According to the principle that a selected population lives in a similar natural environment and has similar lifestyle and diet habits, one township in the endemic fluorosis area and one township in the non-endemic fluorosis area in a certain region were respectively selected as sites for population sampling. In the sites for population sampling, local children aged 8-12 were surveyed. Sampled subjects suffering from emotional disturbance, mental retardation or mental illness were excluded. 268 children aged 8-12 were randomly selected as study subjects. 134 children living in the endemic fluorosis area and 134 children living in the non-endemic fluorosis area were respectively sampled. According to the criteria for classification of fluorosis area (GB 17018-1997), the areas for population sampling were divided into a non-endemic fluorosis area where the prevalence of dental fluorosis (also termed mottled enamel) in local children aged 8-12 is less than 30%, and an endemic fluorosis area where the prevalence of dental fluorosis in local children aged 8-12 is greater than 30%.
An intelligence test was conducted in study subjects using the Combined Raven’s Test (CRT), and then, the polymerase chain reaction (PCR) method was used to identify the correlation between intelligence and COMT gene polymorphism in children in the endemic fluorosis area and non-endemic fluorosis area.
1.3 Statistical method
Statistical analysis was performed using SPSS 17.0.
2.1 Distribution of intelligence quotient (IQ) in children in the endemic fluorosis area and non-endemic fluorosis area In the endemic fluorosis area, the distribution of children’s IQ was significantly shifted to the left, and children’s overall IQ was relatively low with an average value of 91.07, which was lower than that in the non-endemic fluorosis area. The difference was statistically significant (P < 0.05). The rate of mental retardation (IQ<69) in children in the endemic fluorosis area was significantly higher than that in the non-endemic fluorosis area.
The difference was statistically significant (P < 0.05).
2.2 Distribution of intelligence quotient (IQ) by sex in children in the endemic fluorosis area and non-endemic fluorosis area Children in the endemic fluorosis area included 63 males and 71 females, whose average IQ was 93.24 and 91.75, respectively, i.e., the former was slightly higher than the latter, and their difference was not statistically significant (P > 0.05). The rate of mental retardation (IQ < 69) in female children was slightly higher than that in the male children, and their difference was not statistically significant (P > 0.05).
Children in the non-endemic fluorosis area included 69 males and 65 females, whose average IQ was 98.85 and 94.67, respectively, i.e., the former was slightly higher than the latter, and their difference was not statistically significant (P > 0.05).
The rate of mental retardation (IQ < 69) in female children was slightly higher than that in the male children, and their difference was not statistically significant (P > 0.05). The results above indicate that endemic fluorosis causes certain damage to children’s intelligence.
2.3 Levels of intelligence in children in the endemic fluorosis area and nonendemic fluorosis area
Measurements of intelligence showed a great difference between the level of intelligence in children in the endemic fluorosis area and that in the non-endemic fluorosis area, and such difference was statistically significant (P < 0.05). In ability to distinguish consciousness, ability to distinguish similar, comparative reasoning ability, ability to identify relationships among the series and abstract reasoning ability, there were great differences between children in the endemic fluorosis area and those in the non-endemic fluorosis area. All such differences were statistically significant (F = 8.427, P < 0.05; F = 5.164, P < 0.05; F = 6.201, P < 0.01; F = 4.865, P < 0.05; F = 3.473, P < 0.05).
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