Journal of Environmental and Public Health 5 Table 4: Prevalence of Dental Fluorosis (DF) and distribution of Dean’s Fluorosis Index Scores among 370 sampled children in a coal- burning endemic ﬂuorosis area, Zhijin, Guizhou, China, 2004. Dean’s Fluorosis Index Scores Skeletal Fluorosis Status Questionable Very mild n 122 248 370 Normal n (%) 0 10 (2.7) 10 (2.7) n (%) 1 (0.3) 1 (0.3) 2 (0.5) n (%) 3 (0.8) 4 (1.1) 7 (1.9) Mild n (%) 9 (2.4) 32 (8.7) 41 (11.1) Moderate n (%) 71 (19.2) 130 (35.1) 201 (54.3) Severe n (%) 38 (10.3) 71 (19.2) 109 (29.5) Prevalence§ of DF (%) 99.2 95.6 96.8 Yes No Total §Data do not include questionable DF cases. estimate for the entire study population is likely to be lower than the true prevalence among primary school children in Zhijin County. SF diagnoses, type identiﬁcations, and grade classiﬁcations were assigned by one SF expert. While this design feature ensures that all children were diagnosed under the same study criteria, while repeatability can be assessed, any investigator bias will likely be included in the reported results. Also worthy of mention is that disease grade or severity was assessed on a qualitative ordinal scale of mild, moderate, or severe. These categories are open to diagnostic interpretation when SF is subclinical or asymptomatic and participant interpretation when disease is clinical. The degree to which grade misclassiﬁcation occurred is unknown but expected to be minimal. This study did not assess the source of ﬂuoride expo- sure among residents of Zhijin County. Recent studies in literature have assessed the source of ﬂuoride exposure in the southwestern provinces of China and concluded that the majority of ﬂuoride exposure occurs as a result of indoor coal burning [1, 4, 6, 18, 19]. Some regions in southwestern China burn pure high ﬂuoride coal, while in other regions the ﬂuoride levels in coal are compounded by high ﬂuoride levels in clay used as a binder for coal briquettes [1, 6]. In 2004, Dai et al. showed that most coals from western Guizhou Province were actually low ﬂuoride coal, with a ﬂuoride concentration of less than 200 ppm, and reported that average ﬂuoride content of coal in Zhijin county was 89 ppm [19]. However, residents commonly used yellow clay that contained high ﬂuoride content of 324 ppm as a coal- burning additive in furnaces or as a binder in briquette making [19]. The daily primary fuel among residents in the present study is a mixture of coal and clay. Other residents may use solid coal, but many still moisten and grind coal into a paste and subsequently mix it with clay to form briquettes for combustion [18]. Ando et al. considered how ﬂuoride levels in coals from Sichuan Province could result in overexposure to ﬂuoride and later cause ﬂuorosis among local residents [4]. Upon burning, gaseous ﬂuoride and particulate dust-containing ﬂuoride are released from coal briquettes. Ando et al. estimated that only 2% of ﬂuoride exposure was due to inhalation of indoor air in coal- burning households, while 97% of ﬂuoride exposure was due to ingestion of food dried and stored inside the household [4]. Residents of Zhij in in Guizhou Province cook foodstuﬀs in the same fashion as residents of Sichuan Province. Corn and potatoes are the primary food consumed by residents of Zhij in. Corn is hung over open style coal-burning stoves to dry. Wet corn absorbs ﬂuoride volatilized by burning coal [4, 18]. Ando et al. measured ﬂuoride content in corn dried over coal- burning stoves. Daily mean ﬂuoride intake (per person) from corn was 37.6 mg and 3.5 mg from potatoes [4]. More importantly, future research should investigate the level of ﬂuoride intake necessary for the development of skeletal ﬂuorosis in pediatric populations. The two other sources of overexposure to ﬂuoride in China are tea drinking and drinking water. Brick tea drinking is not a customary practice among children in this region of China, so ﬂuoride exposure from this source is unlikely. Results from previous investigations have ruled out contami- nated drinking water as the main source of endemic ﬂuorosis in Zhijin County [4]. One recent study assessed ﬂuoride levels in the drinking water of the two most severe endemic ﬂuorosis villages in Zhijin County, Hehua and Hualuo. Average ﬂuoride levels in the villages’ drinking water is well below the World Health Organization (WHO) guideline value for drinking water [19, 20]. While drinking water levels may not be the main source of ﬂuoride exposure, they should be considered in eﬀorts to reduce ﬂuoride exposure below a certain threshold. Endemic ﬂuorosis develops in response to total ﬂuoride intake. Thus, exposure to all sources of ﬂuoride should be assessed if control measures are to be successful. Two other studies have reported average ﬂuoride intake per day and skeletal radiographic appearances among chil- dren and adolescents with SF in Guizhou province; however, both studies conducted in the late 1980s were small (less than 70 participants) and none of them investigated SF prevalence [17, 21]. Only one large study in China has reported SF prevalence among children. In 1998, Ando et al. investigated SF prevalence among residents of a neighboring coal- burning endemic ﬂuorosis area, Pengsui County in Sichuan Province [4]. However, due to the lapse of time between studies (almost 10 years) and diﬀerences in sampling and SF diagnosis criteria, all comparisons between children in the two provinces should be speculative. The present study assessed SF in a high-risk juvenile population, while the Sichuan Province study assessed a sample of residents from the general population, which included children and adolescents. More important than direct comparison of the two provinces is the fact that in two serious endemic coal- burning ﬂuorosis areas, at two diﬀerent time points in the recent past, children and adolescents were experiencing SF signs and symptoms.