A total of 8663 records were initially retrieved by keywords search on Embase (n=1814), PubMed (n=2341), CNKI (n=1241), the Wan Fang (n=1787), the VIP (n=1398) and CBM (n=79). The second, third, fourth National Oral Health Survey of our country were included as well. After the exclusion of duplicate articles (2136) and irrelevant references (6527) from the pooled database by screening the title and abstracts, a full text review was conducted for 827 studies. In total, 786 articles did not meet the eligibility criteria, in which 86 did not state age clearly or age is not within 8-12, 3 were conducted in quota sampling, 30 studies did not have available data, 638 studies reported irrelevant information, 11 studies did not state survey site, 18 publications repeated survey time and sites. In the end, 41 studies are included in our systematic review and meta-analysis (Fig.1). They comprise 3 national-level, 12 provincial-level, 19 city-level, and 7 county-level studies, which involve 22 provinces, four municipalities and four autonomous regions of the mainland China (Chen et al. 2014, Chen et al. 2004, Chen et al. 2007, Deng et al. 2011, Dong et al. 2011, Fan et al. 2008, Fan et al. 2012, Hu 2008, Huang et al. 2008, Jiang et al. 2010, Li et al. 2004, Li et al. 2012, Li et al. 2018, Liang et al. 2008, Ma &Wei 1998, Ma et al. 2008, Shao et al. 2004, Shen et al. 2007, Shen et al. 2013, Song et al. 2004, Wu et al. 2010, Wu 2010, Xu et al. 2004, Yan et al. 2012, Yao &Wang 2016, Yun et al. 2005, Zhai et al. 2017, Zhang et al. 2017, Zhang et al. 2014, Zhang 2003, Zhao et al. 2017, Zhou et al. 2011, Zhu et al. 2009, Zhu et al. 2006)
The weighted Kappa statistic for examiner consistency during the title and abstract screening was 0.74, and 0.79 in the full-text analysis (Supplementary Table S1). The description of the 41 included studies was summarized in Table 1. Among these, prevalence rates of DF varied from 3.1% to 86.5%. The highest prevalence rate (86.5%) was reported in Yunnan and lowest prevalence rate (3.1%) was reported in Guangxi. Dean method defined by the World Health Organization (WHO) was used as the diagnostic criteria of dental fluorosis in 40 studies and all 41 surveys recruited dentists and trained examiners as investigators. As for quality assessment, the number of positive answers (‘yes’) for the 32 listed items on the Strobe checklist for each study was at least 24 (Supplementary Table S2), which indicating that the quality of the 41 eligible studies was satisfactory.
Prevalence of DF in mainland China
Prevalence of DF over time. The overall estimates of DF prevalence in mainland China was 23.6% (95% CI: 19.3-28.0%, Table 2) using a random-effects model, which involved 433322 participants. A total of 41 studies reported the prevalence of DF during 1995-2020. The prevalence of DF in survey year groups of 1995-1999, 2000-2004, 2005-2009, 2010-2014 and 2015- were 18.8% (95% CI: 4.7%-42.4%), 29.9% (95% CI: 25-34.8%), 16.2% (95% CI: 9.1%-23.3%), 34.3% (95% CI: 25.1-43.5%), 20.5% (95% CI: 8.9-32.1%) respectively. Fig. 2 illustrated an overall slightly ascending trend over time across mainland China, while an apparent reduction in prevalence of DF was observed in 2005-2009 and an apparent increasing in prevalence of DF was observed in 2010-2014.
Prevalence of DF by gender. A total of 17 articles reported prevalence of DF estimates by gender, yielding an overall prevalence of DF in boys and girls were 15.7% (95% CI: 11.9%-19.5%) and 15.2% (95% CI: 11.6%-18.8%) respectively. The final pooled meta-analysis indicated no statistically significant difference in the prevalence of DF between the sexes (RR=1.05, 95% CI: 1.02-1.07, Fig.3).
Prevalence of DF by area. A total of 13 articles reported DF prevalence in children aged 8-12 from both urban and rural. The pooled prevalence of DF in rural and urban China was 14.5% (95% CI: 10.6-18.3%) and 12.7% (95% CI: 10.2-15.2%) respectively. Furthermore, higher prevalence estimates of DF were found in rural, while lower prevalence in urban (RR=0.93, 95% CI: 0.76-1.13, Fig. 4).
Prevalence of DF by water improvement. A total of 7 articles reported DF prevalence in children aged 8-12 years from both water improvement before (WIB) and water improvement after (WIA). The pooled prevalence of DF from WIB and WIA was 63.3% (95% CI: 57-69.6%) and 34.7% (95% CI: 31.3-38.1%) respectively. Furthermore, the prevalence of DF for WIB was significantly higher than that for WIA (RR=1.76, 95% CI: 1.61-1.93, Fig. 5).
Geographical distribution of DF prevalence in mainland China
Figure 6 demonstrates a color-coded map of the distribution of the dental fluorosis in mainland China (data available from most provinces, except Tibet). Five different color distribution areas were created on the map based on the prevalence of DF. And significant variation was observed for geographical region across provinces. The highest prevalence zone showed on the map in the darkest red, including Tianjin (39.9%), Chongqing (41.8%) and Jiangsu (42.5%). And the prevalence of DF in Guangdong (36.5%) and Inner Mongolia (33.0%) was substantial high, as well as in Qinghai, Hebei, Shandong. Other provinces show low prevalence relatively.