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Environment Magazine September/October 2008

 

November-December 2014

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The Hidden Asthma Epidemic in Immigrant Subpopulations

Asthma is a chronic pulmonary condition that affects more than 13% of the U.S. population, up from 11% a decade ago.1 The economic impact of asthma in inflation-adjusted 2013 dollars has grown from over $10 billion in 1990 to over $60 billion in 2007.2 While the burden of asthma in the general population is high, poor housing conditions and proximity to sources of air pollution disproportionately affect certain groups, making these populations more vulnerable to asthma. In particular, minorities and people with lower socioeconomic status are more likely to report having asthma, are more likely to have worse asthma outcomes, and are more likely to be hospitalized for asthma.3 The disparities in asthma prevalence and the inequities in exposure to asthma risk factors across racial and ethnic groups have been broadly viewed as a pervasive social justice issue. However, the crude asthma prevalence among certain racial and ethnic minorities may not tell the whole story due to differences in asthma prevalence among subpopulations. First-generation immigrants from developing countries tend to have lower asthma prevalence than U.S.-born individuals with similar demographic characteristics and environmental exposures. This article discusses the evidence for the association between asthma and environmental risk factors that differentially impact these vulnerable groups. It then addresses the hidden asthma epidemic among immigrant subpopulations with a focus on ways to mitigate these disparities.

The association between asthma and environmental risk factors may be related to disparities in socioeconomic status among immigrant subpopulations. Specifically, socioeconomic status may be a proxy for exposure to environmental or social conditions that are associated with asthma. For example, immigrants make up a large percentage of the community living in public housing and individuals who live in public housing are approximately three times as likely to have asthma as individuals who live in private family dwellings.4 These individuals are also more likely to be exposed to mold, water damage, pests, high concentrations of particulate matter due to cigarette smoking and proximity to roadways, and other risk factors.5

Differential exposure to these environmental conditions might mediate some of the relationship between racial or ethnic minority status and asthma prevalence. In particular, children exposed to cockroach or mite allergens may also be more likely to develop asthma6 or to have asthma symptoms.7 Likewise, controlling for a variety of factors, children exposed to mold indicators in the home are 2.4 times as likely to develop asthma (95% CI = 1.07–5.60 over 10,000 person years).8 Among adults, exposure to mold in the home is associated with a 28% increase in the prevalence of current asthma.9 Furthermore, interventions that reduce exposure to pests and other allergens are associated with symptom reduction among children living in public housing10 and among children living in inner-city locations.11

In contrast to certain other indoor air quality factors, a lower frequency of exposure to secondhand smoke among certain racial and ethnic groups, and particularly Asians and Hispanics,12 may serve as a protective factor against asthma. According to the U.S. Surgeon General's 2010 report, sufficient evidence exists to “infer a causal conclusion” between tobacco smoke exposure and asthma symptoms in children as well as with respiratory symptoms in adults. Moreover, there is a causal connection between exposure to parental smoking and prevalent lifetime asthma among children.13 Exposure to secondhand smoke has also been associated with increased asthma severity14 and a 2.5-fold increase in asthma incidence among children.15

As with risk factors for asthma present in the indoor environment, several environmental asthma risk factors outside of the home also disproportionately affect marginalized groups, including immigrants. Ecologic-level data suggest that there is an increase in the prevalence of asthma hospitalizations among children who live near noxious land uses.16 Exposure to highway-related air pollution seems to be of particular concern. Children who live within 200 meters of a busy road are twice as likely to be hospitalized for asthma, suggesting further environmental justice issues.17 Furthermore, prospective studies suggest that exposure to nitrogen dioxide (NO2) is associated with an increased pediatric asthma incidence, and a case-control study found that the odds of asthma among African American and Latino individuals aged 8–21 years increased by 26% for every 9.4 νg/m3 increase of NO2.18 Likewise, adults who have never smoked are 1.3 times as likely (95% CI = 1.05–1.61) to develop asthma for every 1 νg/m3 increase in empirically measured traffic-related particulate matter near the home over an 11-year period.19 Since 27.4% of non-white individuals live within 250 meters of busy roadways in comparison to the 19.3% of the general population, non-white individuals are disproportionately at risk from traffic-related exposures.20

Beyond the air quality risk factors, there also seem to be links between environmental exposures and exposure to stress that could particularly affect low-income, minority, and immigrant individuals. Children who are exposed to medium or high levels of neighborhood violence are more than 1.5 times as likely to have asthma as kids exposed to low levels of neighborhood violence.21 Moreover, there is an interaction between housing quality and exposure to interpartner violence. Children living in deteriorated homes whose mothers experience chronic interpartner violence are 4.6 times as likely to have asthma as other children.22

While the links between these standard environmental and social risk factors, along with a multitude of other factors, have been extensively considered in the literature, much remains unknown about the etiology of asthma. The relative importance of each of these factors and the interaction of various genetic, epigenetic, and lifestyle factors are not well documented. Although the etiology of asthma is not explored here, it is worth noting that clues into asthma development can be uncovered by studying the development of asthma among some racial and ethnic subgroups, particularly immigrants because they have moved from one environment to another. The noted disparities in exposure to the environmental and social conditions explain only a part of the trends in asthma prevalence.

Differences in Asthma Prevalence Among Ethnic and Racial Subgroups

The distribution of asthma in the population is more nuanced than the previous discussion of environmental and social risk factors may suggest and highlights another critical aspect of the social justice concern. In fact, there is substantial heterogeneity in the prevalence of asthma within ethnic subgroups. Among adolescents who participated in the Healthy Kids Survey, Hispanics with a Cuban or Puerto Rican background had almost twice the prevalence of asthma as Hispanics with a Mexican background. Similarly, among Asian adolescents who participated in this survey, individuals who identified as Filipino had between two and three times the asthma prevalence as individuals who identified as Korean, Laotian, Cambodian, or Vietnamese.23 While the differences in asthma prevalence within racial and ethnic sub-groups are striking, even this underestimates the heterogeneity in asthma burden within populations.

Nativity is an additional factor that affects asthma prevalence within populations with a large proportion of immigrants.24 Foreign-born individuals who immigrate to the United States from low-income countries generally have a lower asthma prevalence than either the general U.S. population or the corresponding U.S.-born population of the same race or ethnicity. This trend has been observed among Hispanic, Asian, and black immigrants to the United States.25 The trend is often referred to as the Latino Paradox, but extends to Asians and other immigrants. The Latino Paradox is a well-documented phenomenon that describes the relative health advantage foreign-born Latinos have over U.S.-born Latinos, including for respiratory outcomes.26 Perhaps not surprisingly, the specific place of birth also matters within each racial and ethnic group. For example, data from the National Health Interview Survey indicate that children born in Mexico were only about half as likely to have asthma as non-Hispanic white children born in the United States. In contrast, children born in Puerto Rico (who are U.S. citizens) were about 2.5 times as likely to have asthma as non-Hispanic white children born in other parts of the United States.27

The nativity trends also apply to non-Hispanic immigrant groups. Among Asian children living in Boston, individuals born in the United States are more than four times as likely to have asthma as children born outside of the United States even controlling for exposure to motor vehicle traffic and a family history of asthma.28 In a study of asthma trends among the black population in Boston, none of the 14 foreign-born children had asthma and foreign-born adults were only about one-third as likely as U.S.-born participants to have asthma (Figure 1). The difference of asthma prevalence by nativity suggests that aggregating asthma data from native- and foreign-born individuals may substantially underestimate the prevalence in the subgroups.29 Furthermore, the influence of a variety of social and environmental risk factors may differ among foreign-born and U.S.-born individuals.30

Figure 1. There are racial disparities in asthma prevalence. First-generation Chinese immigrants living in Boston and individuals living in China have a low asthma prevalence compared to individuals living in the U.S.

Differences in asthma prevalence that reflect country of origin are also observed globally. Generally, people who immigrate from a less developed country to a more developed country have a lower asthma prevalence than people who were born in the more developed country. For example, among adolescents residing in Canada and participating in the International Study of Asthma and Allergies in Childhood, those born in mainland China had lower asthma prevalence than Chinese individuals born in Hong Kong or Canada. Furthermore, length of time residing in Canada was positively associated with asthma symptoms.31 Similarly, among military recruits in Israel, first-generation immigrants from Ethiopia and the former Soviet Union had lower asthma prevalence than the Israeli-born recruits, while immigrants from Western countries had the highest asthma prevalence among the recruits.32

The protective effect seems to be limited to first-generation immigrants, however, and may even erode with time in the host population.33 In Sweden, foreign-born immigrant children were three to four times less likely to have asthma as both international adoptees and children born in Sweden to foreign-born parents. Additionally, the older the children were at the time of immigration, the less likely they were to have asthma.34 Since only first-generation immigrants show low asthma prevalence compared to nonimmigrants, it is possible that early life influences in the host country affect the development of asthma.

The idea that early life exposures play a role in asthma pathophysiology is not especially controversial, although precisely how these environmental factors affect the development of the immune system remains unclear. In what initially seems counterintuitive, the Hygiene Hypothesis, a leading framework to explain the observed variation in the global disparities in disease prevalence, suggests that early life exposure to infectious diseases and parasites is important for proper immune system development. In particular, early life infections that are common in the developing world but are less common in the developed world may help establish immune system regulation and proper balance between T-helper celltypes one and two, which could protect against the development of asthma.35 Lacking early infectious disease exposure might leave the immune system vulnerable to developing sensitivities to other exposures, such as cockroaches, which would otherwise not stimulate a reaction. The Hygiene Hypothesis thus predicts that if immigrants from certain regions of the world grow up with more exposure to infectious disease agents and parasites, they would experience lower asthma prevalence than individuals born in the host country because of the protective effect environmental pathogens have on immune system development.

The decrease in infectious diseases among children in high-income nations may therefore be contributing to an increase in immune disorders.36 However, not all of the evidence supports this theory. For example, one review found that poor hygiene in the host country, defined as early life exposure to the gram-negative bacteria endotoxin, did not seem to protect against asthma onset.37 Additionally, a case-control study of children in California found that exposures in the first year of life to pests, pesticides, solid fuel combustion, farm animals, and crops are all positively associated with asthma onset by 5 years of age.38 However, on an ecologic level, the idea that atopic sensitization is more common in countries that are more developed is supported by several large studies.39

Additionally, an emerging body of literature suggests that the Hygiene Hypothesis may be partially explained by differences in exposures that shape the microbiome. It is known, for example, that intestinal microbiota affect inflammation processes.40 Children who live on farms are exposed to more microorganisms indoors than children who do not live on farms and the diversity of exposure to these microorganisms is inversely correlated with the odds of having asthma.41 Nevertheless, the current understanding of the Hygiene Hypothesis may not fully account for the global asthma prevalence trends.

There are other potential reasons for the low asthma prevalence among immigrant groups. Some evidence suggests that there is a selection bias for who is physically or financially able to immigrate, although other evidence suggests that immigrant health status is similar to that of other individuals in the country of origin.42 In any case, a commonly observed phenomenon is the healthy immigrant effect, in which immigrants have better health upon arrival in the host country and then experience a convergence of health status over time in the host country.43

Regardless of the reasons for the low asthma prevalence among first-generation immigrants, the risk for developing asthma may seem artificially low in immigrant populations if the data are not disaggregated by country of birth. Unfortunately, in communities with potentially vulnerable populations that seem to have a typical or low asthma prevalence because of the combined effect of two subpopulations, one with a high asthma prevalence and one with a low prevalence, resources may not be directed toward offsetting environmental and social risks for the development of asthma based on a failure to notice the high prevalence among the U.S.-born subpopulation. This is especially problematic because of the increased likelihood of exposure to numerous environmental risk factors for asthma among racial and ethnic minorities compared to U.S.-born whites.

Next Steps: Recommendations to Address Asthma as a Social Problem

The existence of hidden subpopulations that suffer disproportionately large asthma burdens is a social problem. From an academic perspective, future research is needed to understand the relative influence of the risk factors associated with asthma in different populations. Furthermore, the approach to asthma research matters. Given the disparities seen by socioeconomic status, nativity, race, and ethnicity with regard to both risk factors for asthma and asthma outcomes, it is essential to meaningfully include typically marginalized groups in asthma research as called for by the principles of environmental justice.

A method to advance these aims is through community-based participatory research (CBPR).44 In CBPR studies, community partners are true collaborators throughout the research process. In Boston, Massachusetts, CBPR projects on asthma have been successfully completed with community partners and key stakeholders from a variety of sectors actively contributing at every stage of the research process. While this approach can be resource intensive, the quality of the science can be excellent and the research findings may result in more direct action to benefit the affected populations, such as contributing to improved pest management practices in public housing.45

Due to the potential for subpopulations to have very different asthma prevalence based on nativity, a critical step is collecting and reporting asthma surveillance data that includes country of birth. Reporting data disaggregated on nativity could help avoid inequities in resource allocation. The repercussions of this were observed in Boston's Chinatown. This community was largely ignored at one time with regard to asthma because there was little data on asthma prevalence or severity and what evidence there was did not seem to indicate serious problems. For example, in an initial survey at a school that had predominately Asian American children, fewer parents of children with Asian surnames reported that their children had asthma than parents of children without Asian surnames. Children living in Chinatown were also less likely to have asthma than children in other nearby neighborhoods.46 However, later work found that the low asthma prevalence among immigrant children living in Chinatown masked the high prevalence of asthma among Asian children born in the United States living in Boston. The availability of data indicating evidence for a much higher prevalence among U.S.-born Chinese children helped lead to the development of asthma programs, including a culturally tailored, English-Chinese education program.47 More broadly, effective targeting of asthma intervention programs should identify and address the unique needs of second- and third-generation immigrant communities.

Nevertheless, until mechanisms exist on municipal, state, and national levels to identify the distinct subgroups that are most at risk, resources will not be allocated as efficiently or equitably as they could be. It is therefore important to disaggregate ethnic and racial groups by country of origin in asthma surveillance and asthma research to bring this demographic factor alongside more commonly recognized factors such as socioeconomic status and race (Figure 2). Another way to begin improving asthma surveillance is through greater awareness among health care providers of the increased asthma risk in children born in the United States to immigrant families.

Figure 2. Pediatric asthma prevalence data from the Behavioral Risk Factor Surveillance System.

Ultimately, however, an increased focus on primary prevention of asthma is needed to reduce exposures associated with asthma incidence, particularly among the most vulnerable populations. There has been surprisingly little attention paid to primary prevention of asthma until very recently, probably because the scientific support for it developed only recently. Nevertheless, during a recent symposium in Massachusetts, representatives from many stakeholder groups met to share advances in the field and to create a roadmap outlining specific asthma primary prevention strategies.

Among the areas that the representatives agreed were ready for action based on the strength of the scientific evidence were: 1) creating asthma-safe housing, schools, early childhood care settings, and workplaces free from exposure to asthmagens such as tobacco smoke, mold, and pests; 2) improving educational and programmatic initiatives, with a special focus on reducing exposure to asthma risk factors in the first 1000 days of child development; and 3) reducing traffic-related air pollution, strengthening community based initiatives to enhance the built environment, improving outdoor air quality, and reducing psychosocial stressors such as neighborhood violence.48 From this, and from the growing body of literature, it seems clear that effective interventions will use multifactorial approaches to reduce disparities in environmental and psychosocial exposures.49

Many of these approaches, such as improving the quality of public housing, will require dedicated resources warranted by the high disease burden in specific subpopulations. By reducing the number of school and work days missed and by reducing the burden on the health care system, multicomponent home-based interventions that decrease exposure to environmental risk factors seem to be a cost-effective policy solution.50 Disaggregated data collection would help public health officials concentrate resources more appropriately to efficiently mitigate racial and ethnic disparities among communities that would otherwise be misidentified as having a low disease burden.

As a final note, health policies aimed at reducing the asthma burden among these immigrant subpopulations should consider the particular opportunities and challenges to implementation when working with marginalized groups. For example, although the implementation of smoke-free laws is associated with reductions in asthma hospitalization rates and the odds of having asthma symptoms,51 varying attitudes about smoking regulations might be expected among immigrants. Researchers, clinicians, policymakers, public health officials, and community groups will need to continue to work together to identify and implement culturally competent measures that reduce the disparities in environmental exposures and asthma burden. By targeting these disparities through further research, implementation of evidence-based primary prevention programs, and the use of more nuanced disease surveillance methods that distinguish U.S.-born and foreign-born subpopulations, we can reduce the burden of chronic disease and mitigate a critical injustice present throughout our society today.

NOTES

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Laura Corlin is an environmental health student in the Department of Civil and Environmental Engineering at Tufts University School of Engineering in Boston, MA. Her research and advocacy focus on characterizing and mitigating health disparities.

Doug Brugge is a professor in the Department of Public Health and Community Medicine at Tufts University School of Medicine in Boston, MA. He became interested in the causes of asthma when his students noticed that foreign-born Chinese immigrant children were less likely to have asthma than U.S.-born children Chinese children.

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