Introduction Income inequality has detrimental effects on health and oral health. However, this effect is doubtfully instantaneous and most studies have measured both income inequality and health contemporaneously. This study examined the association between state-level income inequality and tooth loss among adults in the United States, under different assumptions about lag periods.

Methods This study pooled individual data from the 2008 Behavioural and Risk Factor Surveillance System and state-level data from the US Census Bureau. The Behavioural and Risk Factor Surveillance System collected information on demographic characteristics (age, sex, race/ethnicity and marital status), socioeconomic position (education and annual household income) and tooth loss, which was self-reported on a 4-point scale (none, 1 to 5, 6 or more but not all, and all teeth). Income inequality at state level was measured with the Gini coefficient, based on household income for 1979, 1989 and 1999, respectively. Two-level ordered logit models with individuals nested within states were used to test the association of state Gini coefficient at different lag periods with tooth loss after adjustment for state median household income and individuals' characteristics.

Results In the fully adjusted models, state Gini coefficient was significantly associated with self-reported tooth loss when testing lag periods of 10- (OR: 1.18, 95% CI 1.05 to 1.32) and 20 years (OR 1.16, 95% CI 1.02 to 1.32). However, no association was found when testing the 30-year lag period (OR 1.12, 95% CI 0.92 to 1.35).

Conclusion This study suggests that state income inequality has stronger effects on self-reported tooth loss up to 20 years later.