Asthma is a chronic lung disease that inflames and narrows the airways. Asthma affects nearly 26 million people, including 7.0 million children [1] who may experience recurring episodes of wheezing, chest tightness, shortness of breath, and coughing. These recurring episodes cause sleeplessness, daytime fatigue, reduced activity levels, school and work absenteeism, and increased health care use [1,2]. These, in turn, increase the economic burden on patients and society [3]. Although asthma cannot be cured, with effective asthma care and management, most persons with asthma can be free of symptoms and have a better quality of life. One of the key components of effective asthma care and management is assessing asthma severity to select appropriate therapy and to monitor the health and well-being of people with asthma [4,5].

Although the causes of asthma and the determinants of asthma severity remain poorly understood, multiple environmental and genetic factors play an important role in the development of asthma and the exacerbation of asthma symptoms [6–8]. Asthma severity is the inherent intensity of the disease process while asthma control refers to the degree that symptoms are successfully managed. Disease progression and symptoms can vary among individuals and within an individual’s experience over time. Although we do not know the factors that determine these variations and the underlying asthma severity, knowing asthma severity status is important to determining the type of treatment, its duration, and its effects on the level of asthma control that minimizes the frequency and intensity of symptoms and functional limitations [4,5].

Assessing the health and economic burden of the disease requires that one take both asthma severity and control into account. Several population-based studies have assessed the level of asthma control and examined the factors associated with asthma control [9–12]. However, to the best of our knowledge, limited population-based studies attempt to assess asthma severity [13,14]. One major issue in classifying asthma severity after treatment has started is that the severity of disease is most accurately assessed in patients before the initiation of long-term control medication and once a therapy has been started, the focus is on monitoring the level of control, not the level of severity [4]. Many respondents are already receiving long-term control medication at the time of the survey, further complicating the classification of asthma severity. Other issues impeding the population-based assessment of asthma severity is the varying measures used to classify severity, and the confusion between the underlying severity of disease and the current level of symptom control [15,16].

To evaluate population-based asthma severity, Fuhlbrigge and colleagues [13] defined asthma severity using the NAEPP 2003 guidelines. They showed that 77.3% of the study population (both children and adults with asthma) had moderate to severe persistent asthma. Colice and colleagues [14] used the NAEPP 2007 guidelines (EPR-3) to define asthma severity by using measures for current impairments and risk for future exacerbations among adolescents (aged 12–17 years) and adults (aged ≥18 years). They showed that 79% of patients had persistent asthma. For this study, we used the NAEPP 2007 guidelines for asthma severity classification for clinical research and population-based evaluations—that is, severity can be inferred from the least amount of treatment required to maintain asthma control [4]. We assessed asthma severity and determined the factors associated with it among children and adults with asthma in the states that participated in the Centers for Disease Control and Prevention’s Behavioral Risk Factor Surveillance System (BRFSS) Asthma Call-back Survey (ACBS).

The purpose of this population-based study was to assess asthma severity and identify related potential risk factors among children and adults with current asthma who participated in the ACBS in the years 2006 through 2010. Similar to the findings of previous studies [13,14], persistent asthma prevalence was high among this study population (63.8%); the rate was 60.3% among children and 64.8% among adults with current asthma. However, the rate was not as high as that reported by Fuhlbrigge et al. 2002 (77.3%) [13] and Colice et al. 2012 (79%) [14]. The difference in the rates reported by those studies could be attributable to differences in study methods and the population characteristics. In addition, our analysis finds that 42.5% of those with current asthma were on long-term control medications (higher for children [46%] than adults [41%]), and 47.5% had uncontrolled asthma (higher for adults [50.0%] than children [38.4%]).

Multiple extraneous factors (e.g. inadequate treatment, non-adherence to treatment regimens, reduced responsiveness to therapy, uncontrolled environmental triggers and irritants, and comorbid conditions) could contribute to these findings [6,9,21–25]. However, many of these factors were either not examined because of a lack of data or else examined but did not show an association. In this study, we were able to identify some of the predictors of persistent asthma. Age was a strong predictor of asthma severity for both children and adults. Persistent asthma was more prevalent among children aged 0–4 years and among adults aged 45 years or older. Children, especially preschoolers, are at greater risk of developing illness because of developing immune system, hand-to-mouth behavior, lower body weight and higher intake rate, resulting in a greater dose of hazardous substance per unit of body weight, and dependence on others to meet basic needs such as housing, medical care and protection from hazardous environmental conditions [26]. These characteristics make children prone to frequent upper respiratory infections (common cold, ear infections, and epiglottitis) and make them susceptible to environmental allergens and irritants that may lead to exacerbation of asthma symptoms [24,26]. Although we did not observe the association, asthma duration (time since diagnosed) is associated with having persistent asthma (lower lung function, greater airway responsiveness, more asthma symptoms, and greater use of quick-relief asthma medications) among children [27,29].

As with previous studies [21,22,25,28,29], our study indicates that factors associated with persistent asthma among adults were being 45 years or older, having a household income of less than $15 000, older age at first diagnosis (aged 55 years or older), smoking and also exposure to SHS, and having COPD. However, for both children and adults, none of the known environmental risk factors for symptom exacerbations (cockroaches, mold, pets, and SHS [children only]) were significantly associated with persistent asthma [5,6]. Ours is not an unusual result, since a study of the same population at different time periods or a study of different populations can produce different results [30] and also because of social desirability bias associated with self-report surveys that is over-reporting desirable behaviors (exercise, not smoking) or under-reporting less desirable behaviors (smoking, mold, and cockroaches) [31]. Additional studies can be helpful in identifying additional modifiable predictors of uncontrolled asthma and determining contributing factors for persistent asthma. These efforts can lead to developing targeted interventions, producing strategies that reduce the health risks and economic impact of asthma and improving the health and well-being of people with asthma.

The strength of this study is that we analyzed a large sample size of children and adults with current asthma and assessed asthma severity status and symptom control in states that participated in the ACBS; the ACBS is the only survey providing most of the indicators that allow classification of asthma severity and asthma control by use of the NAEPP guidelines (EPR-3) and can be used to evaluate population-based asthma severity and symptom control.

There are also limitations to our study. One limitation is that the indicators available in the ACBS which can be used to classify asthma control and severity circumscribed our findings. Because of the content of the ACBS questionnaire, we were unable to include all of the required elements in the NAEPP guidelines (e.g. activity limitation, pulmonary function measures, asthma exacerbations that require oral corticosteroid and lung growth status in children) [4]. This limitation may lead to underestimation of persistence asthma prevalence. However, the definition of asthma control and severity in this analysis is consistent with the definition in other reports of population-based estimates [9,13,14]. Another limitation is that the ACBS response rates for the participating states were around 50%. Low response rates may affect the results by introducing non-response bias, if survey respondents differed from non-responders on the characteristics studied. However, the BRFSS sampling procedures and varying response rates among states over the 5-year study period can minimize non-response effects on the results [20,30]. In addition, because of the cross-sectional nature of the survey data, we could not generally determine the temporal sequence or causality. Finally, the findings cannot be generalized to the people with current asthma in states that did not participate in the ACBS.

Our findings indicate that nearly two-third of children and adults with current asthma had persistent asthma. Moreover, despite national guidelines for asthma control and available advanced medical treatments, 38% of children and 50% of adults had uncontrolled asthma. Age (being aged 0–4 or 45 years or older), having a household income of less than $15 000, the age at which asthma is initially diagnosed (if aged 55 years or older), smoking and exposure to SHS, and COPD were significantly associated with having persistent asthma. There is a need for further studies to identify additional modifiable contributing factors for persistent asthma, because asthma severity determines the level of asthma control and responsiveness to therapy. Such additional studies can lead to development of targeted interventions or strategies that will reduce modifiable predictors of increased asthma severity and poor asthma control and, in turn, improve asthma care and quality of life.