4.1. Study strengths and limitations
A full model with main effects and an interaction between air
pollution and indicator variables for each of the stratified income
and education levels-defining lowest income and lowest education
as the reference groups, respectively-provided p-values that
enabled us to test the significance of differences between high and
low SES directly. We found significant interaction terms between
parental income and education levels and air pollutants and traffic
volume as a result of these tests. However, the interaction terms between the factors other than SES and air pollution and traffic
were not significant (p > 0.05).