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Commentary

The Environmental “Riskscape” and Social Inequality: Implications for Explaining Maternal and Child Health Disparities

Rachel Morello-Frosch1,2, Edmond D. Shenassa2,3

1 Center for Environmental Studies, 2 Department of Community Health, School of Medicine and, 3 Centers for Behavioral and Preventive Medicine, Brown University, Providence, Rhode Island, USA

Abstract Top

Background

Research indicates that the double jeopardy of exposure to environmental hazards combined with place-based stressors is associated with maternal and child health (MCH) disparities.

Objective and Discussion

Our aim is to present evidence that individual-level and place-based psychosocial stressors may compromise host resistance such that environmental pollutants would have adverse health effects at relatively lower doses, thus partially explaining MCH disparities, particularly poor birth outcomes. Allostatic load may be a physiologic mechanism behind the moderation of the toxic effect of environmental pollutants by social stressors. We propose a conceptual framework for holistic approaches to future MCH research that elucidates the interplay of psychosocial stressors and environmental hazards in order to better explain drivers of MCH disparities.

Conclusion

Given the complexity of the link between environmental factors and MCH disparities, a holistic approach to future MCH research that seeks to untangle the double jeopardy of chronic stressors and environmental hazard exposures could help elucidate how the interplay of these factors shapes persistent racial and economic disparities in MCH.

A formidable challenge in the field of maternal and child health (MCH) has been explaining the persistent racial and socioeconomic health disparities, particularly in birth outcomes in the United States (Singh and Yu 1995). Despite declines in the overall infant mortality, there remains a significant disparity between black and white infant mortality rates. Nationally, black women are twice as likely as white women to give birth to a very low-birth-weight baby. For preterm births, although the gap between the two racial groups has narrowed recently, the disparity between the two groups remains large: a 6.7% difference (National Center for Health Statistics 2003).

Here we discuss the interplay of environmental hazards with place-based and individual-level psychosocial stressors and its implications for MCH research. Although a strong body of literature has shed much light on the individual-level factors (e.g., health behaviors, inter-pregnancy interval, and access to adequate health care) (Hessol et al. 1998; Hummer 1993; Rawlings et al. 1995; Starfield et al. 1991) and placed-based drivers of MCH disparities (e.g., neighborhood poverty, relative income inequality, poor housing, and segregation) (Buka et al. 2002; Guest et al. 1998; Huynh et al. 2005; Laveist 1993; Matteson et al. 1998; Morenoff 2003; O’Campo et al. 1997; Shenassa et al. 2004), there has been little cross-pollination between this field and the research investigating links between environmental hazard exposures and birth outcomes (Parker et al. 2005; Ritz and Yu 1999; Ritz et al. 2000; Sadler et al. 1999; Whyatt et al. 2004; Wilhelm and Ritz 2003).

Place-based stressors are biologically relevant components of the human environment and can function independently of individual-level stressors to determine health (Diez-Roux 1998; Shenassa 2001). These place-based factors can influence birth outcomes in three ways: a) by affecting birth outcomes directly (Huynh et al. 2005; Rich-Edwards and Grizzard 2005); b) by increasing exposures to environmental hazards, such as air pollutants (Parker et al. 2005; Woodruff et al. 2003); and c) by enhancing susceptibility to the toxic effects of contaminant exposures (Ponce et al. 2005). This third pathway concerning the interaction of place-based stressors with environmental hazards points toward the next generation of studies to understand the combined effects of environmental and psychosocial drivers of MCH disparities. We first discuss the confluence of place-based psychosocial stressors and environmental hazard exposures and its implications for future research on MCH disparities with a focus on birth outcomes. We then propose a possible physiologic link between place-based stressors and environmental hazards in ways that may enhance susceptibility to toxics. We conclude by outlining a conceptual framework for future MCH research.

Social Inequality and Environmental Health Disparities Top

Wide-ranging political, socioeconomic, and discriminatory forces coupled with spatial patterns of industrialization and development have segregated people of color, particularly African Americans, into communities with some of the highest indices of urban poverty and material deprivation (Morello-Frosch and Jesdale 2006; Schultz et al. 2002; Williams and Collins 2004). Researchers and policy makers concerned about environmental justice argue that communities of color and the poor face a higher frequency and magnitude of exposures to environmental as well as psychosocial stressors [Institute of Medicine (IOM) 1999; O’Neill et al. 2003]. Concern has centered on the limited science related to the cumulative impact of multiple exposures to environmental hazards and the potential vulnerability of poor communities to their toxic effects [National Environmental Justice Advisory Council (NEJAC) 2004]. This combination and potential interaction of elevated environmental hazard exposures, on the one hand, and socioeconomic stressors, on the other, have been described as a form of “double jeopardy” (IOM 1999).

Understanding the MCH implications of these “geographies of exposure and susceptibility” (Jerrett and Finkelstein 2005) or “riskscapes” (Morello-Frosch et al. 2001) requires consideration of the timing of exposure to psychosocial stressors as well as environmental hazards during the life course (e.g., during the prenatal years, infancy, adolescence, or adulthood) and socioeconomic, political, cultural, and gender dynamics. For example, the lack of child care for agricultural workers often forces families, mostly mothers, to take their children to the fields while they work, thereby increasing young children’s exposures to pesticides (Natural Resources Defense Council 1999). Many of these pesticides are known neurotoxicants and carcinogens, and the potential long-term effects of childhood and prenatal exposures are just being explored and understood (Berkowitz et al. 2004; Castorina et al. 2003; Eskenazi et al. 2004; Gladen et al. 2003; Perera et al. 2003; Torres-Arreola et al. 2003; Whyatt et al. 2004; Young et al. 2005). Similarly, neighborhood-level factors associated with racial residential segregation may affect health by influencing access to affordable markets with fresh fruits and vegetables and access to health services (Diez-Roux 1997; Morland et al. 2002). Women without access to adequate prenatal care, for example, are likely to have compromised nutritional status, which in turn can heighten the impact of lead exposure both in utero and during early childhood (Lee et al. 2005; Zierold 2004).

State of the Evidence Top

Research on birth outcomes points to the validity of integrating social with environmental health riskscapes in future MCH research (Gee and Payne-Sturges 2004; Morenoff 2003; O’Campo et al. 1997). Evidence shows a consistent relationship between residence in poverty-stricken (Collins et al. 1997; O’Campo et al. 1997; Papacek et al. 2002), segregated (Guest et al. 1998; Laveist 1993) neighborhoods and poor birth outcomes. Moreover, preliminary work suggests substantial racial and ethnic disparities in environmental hazards exposures (Centers for Disease Control and Prevention 2005; IOM 1999; Morello-Frosch 2002; NEJAC 2004), including during pregnancy (Woodruff et al. 2003), and studies have begun to link pollutant exposures and negative birth and developmental outcomes (Dejmek et al. 1999; Ritz and Yu 1999; Ritz et al. 2000, 2002). One recent study of individual factors, pollutant exposures, and neighborhood measures of socioeconomic hardship (Ponce et al. 2005) found that preterm birth risk was affected by the interaction of residential traffic-related air pollution exposure and measures of neighborhood economic hardship.

Distilling the results of this diverse body of MCH research reveals two critical paths for future inquiry. The first is the direct health effects of hazardous social and physical environments to which communities of color and the poor are disproportionately exposed. To date, MCH studies have emphasized this first line of inquiry by analyzing the effects of individual and place-based socioeconomic status (SES) stressors, on one hand, or by assessing the effect of individual factors and environmental hazards, on the other.

The second line of inquiry, as outlined below, examines all of these factors in an integrated fashion by exploring how the multilevel interplay and possible interaction of psychosocial stressors with environmental hazards may shape disparities in birth outcomes. For example, previous pollutant exposures may enhance susceptibility to the toxic effects of current pollutant exposures, particularly if the body’s defense mechanisms and ability to recover or detoxify have been compromised through prior exposures to harmful agents. Similarly, exposure to place-based psychosocial stressors, such as persistent poverty, material deprivation, and a lack of services, may lead to chronic stress, which can weaken the body’s defense systems (Cohen 1999; McEwen 1998).

Physiologic Mechanisms Top

The concept of allostasis provides a framework for measuring the physiologic manifestations of chronic psychosocial and environmental stressors. Allostasis refers to the ability of the body’s stress–response systems to regulate internal physiology in response to psychosocial or physical stressors. The related concept of allostatic load refers to the cumulative physiologic degradation, over the life course, that can result from chronic stress exposure, and the accompanying long-term shift that occurs in the body’s homeostatic functions, with harmful consequences (Geronimus 1992; McEwen 1998; Seeman et al. 1997). The physiologic effect of prolonged stressors can exact a toll on the body that is caused by chronic activation of biologic systems, such as the hypothalamic–pituitary–adrenal axis, which releases hormones (e.g., glucocorticoids) that can have several metabolic and psychological effects, including the mobilization of energy reserves and suppression of the immune system (Seeman et al. 1997; Sterling and Eyer 1988). Chronic activation of this system can lead to “wear and tear” on major organ systems (McEwen 1998).

The mechanism of allostatic load provides a potential pathway by which place-based stressors can modify the toxic effect of environmental hazard exposures to produce disparate patterns of birth outcomes between and within populations. This is also in line with the concept of “weathering” proposed by Geronimus and others, suggesting that chronic stress associated with the combined effects of poverty, racial discrimination, and material deprivation causes the health of African-American mothers to deteriorate particularly rapidly, leading to poorer birth outcomes with increased age (Collins and Williams 1999; Geronimus 1992). The biomechanics of stress and allostasis can be considered a possible mediator of the heightened susceptibility to the adverse effects of pollution exposures observed among people with low SES.

A Framework for Future Research Top

The framework in Figure 1 suggests how area-level and individual-level stressors and buffers may combine to shape environmental hazard exposures, affect individual allostatic load, and in turn enhance susceptibility to the toxic effects of pollution exposures. At the bottom of Figure 1 is a variation of the exposure–health outcome continuum that outlines how environmental toxins might cause disease (National Research Council 1991). Traditionally, the exposure–health outcome continuum includes the emission of a contaminant from an indoor source such as smoking or an outdoor source such as an industrial facility, through human exposure via various media (e.g., air), and the occurrence of an adverse health outcome (e.g., low birth weight). The framework depicted in Figure 1 implies that the presence of an environmental contaminant must first lead to human exposure and then overcome the body’s defense systems to have an adverse health effect. The internal dose may not have an adverse health effect until it achieves a biologically effective dose that depends on rates of bioaccumulation, biotransformation, elimination, and, most relevant to our discussion, an individual’s susceptibility.

Animal studies suggest that stress can moderate a response to environmental toxins and other agents. For example, chronic stress may increase the absorption of environmental toxicants into the body through increased respiration, consumption, or perspiration (Gordon 2003). Similarly, allostatic load may amplify susceptibility to the toxic effects of pollutants, leading to adverse birth outcomes. Indeed, stress may alter physiologic functioning, through stress-dependent hormones that can affect in utero development and shift the threshold for toxicity, thereby leading to adverse birth outcomes at lower exposures (Paarlberg et al. 1995). Moreover, the body’s biotransformation or detoxification systems can remove or nullify toxins, but under conditions of chronic stress, the body’s defense system may be impaired, resulting in compromised organ resistance. Finally, illness caused by chronic stress may compromise a sick individual’s capacity to cope with environmental hazard exposures (Rios et al. 1993).

Conclusion Top

Allostatic load may be a critical physiologic mechanism that explains the excess burden of adverse birth outcomes related to certain pollutants observed among low-SES populations and some communities of color. Maternal immune systems that are shaped by chronic stressors before conception and during pregnancy may enhance particular vulnerabilities to adverse pregnancy outcomes. This is compounded by race- and class-based disparities in exposures to environmental hazards that are driven by the distribution of power, privilege, and economic resources (Morello-Frosch 2002). These environmental health disparities are likely to be moderated by the degree and magnitude of chronic community and individual-level stressors that may be reflected in individuals’ allostatic load. Therefore, a holistic approach to future MCH research that seeks to untangle the double jeopardy of chronic stressors and environmental hazard exposures could help elucidate how the interplay of these individual- and community-level factors shape persistent racial and economic disparities in birth outcomes. Most important, for researchers and practitioners concerned about environmental justice, this line of inquiry could suggest new strategies for alleviating systemic drivers of racial and socioeconomic disparities in birth outcomes.

Figures and Tables Top

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Figure 1.

The interplay of community and individual stressors/buffers that shape exposures and susceptibility to environmental hazards. Thick arrows indicate relationships that have been studied in the epidemiologic and sociology literature; dashed arrows indicate relationships that have not been extensively explored.

References Top

  1. Berkowitz GS, Wetmur JG, Birman-Deych E, Obel J, Lapinski RH, Godbold JH, et al. 2004. In utero pesticide exposure, maternal paraoxonase activity, and head circumference Environ Health Perspect 112:388–391.14998758 Find this article online
  2. Buka S, Brennan RT, Rich-Edwards JW, Raudenbush SW, Earls F 2002. Neighborhood support and the birthweight of urban infants Am J Epidemiol 157:1–8.12505884 Find this article online
  3. Castorina R, Bradman A, McKone TE, Barr DB, Harnly ME, Eskenazi B 2003. Cumulative organophosphate pesticide exposure and risk assessment among pregnant women living in an agricultural community: a case study from the CHAMACOS cohort Environ Health Perspect 111:1640–1648.14527844 Find this article online
  4. Centers for Disease Control and Prevention 2005. Third National Report on Human Exposure to Environmental Chemicals. NCEH Pub. No. 05-0570. Atlanta, GA:Centers for Disease Control and Prevention, National Center for Environmental Health
  5. Cohen S 1999. Social status and susceptibility to respiratory infections Ann NY Acad Sci 896:246–253.10681901 Find this article online
  6. Collins C, Williams D 1999. Segregation and mortality: the deadly effects of racism Social Forum 14:495–523. Find this article online
  7. Collins JW, Herman AA, David RJ 1997. Very-low-birthweight infants and income incongruity among African American and white parents in Chicago Am J Public Health 87:414–417.9096543 Find this article online
  8. Dejmek J, Selevan S, Beneš I, Solansky I, Šrám R 1999. Fetal growth and maternal exposure to particulate matter during pregnancy Environ Health Perspect 107:475–480.10339448 Find this article online
  9. Diez-Roux A 1997. Neighborhood environments and coronary heart disease: a multilevel analysis Am J Epidemiol 146:48–63.9215223 Find this article online
  10. Diez-Roux A 1998. Bringing context back into epidemiology: variables and fallacies in multilevel analysis Am J Public Health 88:216–222.9491010 Find this article online
  11. Eskenazi B, Harley K, Bradman A, Weltzien E, Jewell NP, Barr DB, et al. 2004. Association of in utero organophosphate pesticide exposure and fetal growth and length of gestation in an agricultural population Environ Health Perspect 112:1116–1124.15238287 Find this article online
  12. Gee G, Payne-Sturges D 2004. Environmental health disparities: a framework integrating psychosocial and environmental concepts Environ Health Perspect 1123:1645–1653.15579407 Find this article online
  13. Geronimus AT 1992. The weathering hypothesis and the health of African-American women and infants: evidence and speculations Ethn Dis 2:207–221.1467758 Find this article online
  14. Gladen BC, Shkiryak-Nyzhnyk ZA, Chyslovska N, Zadorozhnaja TD, Little RE 2003. Persistent organochlorine compounds and birth weight Ann Epidemiol 13:151–157.12604157 Find this article online
  15. Gordon C 2003. Role of environmental stress in the physiological response to chemical toxins Environ Res 92:1–7.12706749 Find this article online
  16. Guest AM, Almgren G, Hussey JM 1998. The ecology of race and socioeconomic distress: infant and working-age mortality in Chicago Demography 35:23–34.9512907 Find this article online
  17. Hessol NA, Fuentes-Afflick E, Bacchetti P 1998. Risk of low birth weight infants among black and white parents Obstet Gynecol 92:814–822.9794675 Find this article online
  18. Hummer RA 1993. Racial differences in infant mortality in the U.S.: an examination of social and health determinants Soc Forces 72:529–554. Find this article online
  19. Huynh M, Parker J, Harper S, Pamuk E, Schoendorf K 2005. Contextual effect of income inequality on birth outcomes Int J Epidemiol 34:888–895.15860635 Find this article online
  20. IOM 1999. Toward Environmental Justice: Research, Education, and Health Policy Needs. Washington, DC:Institute of Medicine, Committee on Environmental Justice, Health Sciences Policy Program, Health Sciences Section
  21. Jerrett M, Finkelstein M 2005. Geographies of risk in studies linking chronic air pollution exposure to health outcomes J Toxicol Environ Health 68:1207–1242. Find this article online
  22. Laveist TA 1993. Segregation, poverty, and empowerment: health consequences for African Americans Milbank Q 71:41–64.8450822 Find this article online
  23. Lee M, Chun O, Song W 2005. Determinants of the blood lead levels of US women of reproductive age J Am Coll Nutr 24:1–9.15670978 Find this article online
  24. Matteson DW, Burr JA, Marshall JR 1998. Infant mortality: a multi-level analysis of individual and community risk factors Soc Sci Med 47:1841–1854.9877352 Find this article online
  25. McEwen BS 1998. Protective and damaging effects of stress mediators N Engl J Med 338:171–179.9428819 Find this article online
  26. Morello-Frosch R 2002. The political economy of environmental discrimination Environ Plann C Gov Policy 20:477–496. Find this article online
  27. Morello-Frosch R, Jesdale BM 2006. Separate and unequal: residential segregation and estimated cancer risks associated with ambient air toxics in U.S. metropolitan areas Environ Health Perspect 114:386–393.16507462 Find this article online
  28. Morello-Frosch R, Pastor M, Sadd J 2001. Environmental justice and southern California’s “riskscape”: the distribution of air toxics exposures and health risks among diverse communities Urban Affairs Rev 36:551–578. Find this article online
  29. Morenoff J 2003. Neighborhood mechanisms and the spatial dynamics of birth weight Am J Sociol 108:976–1017.14560732 Find this article online
  30. Morland K, Wing S, Diez Roux A, Poole C 2002. Neighborhood characteristics associated with the location of food stores and food service places Am J Prev Med 22:23–29.11777675 Find this article online
  31. National Center for Health Statistics 2003. National Vital Statistics Report. Washington, DC:National Center for Health Statistics
  32. National Research Council 1991. Human Exposure Assessment for Airborne Pollutants: Advances and Opportunities. Washington, DC:National Academy Press
  33. Natural Resources Defense Council 1999. Trouble on the Farm: Growing Up with Pesticides in Agricultural Communities. San Francisco:Natural Resources Defense Council
  34. NEJAC 2004. Ensuring Risk Reduction in Communities with Multiple Stressors: Environmental Justice and Cumulative Risks/Impacts—Draft Report. New Orleans:National Environmental Justice Advisory Council
  35. O’Campo P, Xue X, Wang M-C, Caughy MOB 1997. Neighborhood risk factors for low birthweight in Baltimore: a multilevel analysis Am J Epidemiol 87:1113–1118. Find this article online
  36. O’Neill MS, Jerrett M, Kawachi I, Levy JI, Cohen AJ, Gouveia N, et al. 2003. Health, wealth, and air pollution: advancing theory and methods Environ Health Perspect 111:1861–1870.14644658 Find this article online
  37. Paarlberg KM, Vingerhoets AJ, Passchier J, Dekker GA, Van Geijn HP 1995. Psychosocial factors and pregnancy outcome: a review with emphasis on methodological issues J Psychosom Res 39:563–595.7490693 Find this article online
  38. Papacek EM, Collins JW, Schulte NF, Goergen C, Drolet A 2002. Differing postneonatal mortality rates of African-American and white infants in Chicago: an ecologic study Matern Child Health J 6:99–105.12092986 Find this article online
  39. Parker J, Woodruff T, Basu R, Schoendorf K 2005. Air pollution and birth weight among term infants in California Pediatrics 115:121–128.15629991 Find this article online
  40. Perera FP, Rauh V, Tsai WY, Kinney P, Camann D, Barr D, et al. 2003. Effects of transplacental exposure to environmental pollutants on birth outcomes in a multiethnic population Environ Health Perspect 111:201–205.12573906 Find this article online
  41. Ponce N, Hoggatt K, Wilhelm M, Ritz B 2005. Preterm birth: the interaction of traffic-related air pollution with economic hardship in Los Angeles neighborhoods Am J Epidemiol 162:140–148.15972941 Find this article online
  42. Rawlings J, Rawlings V, Read J 1995. Prevalence of low birth weight and preterm delivery in relation to the interval between pregnancies among white and black women N Engl J Med 332:69–74.7990903 Find this article online
  43. Rich-Edwards JW, Grizzard T 2005. Psychosocial stress and neuroendocrine mechanisms in preterm delivery Am J Obstet Gynecol 192:S30–S35.15891710 Find this article online
  44. Rios R, Poje G, Detels R 1993. Susceptibility to environmental pollutants among minorities Toxicol Ind Health 9:797–820.8184444 Find this article online
  45. Ritz B, Yu F 1999. The effect of ambient carbon monoxide on low birth weight among children born in southern California between 1989 and 1993 Environ Health Perspect 107:17–25.9872713 Find this article online
  46. Ritz B, Yu F, Chapa G, Fruin S 2000. Effect of air pollution on preterm birth among children born in southern California between 1989 and 1993 Epidemiology 11:502–511.10955401 Find this article online
  47. Ritz B, Yu F, Chapa G, Shaw G, Harris J 2002. Ambient air pollution and risk of birth defects in southern California Am J Epidemiol 155:17–25.11772780 Find this article online
  48. Sadler L, Belanger K, Saftlas A, Leaderer B, Hellenbrand K, McSharry J, et al. 1999. Environmental tobacco smoke exposure and small-for-gestational-age birth Am J Epidemiol 150:695–705.10512423 Find this article online
  49. Schultz A, Williams D, Israel B, Lempert L 2002. Racial and spatial relations as fundamental determinants of health in Detroit Milbank Q 80:677–707.12532644 Find this article online
  50. Seeman TE, Singer BH, Rowe JW, Horwitz RI, McEwen BS 1997. Price of adaptation—allostatic load and its health consequences. MacArthur studies of successful aging Arch Intern Med 157:2259–2268.9343003 Find this article online
  51. Shenassa E 2001. Society, physical health and modern epidemiology Epidemiology 12:467–470.11416784 Find this article online
  52. Shenassa ED, Stubbendick A, Brown MJ 2004. Social disparities in housing and related pediatric injury: a multilevel study Am J Public Health 94:633–639.15054017 Find this article online
  53. Singh GK, Yu SM 1995. Infant mortality in the United States: trends, differentials, and projections, 1950 through 2010 Am J Public Health 85:957–964.7604920 Find this article online
  54. Starfield B, Shapiro S, Weiss J, Liang K-Y, Ra K, Paige D, et al. 1991. Race, family income, and low birth weight Am J Epidemiol 134:1167–1174.1746527 Find this article online
  55. Sterling P, Eyer J 1988. Allostasis: a new paradigm to explain arousal pathology. In: Handbook of Life Stress, Cognition, and Health (Fisher S, Reason JT, eds). Chichester, NY:Wiley, 750
  56. Torres-Arreola L, Berkowitz G, Torres-Sanchez L, Lopez-Cervantes M, Cebrian ME, Uribe M, et al. 2003. Preterm birth in relation to maternal organochlorine serum levels Ann Epidemiol 13:158–162.12604158 Find this article online
  57. Whyatt RM, Rauh V, Barr DB, Camann DE, Andrews HF, Garfinkel R, et al. 2004. Prenatal insecticide exposures and birth weight and length among an urban minority cohort Environ Health Perspect 112:1125–1132.15238288 Find this article online
  58. Wilhelm M, Ritz B 2003. Residential proximity to traffic and adverse birth outcomes in Los Angeles County, California, 1994–1996 Environ Health Perspect 111:207–216.12573907 Find this article online
  59. Williams D, Collins C 2004. Reparations: a viable strategy to address the enigma of African American health Am Behav Sci 47:977–1000. Find this article online
  60. Woodruff T, Parker J, Kyle A, Schoendorf K 2003. Disparities in exposure to air pollution during pregnancy Environ Health Perspect 11:942–946.12782496 Find this article online
  61. Young JG, Eskenazi B, Gladstone EA, Bradman A, Pedersen L, Johnson C, et al. 2005. Association between in utero organophosphate pesticide exposure and abnormal reflexes in neonates Neurotoxicology 26:199–209.15713341 Find this article online
  62. Zierold K 2004. Trends in blood lead levels among children enrolled in the special supplemental nutrition program for women, infants, and children from 1996 to 2000 Am J Public Health 94:1513–1515.15333304 Find this article online
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