Trends in adult current asthma prevalence and contributing risk factors in the United States by state: 2000–2009

Current asthma prevalence among adults in the United States has reached historically high levels. In 2009, 8.4% of U.S. adults (19.5 million persons) reported asthma compared with 7.2% (14.7 million persons) in 2000 [1, 2]. Although national-level estimates indicate that asthma prevalence among adults relatively increased by 33% (4.8 million persons) from 2000 to 2009 (almost 500,000 adults per year) [1, 2], state specific temporal tends in asthma prevalence and their contributing risk factors have not been explored.

To determine the prevalence of current asthma in the United States each year, the Centers for Disease Control and Prevention (CDC) uses national and state-based surveillance systems. Since the late 1970s, the National Health Interview Survey (NHIS), a national-based survey of households from all 50 states and the District of Columbia (D.C.), has provided a national estimate of asthma prevalence by population-based characteristics each year in the United States [3]. However, a systematic state-based survey, the Behavioral Risk Factor Surveillance System (BRFSS), provides state-level estimates of asthma prevalence by population-based characteristics each year.

Estimates of current asthma prevalence from both the national- (NHIS) and the state-based (BRFSS) surveys show that current asthma prevalence among adults is increasing [1, 2, 4‐6] and continuing to vary by certain sociodemographic, behavioral, and geographic factors [7‐12]. More specifically, asthma prevalence can vary by certain modifiable (e.g., weight and smoking status) [13‐25] and non-modifiable (e.g., age and race status) risk factors [7‐11, 26‐30]; however, the extent to which these risk factors contribute to increasing prevalence at the state level over time has not been analyzed or determined. Identifying state-specific changes in asthma prevalence and determining how known risk factors for asthma contribute to changes in prevalence over time are important for public health planning and for generating hypotheses for asthma prevention and control.

From 2000 to 2009, BRFSS collected records on 3,203,280 respondents from the 50 states and D.C. Of these respondents, 159,407 (5.0%) were excluded from the analysis because values for asthma status or sociodemographic factors other than household income, smoking status, or weight status were missing; 3,043,873 respondents were included in the analysis. The average sample size per year from 2000 to 2009 was 304,387 (range: from 174,810 in 2000 to 411,406 in 2007) and per state was 6,000 (range: from 4,621 in WY to 16,112 in WA). Survey response rates varied by year and state; the median state survey response rate for this time period was a little more than 50% (detailed at http://​www.​cdc.​gov/​brfss/​technical_​infodata/​quality.​htm).

Our findings indicate that current asthma prevalence among adults increased significantly from 2000–2001 to 2008–2009 for the majority of states (40/50) and for the U.S. as a whole. Our findings also suggest that place of residence, sociodemographic factors, and modifiable behaviors such as obesity might have contributed to those increases.

We considered population-based sociodemographic factors in the analysis because asthma prevalence is known to vary by certain population subgroups in the United States. Consistent with previous studies [8‐12], asthma prevalence was higher among women than men and among non-Hispanic black than non-Hispanic white persons. Asthma was more prevalent among persons with less than a high school education than among persons with a bachelor degree, and also more prevalent among persons with an annual household income < $15,000 than among persons with a household income ≥ $75,000. Over the last century, the demographic changes for the nation in general reveal a decrease in the men-to-women ratio and an increase in minority populations [35]. These findings, coupled with the pattern of sociodemographic factors among adults with asthma, suggest that the increase in asthma prevalence among adults might be due, in part, to changes in population subgroups. The number of population subgroups with higher proportions of persons with asthma (e.g., women, non-Hispanic black persons) has been increasing at a higher rate than has subgroups with relatively lower proportions of persons with asthma (e.g., men, non-Hispanic white persons). When these changes were accounted for in the analysis, the number of states with a significant linear increasing trends in current asthma prevalence increased from 40 (80%) to 46 (92%) states, including 6 of the10 states (AR, AZ, IA, ND, UT, and WV) and D.C. that had insignificant increasing linear trends when state-specific changes in sociodemographic subgroups were not considered.

In this analysis, the number of states with significant increases in current asthma prevalence did not change when smoking status was added to the model with survey year and sociodemographic factors. It seems that the positive public health effects of tobacco control have become overshadowed by other factors contributing to the increase in current asthma prevalence in the last decade. However, when weight status was considered with survey year, sociodemographic factors, and smoking status, 10 states (AK, IL, KS, ME, WY, AR, AZ, IA, UT, and WV) that had previously shown a significant increase in current asthma prevalence when considering those variables showed no significant increase in prevalence. These findings suggest that the high prevalence of asthma among obese (10.2%, 95% CI 10.0-10.3) and morbidly obese (18.2%, 95% CI 17.7-18.7) persons, coupled with the increasing prevalence of obese persons in the United States [36], could contribute to observed increases in adult asthma prevalence in some states. An analysis comparing two cycles of data from the National Health and Nutrition Examination Survey (NHANES 2001–2002 and 2003–2004) similarly showed that adult asthma prevalence is increasing with greater prevalence among obese and morbidly obese adults [7].

Modifiable factors such as obesity and smoking have been associated with the development of asthma [13‐20, 22‐25], decline in lung function, severity of asthma symptoms, and diminished response to steroid medication [37‐42]. Although the pathophysiologic mechanisms remain unclear, genetic studies indicate that the relationship between asthma and obesity might be explained by independent and combined biologic pathways [42]. An international population-based cohort study that collected DNA samples from 9,167 participants found an independent association between obesity and asthma (OR 2.4, 95% CI 1.7-3.2) and a stronger, combined effect (OR 6.1, 95% CI 2.5-14.4) among nonatopic obese persons [42]. An increased risk of asthma onset and exacerbation has been found also among current or former smokers, especially among women and nonatopic adults [43]. In one study among nonatopic adults, 20% of adult-onset asthma was attributed to current smoking, suggesting that a large proportion of adult-onset asthma could be prevented by smoking cessation [43]. A population-based, nested, case–control study of adults (aged 21–51 years) in Sweden reported an increased risk for adult-onset asthma associated with noninfectious rhinitis that occurred before asthma onset (OR 5.4, 95% CI 4.0-7.2), especially among smoking nonatopic adults (OR 9.1, 95% CI 5.3-15.4), and among persons who smoked before asthma onset (OR 1.5, 95%CI 1.1-21) [44].

The factors accounted for in this analysis as well as other factors that could not be accounted for, such as changes in state public health policies, public and professional awareness of asthma symptoms, air pollution exposures, geographic locations (urban vs rural settings), and occupational exposures, might also be contributing to the observed increase in current asthma prevalence. Although the degree to which public and professional awareness has influenced asthma prevalence estimates could not be determined, the significant independent increase in current asthma prevalence by sociodemographic factors, smoking status, and weight status from this analysis suggests that the contribution might be relatively low, unless increased awareness and reporting of asthma symptoms follow similar patterns of variation. Substantial clinical and epidemiologic evidence suggest that indoor and outdoor air pollutants can increase asthma symptoms [45‐53], and increasing evidence indicates that certain air pollutants can lead to asthma onset [54‐59]. Although fewer studies on new onset asthma have been conducted among adults than children, several cohort studies have found that traffic-related local pollutants can contribute to the onset and manifestation of asthma in adults [55, 58, 59]. Also, a substantial proportion of adult-onset asthma has been attributed to occupational exposures [60‐65]. In a population-based, 10-year prospective study of 6,837 adults (aged 20–44 years at study onset) from 13 countries that participated in the European Community Respiratory Health Surveys (ECRHS and ECRHS II) [66], Kogevinas et al. found large geographic variations in population attributable risk (PAR) for adult-onset asthma due to occupational exposures (PAR range 10-25%); significant excess risk for nurses (relative risk [RR] 2.2, 95% CI 1.3-4.0); and increased risk for participants who experienced an acute symptomatic inhalation event (e.g., fire, mixing cleaning products, or chemical spills) (RR 3.3, 95% CI 1.0-11.1) [61]. Another population-based cohort study of all employed adults aged 25–59 years in Finland from 1986 to 1998 found that a large proportion (400–500 new cases/million persons/year) of adult-onset asthma might have been attributable to occupational factors and, therefore, might have been prevented [64].

The results of this study are based on the analysis of a large representative population-based sample that was obtained by using standardized sampling methods and survey questions for all 50 states and D.C. over a 10-year period. However, three potential sampling frame errors that might have occurred due to nonresponse, noncoverage, and self-reported measures should be considered when interpreting the results. The median response rate for BRFSS each year was ~50%; however, asthma prevalence estimates in this study are similar to estimates in other studies with higher response rates, such as NHIS [67]. Noncoverage may occur because BRFSS does not survey institutionalized adults, the military, or residents without home telephones. We used BRFSS-weighted adjustments to minimize the effect of nonresponse and noncoverage. And, self-reported survey measurements may be less accurate than those based on physical measurements; however, a 1993 review of asthma questionnaires reported a sensitivity of 68% and specificity of 94% when self-reported asthma was compared to a clinical diagnosis of asthma [68].

From 2000 to 2009, current asthma prevalence among adults increased for all 50 states and D.C., with significant increases in 46/50 (92%) states and D.C. After accounting for changes in population-based sociodemographic factors and smoking prevalence, the increasing prevalence of obese and morbidly obese persons in some states is associated with increasing prevalence of asthma, and if causal, could contribute to the increase.

The findings and conclusions in this article are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.