 cases either involved errors in address information or were incomplete. In 140 cases (3.9%), the 1980 census tract of the respondent was also known, and hence the centroid of the intersection of the respondent’s tract and zip code area was used to estimate the respondent’s location. In another 410 (11.3%) cases, only the respondent’s zip code area was known, and thus the zip code centroid was used to locate the respondent. Eight cases in which address information was missing entirely or otherwise insufficient to produce a point location were excluded from our analyses.
Although the TRI database includes latitudes and longitudes for all facilities, these coordinates do not always place the facility in the correct geographic location. For example, the coordinates sometimes place a facility outside its reported zip code area or even outside the United States. Hence, we did our own geocoding of the 1987 TRI facilities, and this coding produced point locations for 14456 (66.0%) facilities.
In cases in which addresses did not produce a geocoded point location, either (1) the latitude and longitude coordinates provided in the TRI database were used to locate the facility, if they produced a point inside the facility’s reported zip code area (4486 cases, or 20.5%), or (2) the centroid of the facility’s zip code area was used (2648 cases, or 12.1%). A sensitivity analysis conducted on the facilities whose addresses produced geocodable point locations showed that when the latitude and longitude coordinates provided in the TRI database correctly placed the facility inside its reported zip code area, the distance between the location we geocoded and the location provided by the reported latitude–longitude coordinates was less than 0.25 mi in most instances. The remaining 304 cases (1.4%) in which information was insufficient to produce a point location were excluded from the analyses.
We used logistic regression to assess the relative importance of race and socioeconomic variables in predicting respondents’ proximity to a polluting facility (the dependent variable). The dependent variable was coded 1 if an ACL respondent lived within 1 mile of a TRI facility and 0 if the respondent lived farther away. Although we also found that disparities existed out to 5 miles (when half-mile increments were used), we present results for a 1-mile radius, as in many previous environmental inequality studies5,24,31 and in previous epidemiological studies examining health outcomes near hazardous waste sites.32,33 We did not disaggregate facilities according to quantity or toxicity of emissions, given that we were interested in the general distributional patterns of TRI facilities, whose physical presence may involve an array of quality-of-life effects, including psychological effects such as anxiety, depression, and social stigmatization.34,35 Furthermore, our analyses assessed only members of residential populations, and these individuals may have been away from pollutant sources during the periods in which facilities were in operation (i.e., during daytime hours).
Our analyses adjusted for potential confounding variables such as age, gender, marital status, region of residence, and residence in a city, suburb, or rural area. We used dichotomous measures of gender and race (Black vs White; we excluded members of other racial/ethnic minority groups because of their small numbers), 3-category measures of age (younger than 45 years, 45–64 years, 65 years or older), 1986 income (less than $15000, $15000–$39999, $40000 or more), and educational level (less than high school, high school and some college, college), and a 4-category measure of marital status (married, widowed, divorced or separated, never married). Results were similar when we examined more-refined categories of these variables. Both the region (Northeast, Midwest, South, West) and type (“urbanicity”) of area (central city, suburb, rural area) in which ACL respondents lived were categorized according to Census Bureau definitions.
Our main analytical goals were to estimate racial and socioeconomic disparities in proximity to polluting industrial facilities after control for age, gender, and the other potentially confounding factors just described and to investigate whether such disparities varied according to region of residence and urbanicity of area of residence by including relevant interaction terms in our models. We initially sought to determine whether racial and socioeconomic disparities were not only independent of confounding factors but also independent of each other—that is, whether racial disparities in the distribution of environmental hazards were mostly a function of socioeconomic disparities between Whites and minority group participants or went beyond socioeconomic differences alone.14,25,36,37
Our analysis was also motivated by recognition that industrial activity tends to concentrate more in metropolitan areas than in rural areas and more so in some regions of the country (such as the Northeast and Midwest) than others. Concentrations of racial/ethnic minority groups also vary by place and region. Although previous inequality studies have examined whether racial and socioeconomic disparities in proximity to polluting industrial facilities vary across region and place, none, to our knowledge, have examined place and region effects simultaneously.
Table 1 shows the breakdown of the ACL sample by unweighted respondent numbers and weighted percentages. Almost one third (29.5%) of the weighted sample lived within 1 mi of a polluting industrial facility; 38.1% of Black respondents and 28.4% of White respondents lived within a mile of such a facility, a statistically significant disparity (χ21=15.91; P≤.001). The composition of the sample was as follows: 11.6% Blacks and 88.4% Whites; 29.8% with incomes of less than $15000, 45.0% with incomes between $15000 and $39999, and 25.2% with incomes of $40000 or more; and 25.2% with no high school diploma, 55.0% with a high school diploma, and 19.8% with a college degree.
Table 2 shows odds ratios (ORs) from our logistic regression analysis of the sample living within a mile of a TRI facility. Model 1 showed a significant racial disparity, after adjustment for gender and age, such that Blacks were significantly more likely than were Whites to reside within 1 mile of such a facility (OR=1.54; 95% confidence interval [CI]=1.24, 1.92). There has been much debate about whether such disparities are a function of socioeconomic differences or of race-related factors such as racial segregation and racialized decisions about land use.2–8,14,21–26 The OR for the race variable was attenuated when the income and education variables were entered into models 2 and 3 but remained statistically significant (model 3: OR=1.38; 95% CI=1.10, 1.72). Thus, racial disparities in proximity were explained partially but not fully by socioeconomic differences.
Income and education were also both statistically significant predictors of respondents’ proximity to polluting facilities in model 3. Those with incomes of less than $15000 (OR=1.46; 95% CI=1.15, 1.85) or between $15000 and $39999 (OR=1.30; 95% CI=1.07, 1.59) were significantly more likely than were those with incomes of $40000 or more to live within a mile of a polluting facility. Those without high school diplomas were significantly more likely to live near such a facility than were those with diplomas or college degrees (OR=1.42; 95% CI=1.10, 1.84). When marital status was added in model 4, education but not income continued to be statistically significant, suggesting that income effects were to a considerable degree a product of the low-income status of divorced, separated, or never-married participants.
Because of historical patterns of industrial development, migration, and housing segregation in the United States, we anticipated variations in racial and socioeconomic disparities in facility locations among the various US regions (Northeast, Midwest, South, West) as well as across city, suburban, and rural locations. We indeed found in model 5 that residents of city and suburban areas were significantly more likely than were residents of rural areas to live near a polluting industrial facility. Also, those living in the Northeast and Midwest were significantly more likely than were those living in other regions to reside near a facility, whereas those living in the South were significantly less likely to reside near a facility. At the same time, racial and socioeconomic variables remained statistically significant in model 5, with little or no reduction in ORs from those in models 3 and 4. In the case of respondents without a high school diploma, the OR increased somewhat in model 5, to 1.71 (95% CI=1.30, 2.25).
We more closely examined the effects of region and urbanicity of residence on racial and socioeconomic disparities in respondents’ proximity to polluting industrial facilities by including in the models all possible interactions between region, urbanicity, and race (or education or income). Table 3 presents all of the significant interactions from these models, in which a pattern variable combining region and urbanicity of residence was used to simplify presentation and interpretation of the results.
For example, regarding place and region interactions, we found no statistically significant differences between city and suburban dwellers in their likelihood of living within 1 mile of polluting industrial facilities in the Northeast, Midwest, or West. Thus, in models 1 and 2 (Table 3), we combined the urban and suburban respondents in each region to create metropolitan-area pattern variables for the Northeast, Midwest, and West. We maintained the distinction between urban and suburban areas in the South because we found statistically significant differences in proximity to polluting industrial facilities across these categories. Because our analyses indicated no regional differences in the effects associated with living in rural areas, these areas were omitted in this analysis.
In model 1 (Table 3), we found that ACL respondents residing in metropolitan areas of the Northeast, Midwest, and West were significantly more likely than were those residing in rural areas to live within 1 mile of a polluting industrial facility. Also, residents of cities (but not suburbs) of the South were significantly more likely than were residents of rural areas to live within 1 mile of such a facility. However, these effects did not eliminate racial, income, or educational disparities in respondents’ proximity to facility locations, although they did reduce racial differences slightly.
In our final step, we examined whether racial and socioeconomic disparities varied geographically according to region and urbanicity of residence by entering interaction terms between race or socioeconomic variables and the region–urbanicity pattern variables. We found no appreciable differences for the income and education variables across region and urbanicity of residence (data not shown). However, we did find substantial geographic differences in the magnitudes of racial disparities in respondents’ proximity to facility locations. As shown in model 2, disparities between Black and White respondents in their proximity to facility locations were greatest in metropolitan areas of the Midwest and West and suburban areas of the South, with statistically significant (P<.05) ORs of 2.63, 2.74, and 2.80, respectively. Racial disparities in proximity to facility locations were not significant in the metropolitan areas of the Northeast or in the cities of the South.
The results of the interactions between region, place, and race can be seen more clearly by examining the actual distribution of Black and White respondents in the ACL sample within 1 mile of a polluting industrial facility in the various regions and city, suburban, and rural locations in the United States. As can be seen in Figure 1, the largest disparities in the percentages of Black and White respondents living within 1 mi of a facility were in metropolitan areas of the Midwest; 58% of Black residents in these areas lived within 1 mi of such a facility, as compared with only one third (35%) of White residents. Substantial racial disparities in the metropolitan West and suburban South were also evident: 50% of Black residents and only 30% of White residents in metropolitan areas of the West lived within 1 mile of a facility, whereas 30% of Black residents and only 14% of White residents in the suburban areas of the South did. No significant racial disparities were evident in rural areas of the United States or metropolitan areas of the Northeast.
The ACL provides new evidence and confirms patterns in a growing body of quantitative research on racial and socioeconomic disparities in exposures to environmental hazards. Racial disparities in the distribution of the ACL sample around polluting industrial facilities remained statistically significant even after we controlled for socioeconomic and other variables. Nevertheless, socioeconomic and other demographic variables were also found to be significantly associated with proximity to a polluting facility.
Lower-income people were found to be significantly more likely than were higher-income people to live near a polluting industrial facility. Similarly, those without high school diplomas were significantly more likely to live near such a facility than were those with higher levels of education. Although we did not find significant gender differences in regard to proximity to a facility, our results suggest that marital status is correlated with the presence of nearby polluting industrial facilities. Participants who were divorced or separated or had never been married were more likely than were participants who were married or widowed to live near such a facility, but at levels not quite reaching statistical significance (i.e., the P<.05 level) once we controlled for region and place.
Racial disparities were also much more pronounced in certain areas of the country than in others. Such disparities were greatest in the metropolitan areas of the Midwest and West and the suburban areas of the South. No significant disparities were found in rural areas or the metropolitan areas of the Northeast. We surmise that these outcomes were the result of differences in historical patterns of racial discrimination, industrial development, and migration across the various regions of the United States and the urban and rural areas of these regions. Clearly, these patterns suggest that region and urbanicity of residence (city, suburb, or rural area) need to be examined more closely in future environmental inequality studies.
This study involved several limitations worth noting. The first is that proximity to hazardous facilities is at best an indirect indicator of exposure. However, proximity has necessarily been widely used in environmental justice studies, given that data regarding volumes, toxicity, and geographic dispersion of air, water, and land pollutants have been scarce, particularly at the national level. This is true as well in unit-hazard coincidence studies, which furthermore conflate geographic units with “communities.” Second, our survey data were based on self-reports, and thus there may have been error in our measurement of income, education, and other variables used in the analyses.
Third, although the baseline address information available for respondents was considered to be of high quality (given that the ACL baseline survey was conducted as a face-to-face interview in people’s homes), there were limitations in converting each address to a latitude–longitude point location. Similar problems were involved in converting TRI facility addresses to point locations. Thus, there was probably measurement error in the dependent variable, the exact distance between a respondent and the nearest polluting facility. Although difficult to determine, the direction of this measurement error most likely resulted in an underestimation of the percentage of respondents living within 1 mi of a facility.
A fourth limitation was that the data used in this study were nearly 20 years old, although replication of our analyses with the 2001 ACL data yielded similar results. Nonetheless, the primary contribution of this work is methodological, given that ours is the first national study, to our knowledge, to demonstrate the potential of combining location and sociodemographic information from surveys with administrative data on environmental hazards while also providing new evidence pertaining to historical racial and socioeconomic disparities in the distribution of environmental hazards.
Survey data such as those of the ACL have a number of advantages over census data, which have been the typical data used in environmental inequality analyses, especially those national in scope. Because survey respondents can be represented as geographic points, their proximity to environmental hazards can be more precisely determined than when 2-dimensional geographic units such as census tracts and zip code areas are used to determine the demographic characteristics of the individuals living around hazardous sites.
Furthermore, surveys allow much more-detailed controls for background factors pertaining to people’s life circumstances than are available from the decennial census. Survey respondents can also be tracked over time so that changes in life circumstances, including those pertaining to living near environmental burdens, can be more directly linked to these individuals’ health and future mortality. Multicollinearity problems in multivariate statistical analyses involving sociodemographic variables are common in analyses of aggregate census data but less likely in assessments of individual-level data obtained from surveys.
Given the significant disparities we found in the way in which the ACL baseline sample was distributed around the nation’s polluting industrial facilities, an important next step in our analyses will be to examine how these disparities affect future health and mortality. Respondents’ proximity to polluting industrial facilities in subsequent waves of the ACL (1989, 1994, and 2001) can be determined. Information about polluting industrial facilities on the TRI database also is being updated annually by the EPA. Along with adding new facilities as reporting requirements change, the EPA has recently begun extensive work to model the quantity, toxicity, and distribution of air and water pollution geographically.
In future studies, it will be possible to model the cumulative residential toxic burdens on ACL respondents since the inception of the study in 1986 and to determine how these cumulative burdens are related to future health and mortality. Because the ACL also contains extensive information on respondents’ social, economic, and psychological states and other life circumstances, it will be possible as well to weigh the relative importance of these factors along with incidence of environmental burdens in understanding racial and socioeconomic disparities in health and mortality.
This research was supported by the National Institute on Aging (grants PO1AG05561, RO1AG09978, and RO1AG018418).
We thank the technical sections of the University of Michigan Survey Research Center for conducting the sampling, interviewing, and coding for the Americans’ Changing Lives Study (ACL) and the School of Natural Resources and Environment Geographic Information Systems Lab for geocoding the locations of ACL respondents and polluting industrial facilities in the Environmental Protection Agency’s Toxic Release Inventory database.
P. Mohai originated the study, conducted the statistical analyses, and led the writing of the article. P.M. Lantz helped to conceptualize ideas and contributed to the design of the analyses and writing of the article. J. Morenoff and J.S. House helped to conceptualize ideas and contributed to the design of the analyses. R.P. Mero provided data management and statistical support. All of the authors helped to interpret findings, reviewed drafts of the article, and participated in making revisions.
Human Participant Protection
This research was approved by the institutional review board of the University of Michigan. All study respondents provided informed verbal consent.