4 Age (yr) 7–11 12–16 Total Table 2: Detection of skeletal ﬂuorosis (SF) among 370 sampled children according to gender and age, Zhijin, Guizhou, China, 2004. Boys Number examined 78 110 188 SF n (%) 21 (26.9) 49 (44.5) 70 (37.2) Girls Number examined 76 106 182 SF n (%) 18 (23.7) 34 (32.1) 52 (28.6) Total Number examined 154 216 370 SF n (%) 39 (25.3) 83 (33.4)§ 122 (33.0) §Older children (12–16 years) were signiﬁcantly more likely to be diagnosed with skeletal ﬂuorosis than younger children (7–11 years) (OR = 1.84, 95% CI: 1.17-2.90; P = .0082). Table 3: Distribution of type and grade of Skeletal ﬂuorosis (SF) detected among children in a coal-burning endemic ﬂuorosis area, Zhijin, Guizhou, China, 2004. Constrictive type Raritas type Mixed type Soft type Total Mild Moderate Severe Total Boys n (%) Type of SF 46 (37.7) 9 (7.4) 8 (6.6) 7 (5.7) 70 (57.4) Girls n (%) 28 (23.0) 10 (8.2) 12 (9.8) 2 (1.6) 52 (42.6)§ Grade of SF 28 (23.0) 26 (21.3) 34 (27.9) 23 (18.8) 8 (6.6) 3 (2.5) 70 (57.4) 52 (42.6)† Total n (%) 74 (60.7) 19 (15.6) 20 (16.4) 9 (7.4) 122 (100.0) 54 (44.3) 57 (46.7) 11 (9.0) 122 (100.0) §Type of SF does not diﬀer by gender (χ2 = 5.472, df = 3; P = .140). †Grade of SF does not diﬀer by gender (χ2 = 1.854, df = 2; P = .396). Figure 2: X-ray for another 7-year old boy. and 2). The degree of severity did not appear increase with age (data not shown). On average, SF severity was worse (grade was higher) among boys compared to girls; however, severity diﬀerences according to gender were not statistically signiﬁcant (P = .396). In addition, no signiﬁcant linear trend across grade of SF by gender was observed (P = .183). All 370 children, regardless of SF disease status, were screened for DF. A total of 358 children (96.8%) were Journal of Environmental and Public Health diagnosed with some form of DF (Table 4). Boys were slightly more likely to be diagnosed with DF than girls (data not shown). Only 10 (2.7%) children had normal teeth with no signs of ﬂuorosis. Two children were diagnosed as questionable and not included in the DF prevalence estimates. Nearly all (99.2%) of children with SF were also diagnosed with DF, while 95.6% of children without SF showed clinical signs of DF. In our sample, the majority of Zhijin County children had either moderate or severe DF index scores. 4. Discussion This study provides important evidence on the percentage of SF among children and adolescents in southwest China and creates a baseline to assess future endemic ﬂuorosis control eﬀorts in this region. As expected, percentage of SF was higher among older children compared to children under 12 years old, but no gender diﬀerences were observed. Constrictive-type SF was the most common type of SF although raritas, mixed, and soft type were also diagnosed. Nearly half of children with SF were classiﬁed as moderate and 9% suﬀered from severe SF. The strengths of this study include methodological steps taken to ensure that the estimates were valid and representative of children in this area and all SF diagnoses were made consistently. All children attending every one of the compulsory 9-year schools in the region were included in the sampling frame. Data collection occurred during the school year at local schools and coincided with mandatory exams. School attendance rates are at a peak during this time of the school year, helping to ensure high participation rates. To ensure consistent and valid disease diagnosis, a single experienced technician took X-ray photos of all 370 high-risk children and one experienced SF expert analyzed all X-ray photos and made oﬃcial SF diagnoses. The study has several weaknesses or limitations worthy of comment. A survey can only assess disease distribution in a population at one moment in time. SF occurs as a result of cumulative exposure to excess ﬂuoride levels and a minimum threshold of exposure has yet to be determined. Thus, students that showed no X-ray signs of SF during our study could conceivably be diagnosed as mild cases six months later. Furthermore, X-ray diagnosis is expensive and individual diagnosis of the entire study population was not feasible. It is possible that students at low risk for SF, at the time of the study, indeed would have been diagnosed with SF if examined further by X-ray. Thus, our