Income and oral health relationship in Brazil: is there a threshold?

We explored the relationship between income and two oral health outcomes in Brazil, in order to assess the shape of this relationship.

Individual-level data from a national oral health survey were obtained for 22 634 15- to 19-year-old subjects from 330 municipalities. Relationships between income (equivalized household income) and oral health were smoothed using the locally weighted ordinary least squares regression (LOWESS) technique in order to assess the relationship between material circumstances and oral health. We also ranked individuals based on equivalized household income, supplemented by information from total household income, interviewees' earnings, number of cars in the household and years of education, in order to assess the relationship between social position and oral health.

The relationship between oral health and equivalized household income showed a threshold and, assuming causality, income levels higher than R$850 per month did not improve oral health further. The relationship between oral health and social position was linear. Correlations of oral health with the ranking variable (social position) were stronger than with equivalized household income, regardless of the income level, and did not decrease after controlling for income.

The relation of oral health in teenagers with equivalized household income (material circumstances) showed a threshold, but the relation with a ranking variable (social position) was linear. Maybe differences in oral health between individuals are influenced by both their material circumstances (up to a certain level of income) and their social position in relation to others, i.e. social status (at any income level).