Background: Dental caries is the most prevalent lifestyle-related disease worldwide for children. A low socio-economic status (SES) is significantly associated with a higher risk of having dental caries. Since children spend most of their day at schools and are susceptible to the school context, oral health promotion activities in schools are recommended. Tooth-brushing after school lunch, which is one of the oral health programs in school, may help prevent dental caries regardless of their SES level.

Objectives: The purpose of this study was to examine the association between dental caries, SES and the time of tooth-brushing after school lunch among elementary school students.

Methods: The cross-sectional study was conducted of 43 public elementary schools under the jurisdiction of two education board offices in Okinawa, Japan in 2018. The participants were 1,248 students of fifth graders who had obtained consent from their parents or caregivers. The students' dental caries experiences were extracted from personal dental records of medical checkup in the schools. The indicator of SES was the financial assistance for education by the municipalities. The data of dental caries and SES was provided by the schools. We confirmed with school nurse teachers at each school whether tooth-brushing after school lunch was conducted or not. In addition, a self-administered anonymous questionnaire was conducted for students to obtain information on socio-demographics, lifestyles, and family relationships. Using multi-level logistic regression models, the association of SES and tooth-brushing after school lunch with dental caries were analyzed. In that case, SES, socio-demographic variables, and lifestyles were used as the individual-level variables; tooth-brushing after school lunch was used as the school-level variable.

Results: Students with lower SES were more likely to experience dental caries than those with higher SES (OR: 1.7, 95%CI: 1.24-2.38). Students in the schools not carrying out tooth-brushing after school lunch were more likely to experience dental caries than those in the schools doing it (OR: 1.8, 95%CI: 1.05-3.02). Adjusted for the individual-level and school-level variables, these findings were in the same direction.

Conclusions: This study showed that dental caries of students were associated with SES. Additionally, toothbrushing at schools may contribute to the prevention of caries in all students attending schools.