Interaction of Smoking, Socioeconomic Status Defines Respiratory Disease Risk

Risk of respiratory diseases in adults in high income countries may be defined by the interaction of socioeconomic status (SES) and smoking, according to study findings published in Respiratory Medicine.

Although both smoking and SES are known to independently affect airway diseases, little is known about the interaction of these 2 factors in respiratory diseases. Moreover, the definitions of SES is often inconsistent between studies. Investigators in Sweden therefore chose to assess whether and to what extent smoking and social class interact relative to risk of respiratory diseases in adults in high income countries. SES was defined by a combination of occupation and educational level.

The researchers conducted a retrospective study using data from 2 population-based cross-sectional studies: the West Sweden Asthma Study (WSAS, n=23,753) and Obstructive Lung Disease in Northern Sweden study (OLIN, n=6,519). Both studies included randomly selected adults 20 to 75 years of age. The probability for the interaction between socioeconomic status and smoking in relation to respiratory outcomes was estimated using Bayesian network analysis.

Participants from both studies completed a modified version of a British Medical Research Council questionnaire (50.1% response rate for WSAS; 56.4% response rate for OLIN). The email questionnaire included questions on demographic characteristics, social characteristics including education, and respiratory symptoms.

Respondents were categorized by smoking status as either “former smokers” (those who quit at least 12 months prior to completing the questionnaire), “current smokers” (including those who smoked within the 12 months prior to completing the questionnaire), or “never smokers.” The researchers evaluated SES based occupation/job titles, using 2 Swedish SES classification systems (the SEI, published in 1982, and the SSYK, published in 2012) that collectively allowed researchers to take into both job titles and the years of education associated with those job titles.

In the combined study sample (N=30,123; 54% female), those with current asthma vs those without asthma were more likely to be female (60.7%), highly educated, and have a high prevalence of comorbidities. Respondents with non-allergic asthma were mostly non-manual workers (19.5%) and manual service workers with high educational attainment. Allergic asthma respondents were mostly manual service workers (20.8%) with high home smoking exposure (23.3%) and high hereditary lung diseases (97%).

The researchers also did separate analyses of the independent association of various outcomes with SES and smoking. In the independent analysis of smoking, researchers found that the probability of having non-allergic asthma and chronic bronchitis and/or chronic obstructive pulmonary disease (COPD) was higher among current smokers vs never smokers. The probability of having current asthma, allergic asthma, non-allergic asthma, and chronic bronchitis and/or COPD was higher among former smokers vs never smokers.

In the independent analysis of SES, the researchers found that people in manual jobs and intermediate non-manual occupations were more likely to have allergic asthma and non-allergic asthma vs professionals and executives. No association was found between educational level and the probability of COPD and/or chronic bronchitis. Those with higher education had higher probability of having current asthma and allergic asthma, but lower probability of non-allergic asthma vs those with a low education level.

The investigators found the association between smoking and the probability of allergic and non-allergic asthma was modified by educational and occupational SES. Former smokers who were manual workers in service and intermediate non-manual employees had higher probability of allergic asthma compared with executives and professionals. Former smokers among executives and professionals had higher probability of non-allergic asthma than those with manual occupations, those who worked in homes, and those with a primary level of education only.

Former smokers with primary-level education had higher probability of non-allergic asthma compared with individuals with secondary- and tertiary-level education. The researchers noted allergic asthma due to former smoking was higher among highly educated individuals compared with less educated individuals.

Study limitations include the cross-sectional design, the use of retrospective, self-reported data, and the use of allergic rhinitis as a marker for asthma sensitivity.

“Beyond their independent roles, socioeconomic status and smoking interact in defining the risk of respiratory diseases. Clearer understanding of this interaction can help to identify population subgroups at most need of public health interventions,” the investigators concluded.