Country of Birth as a Risk Factor for Asthma among Mexican Americans
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美国呼吸和危急护理医学 2005年第1期
Centers for Disease Control and Prevention
Department of Pediatrics, Emory University School of Medicine, Atlanta, Georgia
Municipal Institute of Medical Research, Barcelona, Spain
Instituto Nacional de Salud Pe瞓lica, Cuernavaca Morelos, Mexico
ABSTRACT
In the United States, among Hispanics, Mexican Americans have the lowest rate of asthma. However, this population includes Mexican Americans born in the United States and in Mexico, and risk factors that might impact the prevalence of asthma differ between these groups. To determine the prevalence of and risk factors for asthma among U.S.- and Mexican-born Mexican Americans, we analyzed data from two U.S. surveys that included 4,574 persons who self-reported their ethnicity as Mexican American from the Third National Health and Nutrition Examination Survey (NHANES III) 1998eC1994 and 12,980 persons who self-reported their ethnicity as Mexican American from National Health Interview Survey (NHIS) 1997eC2001. U.S.-born Mexican Americans were more likely than Mexican-born Mexican Americans to report ever having asthma in both the NHANES III (7% [SE 0.5] vs. 3% [SE 0.3], p < 0.001) and NHIS surveys (8.1% [0.4] vs. 2.5% [0.2], p < 0.001). In a multivariate regression model controlling for multiple demographic variables and health care, the risk for asthma was higher among U.S.-born Mexicans in NHANES III (odds ratio 2.1, 95% confidence interval 1.4eC3.3) and NHIS (odds ratio 2.7, 95% confidence interval 1.6eC5.5). In conclusion, the prevalence of asthma was higher in U.S.-born than in Mexican-born Mexican Americans. This finding highlights the importance of environmental exposures in developing asthma in a migratory population.
Key Words: asthma Mexican Americans migration
In the United States, the prevalence of asthma varies significantly among different ethnic groups, with Mexican Americans having the lowest asthma prevalence among Hispanics (1, 2). Higher rates of respiratory diseaseeCrelated mortality and higher prevalence of chronic medical conditions have also been observed in U.S.-born Hispanics than in foreign-born Hispanics residing in the United States (3, 4). The reason for these disparities are unknown; however, these observations suggest that place of birth and early life exposure to and length of residence in the United States might influence exposure to environmental factors that contribute to the development of chronic health problems (3, 5). The prevalence of asthma among Mexican Americans might therefore differ according to their place of birth and change in lifestyle associated with the acculturation processes over several years of residence in the United States. To test this hypothesis, the present authors compared the prevalence of asthma among U.S.-born Mexican Americans and Mexicans who immigrated to the United States using data from the Third National Health and Nutrition Examination Survey (NHANES III), 1988eC1994, and the National Health Interview Survey (NHIS) for 1997eC2001. Some of the results of this study have been previously reported in the form of an abstract (6).
METHODS
The present authors used data from two large and independent population surveys to determine factors that influence the prevalence of asthma among U.S.- and Mexican-born Mexican Americans. NHANES III, which comprises interview and medical examination data, and the NHIS survey, which comprises interview data only, were used to examine the prevalence of asthma and respiratory symptoms. In both surveys, interviews were conducted in the language spoken in the household (English or Spanish). NHANES III was conducted in two phases from October 1988 through October 1994 (7). NHIS is an annual, national health survey conducted by personal interview in the home throughout the year; for this study, the authors used data compiled from 1997 through 2001 (8). Both surveys use a complex multistage, stratified survey of civilian, noninstitutionalized populations. The authors restricted analyses to survey participants reporting a Mexican origin who were 18 years or older. They calculated all estimates using the sampling weight to provide population estimates that adjusted for unequal probabilities of selection and that accounted for nonresponse.
Questionnaire
The authors obtained information on variables from the NHANES III and NHIS questionnaires and defined them as follows:
Asthma.
From NHANES III, we used the following questions: "Has a doctor ever told you that you had asthma" and "Have you had wheezing or whistling in your chest at any time in the past 12 months" From NHIS, we used the following questions: "Has a doctor or other health professional ever told you that you had asthma" and "Have you had an asthma attack/episode in the last 12 months"
Covariates.
Age, sex, smoking status (current, former, and never), and education (number of years lower than, equivalent to, or greater than high school [< 12, = 12, > 12 years]) were available in both questionnaires. Body mass index (BMI) was defined as weight in kilograms divided by height in meters squared and was categorized as normal (18.5 to < 25), overweight (25 to < 30), and obese ( 30) (9). PovertyeCincome ratio (PIR) was computed as the ratio of the midpoint family income divided by the poverty level in dollars as defined by the Census Bureau for the corresponding survey year (8). Acculturation status was defined by the language used during the survey interview (10). Citizenship status was available only for NHIS, and time since immigration to the United States was available only for NHANES III. We defined health care access as using medical insurance coverage and having a regular source of medical care (4). For having a regular source of medical care, we used the following question in NHANES: "Is there a particular clinic, health center, doctor's office, or other place that you usually go to if you are sick, need advice about your health, or for routine care" In NHIS, the question was "Do you have a place you usually go when sick"
Analysis.
The frequency distribution of specific variables from NHANES III and NHIS were determined among U.S.-born and Mexican-born Mexican Americans. We compared proportions of categorical variables using a 2 test for independence. Univariate and multivariate logistic regression models were performed separately for NHIS and NHANES III to determine the crude and adjusted odds ratios (ORs) of ever having been diagnosed with asthma. Stratified analyses by place of birth were performed to determine the risk factors of asthma diagnosis. Models were adjusted for the following covariates: gender, BMI, smoking status, age, PIR, insurance coverage, regular source of medical care, region of survey, language of interview, and citizenship status (only for NHIS data). We also tested independently in each survey the interactions of specific covariates (BMI, age, gender, smoking, regular access to health care, and PIR) with the exposure (place of birth). The prevalence and OR for asthma in Mexican-born Mexican Americans, according to their time of residence in the United States, were determined using NHANES III. A level of p < 0.05 was considered significant. For the analyses, we used SAS (SAS Institute, Cary, NC) and SUDAAN (SAS Version 8; Research Triangle Institute, Research Triangle Park, NC) to adjust for complex sample design when variance estimates ware calculated.
RESULTS
The analysis comprised 17,554 adults. From NHANES III, 4,574 of 20,050 adults self-reported their ethnicity as Mexican American (22.8%): 2,422 (12.1%) were U.S.-born Mexican Americans, and 2,152 (10.7%) were Mexican-born Mexican Americans. We excluded 732 respondents for the following reasons: 371 were not born in the United States or Mexico and 361 had no medical examination data. From NHIS, we analyzed data from 12,980 persons who self-reported their ethnicity as Mexican American (from 159,376 adults, or 8.1%): 5,860 (3.6%) were U.S.-born and 7,120 (4.5%) were Mexican-born.
In both surveys, Mexican-born participants were significantly younger and had a lower education level and higher PIR than did U.S.-born participants (Tables 1 and 2). Also, Mexican-born participants were on average diagnosed with asthma at an older age when compared with U.S.-born participants (33 years [SE 1.9] vs. 21 years [SE 2.8], p < 0.001). The prevalence of ever having asthma was significantly higher in U.S.-born participants in both NHANES III (7.0% [SE 0.5] and 3.0% [SE 0.3], p < 0.001) and NHIS surveys (8.1% [SE 0.4] ves. 2.5% [SE 0.2], p < 0.001) than in Mexican-born participants. Figure 1 presents the prevalence of asthma for both surveys by year. In the group of excluded participants from NHANES III (n = 732), the prevalence of ever having asthma was 4%, and their exclusion from the analysis did not modify the difference in asthma prevalence among U.S.- and Mexican-born participants in NHANES III.
Among lifestyle risk factors potentially associated with asthma, the prevalence of current smoking was slightly higher in U.S.-born participants. Although more Mexican-born participants were overweight (BMI > 25 and < 30), U.S.-born participants had a significantly higher prevalence of obesity (BMI 30).
A larger proportion of U.S.-born participants had access to health care; however, when we compared the prevalence of asthma diagnosis between U.S.- and Mexican-born participants with health insurance or a regular source for health care, U.S.-born participants had significantly higher asthma prevalence in both NHANES III and NHIS. In NHANES III, the prevalence of asthma among participants with and without a regular source of health care was, respectively, 7.4% and 5.8% for U.S.-born participants and 4.3% and 1.4% for Mexican-born participants (p < 0.001). In NHIS, the prevalence of asthma among participants with and without a regular source of health care was, respectively, 7.4% and 5.8% in U.S.-born participants and 3.5% and 0.7% in Mexican-born participants (p < 0.001). In NHANES III, in the subpopulation with health care insurance, the prevalence of asthma was 7.0% in U.S.-born and 3.7% in Mexican-born participants (p < 0.001). In NHIS, the prevalence of asthma in participants with insurance was also higher in U.S.- compared with Mexican-born participants (5.5% vs. 2.1%, p < 0.001). The prevalence of asthma in uninsured participants remained higher for U.S.- versus Mexican-born participants (6.6% vs. 2.0% in NHANES III and 8.9% vs. 3.1% in NHIS, p < 0.001).
After adjustment for age, smoking status, BMI, region of survey, language, and health care access, U.S.-born Mexican Americans were at higher risk for asthma than were Mexican-born Mexican Americans in NHANES III (odds ratio [OR] 2.12, 95% confidence interval [CI] 1.38eC3.28) and NHIS (OR 2.73, 95% CI 1.63eC5.50). Obesity (BMI 30) also was associated with higher odds for asthma in both U.S.- and Mexican-born participants only in NHIS (Tables 3 and 4).
We did not find significant interactions between place of birth with gender, BMI, smoking status, regular source for medical care, and poverty level in NHANES III or NHIS. The interaction between age and place of birth was significant in both surveys. The results of this interaction are shown in the stratified analysis by place of birth in each survey (Table 4) where the risk for asthma increases with age in Mexican-born participants and decreases in U.S.-born participants.
The average age at immigration to the United States was 21 years (interquartile range 16eC29 years). The prevalence of asthma in Mexican-born participants with fewer than 10 years of residence in the United States was 2.0% (SE 0.5) compared with 4.0% (SE 0.5) in those who had resided more than 10 years in the United States (p < 0.01). After adjusting for age, the OR of having asthma associated with more than 10 years of U.S. residence was 1.60 (0.74eC3.07) compared with residing in the U.S. less than 10 years.
DISCUSSION
U.S.-born Mexican Americans had a higher prevalence of asthma than did Mexican-born Mexican Americans, independent of access to health care and other potential confounders. Also, with prolonged times of residence in the United States, the prevalence of asthma increased in Mexican-born participants, suggesting that either cumulative environmental exposure and/or lifestyle changes after immigration may influence the risk of developing asthma.
The authors' results agree with reports from smaller studies conducted in European countries, Israel, and Australia where foreign-born populations arriving from countries with lower asthma prevalence experience a time-dependent increase in asthma and allergy prevalence after immigrating (11eC19). This phenomenon does not appear to be limited to international immigration but has also been described in populations that emigrate from different regions within the same country (20, 21). Although the mechanisms are not fully understood, immigrating populations may become more susceptible to asthma through exposure to new allergens that are ubiquitous in a particular region. The role of environmental exposure is supported by studies showing that the spectrum of allergic reactions and skin-test reactivity in immigrants can change in relation to time of residence and become more similar to the pattern of allergic responses seen in the native population (21, 22). In the Tucson epidemiologic study, adults who immigrated into the Tucson area had a significantly higher prevalence of skin-test reactivity after 8 years of follow-up, compared with Tucson adult natives who were followed up for the same period of time (20). In a study of adolescent immigrants in Australia, there was an 11% increase in the odds of current wheeze for every additional year after immigration; furthermore, the largest increase in respiratory symptom prevalence was observed in the group of subjects arriving from countries with the lowest asthma prevalence; however, in this study there was no association between respiratory symptom prevalence and either atopy or sputum eosinophilia (23).
In contrast, the European Community Respiratory Health Survey (ECRHS) reported more asthma symptoms in immigrants (OR 1.21, 95% CI 1.0eC1.51) and emigrants (OR 1.31, 95% CI 0.96eC1.51) compared with nonimmigrants, and no consistent pattern was observed when migrants moved to and from countries with high or low asthma prevalence and their prevalence of bronchial hyperresponsiveness did not differ; however, the authors acknowledged that the small number of immigrants in this study limited its power. The length of residence in the host country also was not considered (24).
Epidemiologic studies suggest that the environment during early childhood is an important factor for the risk of developing asthma and atopic disorders (25). Approximately 45% of Mexican immigration into the United States is derived from nonurban areas (localities with < 15,000 persons) and from predominantly rural states (26) where higher rates of early childhood infections (27) may confer development of an immunologic response that reduces subsequent risk for developing atopy and asthma (28). However, this early protection may fade over time with prolonged exposure to new environmental factors (11, 21) and with subsequent lifestyle changes (5). In both NHANES III and NHIS, the proportion of Mexican-born participants living in an urban environment was 68% and 55%, respectively; it is therefore possible that exposure to a more polluted outdoor environment may contribute to the increasing asthma prevalence (29) in Mexican immigrants who arrive primarily from less urbanized areas. This possibility is further supported by an Environmental Protection Agency (EPA) study showing that approximately 80% of Hispanics in the United States live in areas failing to meet at least one EPA air-quality standard (30). Also, the air pollution in counties with a predominant Hispanic population could account for some of the disparities in asthma outcomes in this population (31). Although the prevalence of obesity was higher in U.S.-born participants than in Mexican-born participants, this difference did not explain the increased risk for asthma observed in the U.S.-born participants. However, 17% (SE 1.2) of the Mexican-born participants who had resided fewer than 10 years in the United States had a BMI of 30 or greater, compared with 26% (SE 2.1) of those who had lived more than 10 years in the United States (p < 0.002). This increase in obesity may partly explain the higher asthma prevalence in Mexican-born participants with a longer time of residence in the United States. On average, Mexican-born participants with asthma were diagnosed at an older age when compared with U.S.-born participants (34 vs. 21, p < 0.001) and most were diagnosed after immigrating into the United States, which suggests that prolonged exposure to environmental factors occurring after immigration plays an important role in increasing the odds for asthma. However, it is also possible that Mexican-born participants with a longer time of residence in the United States were also more likely to have health care access (32) and therefore of being diagnosed with asthma. In NHANES III, 54% (SE 3.1) of Mexican-born participants with more than 10 years of residence in the United States had a regular source for health care compared with 46% (SE 3.1) of those with fewer than 10 years (p < 0.05). Other unmeasured confounders that could have contributed to the higher odds for asthma in Mexican-born participants with longer periods of residence in the United States may include the following:
Although little is known about whether diagnostic practices differ between health care providers in the United States and Mexico, health care providers in the United States may be more likely to diagnose asthma than their colleagues in Mexico.
Immigrants who obtain legal status are more likely to seek health care. In NHIS, 75% (SE 1.1) of Mexican-born participants with citizenship had a regular source for health care, compared with only 50% (SE 1.1) in those with no citizenship (p < 0.05).
Mexican-born immigrants may have refrained from seeking health care for their asthma because of their legal status; however, if their asthma became severe enough, they may have had to access the health care system.
Several limitations must be considered when interpreting the present authors' results. First, there was a lack of consistency in some of the results across surveys. The association of obesity (BMI 30) with increased odds for asthma was significant in NHIS and not in NHANES III (Tables 3 and 4). It is possible that a larger sample of obese participants in NHIS and differences between BMI determined from self-reported weight and height (NHIS) versus measured BMI could have accounted for the discrepancy in the results (33). Also, the lack of insurance coverage was associated with increased odds for asthma in NHIS and not in NHANES III (Table 4); this finding could partly be explained by the fact that, in NHANES III, insurance coverage was estimated as having coverage in the last month by any of the following: Medicare, Medicaid, CHAMPUS, the Veterans Administration, private, or other, whereas NHIS relied on the more general question, "Do you have insurance" Second, both surveys used questionnaires that rely on self-referred asthma and are limited by a low sensitivity and may therefore not accurately reflect the true asthma prevalence (34). Further, it is possible that U.S.- and Mexican-born Mexicans responded differently in both surveys because of cultural differences; however, as suggested by the multilingual ECRHS, international comparisons are not affected by errors caused by cross-cultural variations in the reporting of symptoms, which supports that a differential measurement error between U.S.- and Mexican-born participants is unlikely (35). Third, changes in the design of NHIS in 1997 (36) may have accounted for the higher prevalence observed in 1998 (Figure 1). Also, the increase trend in asthma prevalence in U.S.-born participants after 1998 should be viewed with caution, particularly because some of the observed variability between sources and across periods may be related to insufficient reliability of the asthma-related questions (37). However, the increase trend in asthma prevalence among the U.S.-born participants is consistent with an increase in asthma prevalence among Hispanics in the Behavior Risk Factor Surveillance System in which the yearly asthma prevalence was as follows: for 1999, 6.8 (95% CI 2.9eC10.7); for 2000, 8.3 (95% CI 7.5, 9.1); for 2001, 9.4 (95% CI 8.5, 10.2); and for 2002, 9.7 (95% CI 8.7, 10.6) (38).
Our study suggests that both place and duration of residency should be considered when interpreting the prevalence of asthma among Mexican Americans. In addition, the increasing rate of asthma in Mexicans after immigration to the United States underlines the importance of lifestyle and environmental exposure in the development of these diseases and offers an opportunity to examine the role of geneeCenvironment interactions.
Acknowledgments
The authors thank Marc Moss, MD, and Roland H. Ingram, MD, for their input and critical review of this manuscript.
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Department of Pediatrics, Emory University School of Medicine, Atlanta, Georgia
Municipal Institute of Medical Research, Barcelona, Spain
Instituto Nacional de Salud Pe瞓lica, Cuernavaca Morelos, Mexico
ABSTRACT
In the United States, among Hispanics, Mexican Americans have the lowest rate of asthma. However, this population includes Mexican Americans born in the United States and in Mexico, and risk factors that might impact the prevalence of asthma differ between these groups. To determine the prevalence of and risk factors for asthma among U.S.- and Mexican-born Mexican Americans, we analyzed data from two U.S. surveys that included 4,574 persons who self-reported their ethnicity as Mexican American from the Third National Health and Nutrition Examination Survey (NHANES III) 1998eC1994 and 12,980 persons who self-reported their ethnicity as Mexican American from National Health Interview Survey (NHIS) 1997eC2001. U.S.-born Mexican Americans were more likely than Mexican-born Mexican Americans to report ever having asthma in both the NHANES III (7% [SE 0.5] vs. 3% [SE 0.3], p < 0.001) and NHIS surveys (8.1% [0.4] vs. 2.5% [0.2], p < 0.001). In a multivariate regression model controlling for multiple demographic variables and health care, the risk for asthma was higher among U.S.-born Mexicans in NHANES III (odds ratio 2.1, 95% confidence interval 1.4eC3.3) and NHIS (odds ratio 2.7, 95% confidence interval 1.6eC5.5). In conclusion, the prevalence of asthma was higher in U.S.-born than in Mexican-born Mexican Americans. This finding highlights the importance of environmental exposures in developing asthma in a migratory population.
Key Words: asthma Mexican Americans migration
In the United States, the prevalence of asthma varies significantly among different ethnic groups, with Mexican Americans having the lowest asthma prevalence among Hispanics (1, 2). Higher rates of respiratory diseaseeCrelated mortality and higher prevalence of chronic medical conditions have also been observed in U.S.-born Hispanics than in foreign-born Hispanics residing in the United States (3, 4). The reason for these disparities are unknown; however, these observations suggest that place of birth and early life exposure to and length of residence in the United States might influence exposure to environmental factors that contribute to the development of chronic health problems (3, 5). The prevalence of asthma among Mexican Americans might therefore differ according to their place of birth and change in lifestyle associated with the acculturation processes over several years of residence in the United States. To test this hypothesis, the present authors compared the prevalence of asthma among U.S.-born Mexican Americans and Mexicans who immigrated to the United States using data from the Third National Health and Nutrition Examination Survey (NHANES III), 1988eC1994, and the National Health Interview Survey (NHIS) for 1997eC2001. Some of the results of this study have been previously reported in the form of an abstract (6).
METHODS
The present authors used data from two large and independent population surveys to determine factors that influence the prevalence of asthma among U.S.- and Mexican-born Mexican Americans. NHANES III, which comprises interview and medical examination data, and the NHIS survey, which comprises interview data only, were used to examine the prevalence of asthma and respiratory symptoms. In both surveys, interviews were conducted in the language spoken in the household (English or Spanish). NHANES III was conducted in two phases from October 1988 through October 1994 (7). NHIS is an annual, national health survey conducted by personal interview in the home throughout the year; for this study, the authors used data compiled from 1997 through 2001 (8). Both surveys use a complex multistage, stratified survey of civilian, noninstitutionalized populations. The authors restricted analyses to survey participants reporting a Mexican origin who were 18 years or older. They calculated all estimates using the sampling weight to provide population estimates that adjusted for unequal probabilities of selection and that accounted for nonresponse.
Questionnaire
The authors obtained information on variables from the NHANES III and NHIS questionnaires and defined them as follows:
Asthma.
From NHANES III, we used the following questions: "Has a doctor ever told you that you had asthma" and "Have you had wheezing or whistling in your chest at any time in the past 12 months" From NHIS, we used the following questions: "Has a doctor or other health professional ever told you that you had asthma" and "Have you had an asthma attack/episode in the last 12 months"
Covariates.
Age, sex, smoking status (current, former, and never), and education (number of years lower than, equivalent to, or greater than high school [< 12, = 12, > 12 years]) were available in both questionnaires. Body mass index (BMI) was defined as weight in kilograms divided by height in meters squared and was categorized as normal (18.5 to < 25), overweight (25 to < 30), and obese ( 30) (9). PovertyeCincome ratio (PIR) was computed as the ratio of the midpoint family income divided by the poverty level in dollars as defined by the Census Bureau for the corresponding survey year (8). Acculturation status was defined by the language used during the survey interview (10). Citizenship status was available only for NHIS, and time since immigration to the United States was available only for NHANES III. We defined health care access as using medical insurance coverage and having a regular source of medical care (4). For having a regular source of medical care, we used the following question in NHANES: "Is there a particular clinic, health center, doctor's office, or other place that you usually go to if you are sick, need advice about your health, or for routine care" In NHIS, the question was "Do you have a place you usually go when sick"
Analysis.
The frequency distribution of specific variables from NHANES III and NHIS were determined among U.S.-born and Mexican-born Mexican Americans. We compared proportions of categorical variables using a 2 test for independence. Univariate and multivariate logistic regression models were performed separately for NHIS and NHANES III to determine the crude and adjusted odds ratios (ORs) of ever having been diagnosed with asthma. Stratified analyses by place of birth were performed to determine the risk factors of asthma diagnosis. Models were adjusted for the following covariates: gender, BMI, smoking status, age, PIR, insurance coverage, regular source of medical care, region of survey, language of interview, and citizenship status (only for NHIS data). We also tested independently in each survey the interactions of specific covariates (BMI, age, gender, smoking, regular access to health care, and PIR) with the exposure (place of birth). The prevalence and OR for asthma in Mexican-born Mexican Americans, according to their time of residence in the United States, were determined using NHANES III. A level of p < 0.05 was considered significant. For the analyses, we used SAS (SAS Institute, Cary, NC) and SUDAAN (SAS Version 8; Research Triangle Institute, Research Triangle Park, NC) to adjust for complex sample design when variance estimates ware calculated.
RESULTS
The analysis comprised 17,554 adults. From NHANES III, 4,574 of 20,050 adults self-reported their ethnicity as Mexican American (22.8%): 2,422 (12.1%) were U.S.-born Mexican Americans, and 2,152 (10.7%) were Mexican-born Mexican Americans. We excluded 732 respondents for the following reasons: 371 were not born in the United States or Mexico and 361 had no medical examination data. From NHIS, we analyzed data from 12,980 persons who self-reported their ethnicity as Mexican American (from 159,376 adults, or 8.1%): 5,860 (3.6%) were U.S.-born and 7,120 (4.5%) were Mexican-born.
In both surveys, Mexican-born participants were significantly younger and had a lower education level and higher PIR than did U.S.-born participants (Tables 1 and 2). Also, Mexican-born participants were on average diagnosed with asthma at an older age when compared with U.S.-born participants (33 years [SE 1.9] vs. 21 years [SE 2.8], p < 0.001). The prevalence of ever having asthma was significantly higher in U.S.-born participants in both NHANES III (7.0% [SE 0.5] and 3.0% [SE 0.3], p < 0.001) and NHIS surveys (8.1% [SE 0.4] ves. 2.5% [SE 0.2], p < 0.001) than in Mexican-born participants. Figure 1 presents the prevalence of asthma for both surveys by year. In the group of excluded participants from NHANES III (n = 732), the prevalence of ever having asthma was 4%, and their exclusion from the analysis did not modify the difference in asthma prevalence among U.S.- and Mexican-born participants in NHANES III.
Among lifestyle risk factors potentially associated with asthma, the prevalence of current smoking was slightly higher in U.S.-born participants. Although more Mexican-born participants were overweight (BMI > 25 and < 30), U.S.-born participants had a significantly higher prevalence of obesity (BMI 30).
A larger proportion of U.S.-born participants had access to health care; however, when we compared the prevalence of asthma diagnosis between U.S.- and Mexican-born participants with health insurance or a regular source for health care, U.S.-born participants had significantly higher asthma prevalence in both NHANES III and NHIS. In NHANES III, the prevalence of asthma among participants with and without a regular source of health care was, respectively, 7.4% and 5.8% for U.S.-born participants and 4.3% and 1.4% for Mexican-born participants (p < 0.001). In NHIS, the prevalence of asthma among participants with and without a regular source of health care was, respectively, 7.4% and 5.8% in U.S.-born participants and 3.5% and 0.7% in Mexican-born participants (p < 0.001). In NHANES III, in the subpopulation with health care insurance, the prevalence of asthma was 7.0% in U.S.-born and 3.7% in Mexican-born participants (p < 0.001). In NHIS, the prevalence of asthma in participants with insurance was also higher in U.S.- compared with Mexican-born participants (5.5% vs. 2.1%, p < 0.001). The prevalence of asthma in uninsured participants remained higher for U.S.- versus Mexican-born participants (6.6% vs. 2.0% in NHANES III and 8.9% vs. 3.1% in NHIS, p < 0.001).
After adjustment for age, smoking status, BMI, region of survey, language, and health care access, U.S.-born Mexican Americans were at higher risk for asthma than were Mexican-born Mexican Americans in NHANES III (odds ratio [OR] 2.12, 95% confidence interval [CI] 1.38eC3.28) and NHIS (OR 2.73, 95% CI 1.63eC5.50). Obesity (BMI 30) also was associated with higher odds for asthma in both U.S.- and Mexican-born participants only in NHIS (Tables 3 and 4).
We did not find significant interactions between place of birth with gender, BMI, smoking status, regular source for medical care, and poverty level in NHANES III or NHIS. The interaction between age and place of birth was significant in both surveys. The results of this interaction are shown in the stratified analysis by place of birth in each survey (Table 4) where the risk for asthma increases with age in Mexican-born participants and decreases in U.S.-born participants.
The average age at immigration to the United States was 21 years (interquartile range 16eC29 years). The prevalence of asthma in Mexican-born participants with fewer than 10 years of residence in the United States was 2.0% (SE 0.5) compared with 4.0% (SE 0.5) in those who had resided more than 10 years in the United States (p < 0.01). After adjusting for age, the OR of having asthma associated with more than 10 years of U.S. residence was 1.60 (0.74eC3.07) compared with residing in the U.S. less than 10 years.
DISCUSSION
U.S.-born Mexican Americans had a higher prevalence of asthma than did Mexican-born Mexican Americans, independent of access to health care and other potential confounders. Also, with prolonged times of residence in the United States, the prevalence of asthma increased in Mexican-born participants, suggesting that either cumulative environmental exposure and/or lifestyle changes after immigration may influence the risk of developing asthma.
The authors' results agree with reports from smaller studies conducted in European countries, Israel, and Australia where foreign-born populations arriving from countries with lower asthma prevalence experience a time-dependent increase in asthma and allergy prevalence after immigrating (11eC19). This phenomenon does not appear to be limited to international immigration but has also been described in populations that emigrate from different regions within the same country (20, 21). Although the mechanisms are not fully understood, immigrating populations may become more susceptible to asthma through exposure to new allergens that are ubiquitous in a particular region. The role of environmental exposure is supported by studies showing that the spectrum of allergic reactions and skin-test reactivity in immigrants can change in relation to time of residence and become more similar to the pattern of allergic responses seen in the native population (21, 22). In the Tucson epidemiologic study, adults who immigrated into the Tucson area had a significantly higher prevalence of skin-test reactivity after 8 years of follow-up, compared with Tucson adult natives who were followed up for the same period of time (20). In a study of adolescent immigrants in Australia, there was an 11% increase in the odds of current wheeze for every additional year after immigration; furthermore, the largest increase in respiratory symptom prevalence was observed in the group of subjects arriving from countries with the lowest asthma prevalence; however, in this study there was no association between respiratory symptom prevalence and either atopy or sputum eosinophilia (23).
In contrast, the European Community Respiratory Health Survey (ECRHS) reported more asthma symptoms in immigrants (OR 1.21, 95% CI 1.0eC1.51) and emigrants (OR 1.31, 95% CI 0.96eC1.51) compared with nonimmigrants, and no consistent pattern was observed when migrants moved to and from countries with high or low asthma prevalence and their prevalence of bronchial hyperresponsiveness did not differ; however, the authors acknowledged that the small number of immigrants in this study limited its power. The length of residence in the host country also was not considered (24).
Epidemiologic studies suggest that the environment during early childhood is an important factor for the risk of developing asthma and atopic disorders (25). Approximately 45% of Mexican immigration into the United States is derived from nonurban areas (localities with < 15,000 persons) and from predominantly rural states (26) where higher rates of early childhood infections (27) may confer development of an immunologic response that reduces subsequent risk for developing atopy and asthma (28). However, this early protection may fade over time with prolonged exposure to new environmental factors (11, 21) and with subsequent lifestyle changes (5). In both NHANES III and NHIS, the proportion of Mexican-born participants living in an urban environment was 68% and 55%, respectively; it is therefore possible that exposure to a more polluted outdoor environment may contribute to the increasing asthma prevalence (29) in Mexican immigrants who arrive primarily from less urbanized areas. This possibility is further supported by an Environmental Protection Agency (EPA) study showing that approximately 80% of Hispanics in the United States live in areas failing to meet at least one EPA air-quality standard (30). Also, the air pollution in counties with a predominant Hispanic population could account for some of the disparities in asthma outcomes in this population (31). Although the prevalence of obesity was higher in U.S.-born participants than in Mexican-born participants, this difference did not explain the increased risk for asthma observed in the U.S.-born participants. However, 17% (SE 1.2) of the Mexican-born participants who had resided fewer than 10 years in the United States had a BMI of 30 or greater, compared with 26% (SE 2.1) of those who had lived more than 10 years in the United States (p < 0.002). This increase in obesity may partly explain the higher asthma prevalence in Mexican-born participants with a longer time of residence in the United States. On average, Mexican-born participants with asthma were diagnosed at an older age when compared with U.S.-born participants (34 vs. 21, p < 0.001) and most were diagnosed after immigrating into the United States, which suggests that prolonged exposure to environmental factors occurring after immigration plays an important role in increasing the odds for asthma. However, it is also possible that Mexican-born participants with a longer time of residence in the United States were also more likely to have health care access (32) and therefore of being diagnosed with asthma. In NHANES III, 54% (SE 3.1) of Mexican-born participants with more than 10 years of residence in the United States had a regular source for health care compared with 46% (SE 3.1) of those with fewer than 10 years (p < 0.05). Other unmeasured confounders that could have contributed to the higher odds for asthma in Mexican-born participants with longer periods of residence in the United States may include the following:
Although little is known about whether diagnostic practices differ between health care providers in the United States and Mexico, health care providers in the United States may be more likely to diagnose asthma than their colleagues in Mexico.
Immigrants who obtain legal status are more likely to seek health care. In NHIS, 75% (SE 1.1) of Mexican-born participants with citizenship had a regular source for health care, compared with only 50% (SE 1.1) in those with no citizenship (p < 0.05).
Mexican-born immigrants may have refrained from seeking health care for their asthma because of their legal status; however, if their asthma became severe enough, they may have had to access the health care system.
Several limitations must be considered when interpreting the present authors' results. First, there was a lack of consistency in some of the results across surveys. The association of obesity (BMI 30) with increased odds for asthma was significant in NHIS and not in NHANES III (Tables 3 and 4). It is possible that a larger sample of obese participants in NHIS and differences between BMI determined from self-reported weight and height (NHIS) versus measured BMI could have accounted for the discrepancy in the results (33). Also, the lack of insurance coverage was associated with increased odds for asthma in NHIS and not in NHANES III (Table 4); this finding could partly be explained by the fact that, in NHANES III, insurance coverage was estimated as having coverage in the last month by any of the following: Medicare, Medicaid, CHAMPUS, the Veterans Administration, private, or other, whereas NHIS relied on the more general question, "Do you have insurance" Second, both surveys used questionnaires that rely on self-referred asthma and are limited by a low sensitivity and may therefore not accurately reflect the true asthma prevalence (34). Further, it is possible that U.S.- and Mexican-born Mexicans responded differently in both surveys because of cultural differences; however, as suggested by the multilingual ECRHS, international comparisons are not affected by errors caused by cross-cultural variations in the reporting of symptoms, which supports that a differential measurement error between U.S.- and Mexican-born participants is unlikely (35). Third, changes in the design of NHIS in 1997 (36) may have accounted for the higher prevalence observed in 1998 (Figure 1). Also, the increase trend in asthma prevalence in U.S.-born participants after 1998 should be viewed with caution, particularly because some of the observed variability between sources and across periods may be related to insufficient reliability of the asthma-related questions (37). However, the increase trend in asthma prevalence among the U.S.-born participants is consistent with an increase in asthma prevalence among Hispanics in the Behavior Risk Factor Surveillance System in which the yearly asthma prevalence was as follows: for 1999, 6.8 (95% CI 2.9eC10.7); for 2000, 8.3 (95% CI 7.5, 9.1); for 2001, 9.4 (95% CI 8.5, 10.2); and for 2002, 9.7 (95% CI 8.7, 10.6) (38).
Our study suggests that both place and duration of residency should be considered when interpreting the prevalence of asthma among Mexican Americans. In addition, the increasing rate of asthma in Mexicans after immigration to the United States underlines the importance of lifestyle and environmental exposure in the development of these diseases and offers an opportunity to examine the role of geneeCenvironment interactions.
Acknowledgments
The authors thank Marc Moss, MD, and Roland H. Ingram, MD, for their input and critical review of this manuscript.
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