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Secondhand smoke and incidence of dental caries in deciduous teeth among children in Japan: population based retrospective cohort study

BMJ 2015; 351 doi: (Published 21 October 2015) Cite this as: BMJ 2015;351:h5397
  1. Shiro Tanaka, associate professor,
  2. Maki Shinzawa, assistant professor,
  3. Hironobu Tokumasu, student,
  4. Kahori Seto, assistant professor,
  5. Sachiko Tanaka, lecturer,
  6. Koji Kawakami, professor
  1. 1Department of Pharmacoepidemiology, Graduate School of Medicine and Public Health, Kyoto University, Yoshida-Konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan
  1. Correspondence to: K Kawakami kawakami.koji.4e{at}
  • Accepted 22 September 2015


Study question Does maternal smoking during pregnancy and exposure of infants to tobacco smoke at age 4 months increase the risk of caries in deciduous teeth?

Methods Population based retrospective cohort study of 76 920 children born between 2004 and 2010 in Kobe City, Japan who received municipal health check-ups at birth, 4, 9, and 18 months, and 3 years and had information on household smoking status at age 4 months and records of dental examinations at age 18 months and 3 years. Smoking during pregnancy and exposure of infants to secondhand smoke at age 4 months was assessed by standardised parent reported questionnaires. The main outcome measure was the incidence of caries in deciduous teeth, defined as at least one decayed, missing, or filled tooth assessed by qualified dentists without radiographs. Cox regression was used to estimate hazard ratios of exposure to secondhand smoke compared with having no smoker in the family after propensity score adjustment for clinical and lifestyle characteristics.

Study answer and limitations Prevalence of household smoking among the 76 920 children was 55.3% (n=42 525), and 6.8% (n=5268) had evidence of exposure to tobacco smoke. A total of 12 729 incidents of dental caries were observed and most were decayed teeth (3 year follow-up rate 91.9%). The risk of caries at age 3 years was 14.0% (no smoker in family), 20.0% (smoking in household but without evidence of exposure to tobacco smoke), and 27.6% (exposure to tobacco smoke). The propensity score adjusted hazard ratios of the two exposure groups compared with having no smoker in the family were 1.46 (95% confidence interval 1.40 to 1.52) and 2.14 (1.99 to 2.29), respectively. The propensity score adjusted hazard ratio between maternal smoking during pregnancy and having no smoker in the family was 1.10 (0.97 to 1.25).

What this study adds Exposure to tobacco smoke at 4 months of age was associated with an approximately twofold increased risk of caries, and the risk of caries was also increased among those exposed to household smoking, by 1.5-fold, whereas the effect of maternal smoking during pregnancy was not statistically significant.

Funding, competing interests, data sharing This study was supported by a grant in aid for scientific research 26860415. The authors have no competing interests or additional data to share.


  • We thank the Child and Family Bureau and Public Health and Welfare Bureau of Kobe City for providing the health check-up data and advice; C Wilunda and C Hongyan (Kyoto University) for their advice; and K Fujii (Kyoto University) for secretarial assistance.

  • Contributors: ShT performed statistical analysis and had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. MS, HT, and KK contributed to the design and conduct of the study. SK and SaT contributed to the writing of the manuscript. KK is the principal investigator and the guarantor of the study. The sponsor of the study had no role in the study design, data collection, data analysis, data interpretation, or writing of the report.

  • Funding: This study was supported by a grant in aid for scientific research 26860415.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at and declare: no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Ethical approval: This study was exempt from obtaining individual informed consent based on the Ethical Guidelines for Epidemiological Research by Ministry of Health, Labour, and Welfare. The study protocol was approved by the Ethics Committee, Kyoto University Graduate School and Faulty of Medicine (E2045). We managed the data based on the Act of Personal Information Protection in Kobe City and take responsibility for their integrity.

  • Data sharing: No additional data available.

  • Transparency: The lead author (KK) affirms that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.

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