Smoking Trajectories among Koreans in Seoul and California: Exemplifying a Common Error in Age Parameterization

Immigration to a nation with a stronger anti-smoking environment has been hypothesized to make smokingless common. However, little is known about how environments influence risk of smoking across the lifecourse.Research suggested a linear decline in smoking over the lifecourse but these associations, in fact, might not belinear. This study assessed the possible nonlinear associations between age and smoking and examined how theseassociations differed by environment through comparing Koreans in Seoul, South Korea and Korean Americansin California, United States. Data were drawn from population based telephone surveys of Korean adults in Seoul(N=500) and California (N=2,830) from 2001-2002. Locally weighted scatterplot smoothing (lowess) was used toapproximate the association between age and smoking with multivariable spline logistic regressions, includingadjustment for confounds used to draw population inferences. Smoking differed across the lifecourse betweenKorean and Korean American men. The association between age and smoking peaked around 35 years amongKorean and Korean American men. From 18 to 35 the probability of smoking was 57% higher (95%CI, 40 to71) among Korean men versus 8% (95%CI, 3 to 19) higher among Korean American men. A similar differencein age after 35, from 40 to 57 years of age, was associated with a 2% (95%CI, 0 to 10) and 20% (95%CI, 16to 25) lower probability of smoking among Korean and Korean American men. A nonlinear pattern was alsoobserved among Korean American women. Social role transitions provide plausible explanations for the declinein smoking after 35. Investigators should be mindful of nonlinearities in age when attempting to understandtobacco use.