There is no universally accepted definition for SES,30 and the studies used various criteria. SES is most commonly stratified by occupation, educational level, income, and residential area (measured as the area deprivation index). For studies involving children, the criteria included parental income, occupation, and educational level. Regardless of how SES is defined, most studies have demonstrated a positive correlation between higher SES and pulmonary function. The relationship remains significant even when adjusting for various confounders including anthropometric variables (height, weight, and body mass index), age, race, sex, smoking status, and respiratory illness. The magnitude of the effect of SES on lung function is variable. In the largest study to date, Wheeler and Ben-Shlomo10 evaluated the effects of SES, determined by occupation, on lung function in 32,905 subjects. FEV1 in the lower SES group was 2.7% lower than that in the higher SES group after correcting for height, age, smoking status, and respiratory illness. The effect of SES was larger than the effect of poor air quality (1.9 to 2.3%) but was smaller than the effect of active smoking (5.2 to 6.4%). Prescott et al,17 evaluated the effect of adult SES, based on education and household income, and adjusted for age, height, and smoking status, and found reductions in FEV1 (men, 363 mL; women, 221 mL) and FVC (men, 342 mL; women, 221 mL) between the groups with the lowest and highest SES.