Figure 2. Directed acyclic graph (DAG) at visits j = J and j = J+1. We present the directed acyclic graph (DAG) at adjacent visits j = J and j = J+1 to illustrate the relationships we assumed between the variables included in the regression models. AP^i,j represents 1st, 2nd, 3rd, and 4th weeks averages of air pollutant concentration before the j^th visit of the i^th participant. Y^i,j represents the i^th participant gene-specific DNA methylation at visit j. C₁^i,j and C₂^i,j corresponds to the potential confounding variables and risk factors of DNA methylation for participant i^th at visit j, respectively. uⁱ represents the random intercept of participant i.