Are Frequent Asthma Symptoms Among Low-Income Individuals Related to Heavy Traffic Near Homes, Vulnerabilities, or Both?
Introduction
Asthma is one of the most common chronic health conditions in the United States. In 2005, 32.6 million Americans (11.2% of the population) were diagnosed with asthma (1). Asthma symptoms can be controlled effectively with clinical treatment and avoidance of environmental triggers. In the past two decades, major advances in the development of anti-inflammatory medications for asthma were made. However, many Americans are still suffering from daily/weekly symptoms, which we define herein as frequent asthma symptoms. In California, more than 620,000 adults (nearly one in every four with asthma) and 124,000 children (one in every ten with asthma) experienced frequent asthma symptoms in 2001 (2). People of low income disproportionately suffer from frequent asthma symptoms. Among Californian adults with asthma, those in poverty (with a family income below the federal poverty level [FPL]) were almost twice as likely to experience frequent asthma symptoms as those with incomes at or above three times the FPL (34.2% and 18.6%, respectively) (2). The extra burden of asthma among populations with low socioeconomic status has also been documented in other studies 3, 4, 5, 6.
In developed countries, traffic exhaust is one of the most important sources of outdoor air pollution. Truck, car, bus, and other vehicle emissions produce a complex mixture of toxic chemicals (e.g. benzene, particulate matter) and a variety of irritant gases, including nitrogen dioxide (NO2), sulfur dioxide and ozone (O3) (7). In California, the increase in vehicle miles traveled (VMT) on California roadways (97%) outpaced population growth (40%) between 1982 and 2001 (8). Previous studies reported associations between proximity to heavy traffic and frequent asthma symptoms 9, 10, 11. It has also been reported that environmental risks are inversely correlated with income (12). For example, Californian block groups in the lowest quartile of median family income were three times more likely to be high-traffic areas than were block groups in the highest income quartile (13). A study also showed that minority and high-poverty neighborhoods bore over two times the levels of traffic density compared with the rest of the Southern California region (14). High prevalence of severe asthma among low-income groups may also reflect differences in access to care, such as lack of insurance, and presence of other risk factors, such as smoking, second-hand smoking, or obesity 15, 16, 17, 18, 19, 20.
However, there is limited information regarding whether the higher prevalence of frequent asthma symptoms among low-income individuals is related to higher exposure to air pollutants, greater vulnerability because of poverty, and associated factors such as compromised health status, poor access to care, and behavioral risk factors, or to a combination of these factors 21, 22. As the largest state-level survey in the nation, the California Health Interview Survey (CHIS) provides a unique opportunity to examine the interactions among these factors. Using data from Los Angeles and San Diego County respondents of CHIS 2001 and traffic count data from the California Department of Transportation (Caltrans), we addressed the following questions: (1) Do people in poverty have higher exposures to vehicle-related air pollution (as measured by traffic density near homes) and/or greater vulnerabilities (as measured by overall health status, being obese or overweight, smoking status, insurance status, delays in care, and employment status)? (2) Is the higher prevalence of frequent asthma symptoms among those in poverty related to greater traffic exposures, vulnerabilities, or both? (3) Are individuals in poverty likely to suffer greater health effects from traffic exposures due to a combination of greater exposure and greater vulnerability?
Section snippets
Methods
Eligible study participants were respondents from whom health data were collected between November 2000 and September 2001 as part of CHIS 2001, who (1) resided in Los Angeles or San Diego Counties during this period and (2) reported ever having been diagnosed with asthma by a physician. CHIS is a two-stage, geographically stratified random-digit-dial (RDD) telephone survey that has been conducted biannually in California since 2001. CHIS 2001 included adults, adolescents, and children from
Results
Table 1 lists the distribution of CHIS respondents with asthma by sociodemographic characteristics, traffic exposures, and vulnerabilities. The prevalence of frequent asthma symptoms was 26.0% among those in poverty versus 15.9% among those above the FPL. Asthmatic respondents below the FPL were more likely to be children and adolescents (ages 1–17), women, and Latino; they were also more likely to live near heavy traffic, to be unemployed, uninsured, and in overall fair or poor health.
Discussion
Our results suggest that the higher prevalence of frequent asthma symptoms among low-income individuals is related to both higher traffic exposures and increased susceptibility to environmental challenges by virtue of differences in underlying health status and access to medical care. Furthermore, those in poverty appeared to be more affected by heavy traffic near homes than those above the poverty level, although the strength of this conclusion is limited by a lack of statistical power to
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