Dental fluorosis

The Region of Pingxiang in the Peoples' Republic of China consists mostly of hilly land with some mountainous areas. The geology of the region is lime rock and granite and there is abundant coal, which residents of the hilly areas use for fuel. People living in the mountainous areas use firewood for heating and cooking.

During 1987 to 1990, the epidemiology of fluorosis, environmental factors, geological features, and total amount of fluoride intake by residents of Pingxiang region were studied. The results of those studies have been described individually in previous reports[1,3]; this study is a comprehensive report of those findings.

Materials and methods

Ten households in coal-burning areas and 10 households in wood-burning areas were surveyed to determine the fluoride intake of the residents. Samples of air, water, grain, vegetables, soil, and coal used for cooking and heating were collected according to unified methods of sampling. Air and water samples were analyzed according to the Standard Method of Analysis of Environment Monitoring; grain and vegetable samples were analyzed according to the Process of Examination of Food Hygiene approved by the Peoples' Republic of China; and soil and coal samples were analyzed according to the Method of Monitoring and Examination of Soil Hygiene. Fluorine levels in all samples were determined with the fluorine ion selective electrode method. The total amount of fluoride intake was determined, using the methods of calculation described in a previous report[4] and weighing. The weighing method was conducted by an investigator who was assigned to collect food samples and record the weight of each subject's intake of cooked food on 5 consecutive days.

Results

Incidence of dental and skeletal fluorosis and fluoride content in urine. A general survey to determine the incidence of dental fluorosis was conducted among residents who were 8 y of age and older. Dental fluorosis was detected in 46.96% of the residents in the entire region. In the coal-burning areas, the incidence was 51.77%; areas with mostly fire-burning residences and fewer coal-burning residences had an incidence rate of 13.79%. The incidence of dental fluorosis in residences where only firewood was burned was less than 6.00%.

Sixty-seven adults (34 men, 33 women) from coal-burning areas were selected for bone radiographs. Five cases of skeletal fluorosis of the sclerosis type were detected, i.e., an incidence rate of 7.50%.

Fluoride content in urine was determined for children (226 subjects with dental fluorosis from coalburning areas, 150 from wood-burning areas [age range = 10-14 y]). The fluoride content in urine was 1.45 [+ or -] 0.71 mg/l for subjects from the coalburning areas and 0.69 [+ or -] 0.60 mg/l for subjects from wood-burning areas (p < .01). In addition, urine fluoride was determined for 54 adults from coal-burning areas. The average content was 2.07 [+ or -] 1.18 mg/l.

Fluoride in drinking water. Fluoride content of drinking water was analyzed for every village in the region. The average fluoride level was 0.14 [+ or -] 0.08 mg/l, which is within the national permissible standards. Drinking water stored in water vats for 24 h was examined in 48 coal-burning households. The fluoride content of the stored water was 0.23 [+ or -] 0.12 mg/l, indicating that the fluoride content tends to rise in stored water.

Fluorine in indoor air. The fluorine content of indoor air for coal-burning areas and wood-burning areas is shown in Table 1. Fluorine content in the coalburning areas was much greater than that in the woodburning areas and exceeded the national permissible standard. Fluorine content in the wood-burning areas was well within the national standards.

[TABULAR DATA OMITTED]

Fluoride in food. Comparisons were made between the fluoride content in summer and winter vegetables. Fluoride content of summer gourd-type vegetables was similar in the coal- and wood-burning areas; however, winter vegetables, which are mostly broadleaf plants, had higher fluoride levels in the coal-burning areas than in the wood-burning areas, as was the case for tea, pickled vegetables, and smoked meat (Table 2).

[TABULAR DATA OMITTED]

Rice and red peppers are often stored, and the effects of storage on fluoride content were analyzed in the coal-burning areas. Fresh rice and red peppers were compared with those stored for 5 mo. in both cases, the fluoride content was greater in the stored samples (Table 3).

[TABULAR DATA OMITTED]

Fluorine in soil. Total fluorine content and watersoluble fluoride content were determined in both cultivated soil (used for gardening) and natural soil (used for mixing coal). The water-soluble fluoride content was higher in cultivated soil than in uncultivated soil (Table 4).

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Fluorine in coal and smoke dust. Coal samples from 19 villages in the coal-burning areas were analyzed for fluorine content. The average fluorine content was 439.7 [+ or -] 147.9 mg/kg. Smoke dust samples from 9 coal-burning households were also analyzed; the total fluorine content was 31 787 [+ or -] 2 719 mg/kg; the watersoluble fluoride content was 17 873 [+ or -] 9 998 mg/kg.

Fluorine from brick and tile kilns. During the past decade, the baking of bricks and tiles in kilns has become a small industry in rural areas. The content of fluorine in the adobe used for baking the bricks and tiles was determined. The average fluorine contents in the adobe for baking bricks and for baking tiles were 668.5 [+ or -] 75.0 mg/kg and 12 365.1 [+ or -] 759.9 mg/kg, respectively. The fluorine content in the baked bricks was 410.4 [+ or -] 277.7 mg/kg and in the baked tiles it was 702.7 [+ or -] 29.4 mg/kg; therefore, during the baking process, 258 mg of fluorine was released for each 1.0 kg of mud used for baking bricks, and 11 663 mg fluorine was released for each 1.0 kg of mud used for baking tiles. Fluorine content of air surrounding brick and tile kilns (size range = 5-30 m) was 0. 139 [+ or -] 0.081 mg/[m.sup.3] and 1.442 [+ or -] 1.282 mg/[m.sup.3], respectively.

In an examination of a village with 1 500 residents, it was found that an average of 293.83 kg of fluorine was released per year during the past 10 y. The highest level released was 505 kg. Approximately 50% of the released fluorine was water-soluble, which could have had harmful effects on the environment and human health.

Geological distribution of soil fluorine. The geochemical environment of Pingxiang region is rich in iron and aluminum and has neutral or acid soils. Fluorine content in clay and humus, which are formed from weathering of the permian Xiao jiang Bian rock, is the highest, followed by the content in the rock itself. The fluorine content in the Triassic Da Ye soil is low. High fluorine content exists primarily in malm and calcium-magnesium mudstone, which is the adobe used for bricks and tiles.

Residents' fluoride intake. Results showed that the total fluoride intake in the coal-burning areas was higher than that in the wood-burning areas (Table 5). The proportion of fluoride intake via the digestive tract was no different from that via the respiratory tract.

[TABULAR DATA OMITTED]

Sulfur dioxide in indoor air. The average concentration of sulfur dioxide in indoor air was higher than than the national standard (Table 6).

Discussion

There was a 46.96% incidence of dental fluorosis in the region of Pingxiang, which indicates endemic fluorosis. Fluoride in drinking water in the whole region was within permissible standards and, therefore, water was excluded as an etiological agent. The rate of dental fluorosis in coal-burning areas was much greater than that in wood-burning areas, suggesting that endemic fluorosis is related to fluorine content in air from burning coal. In addition, water stored in kitchens with coal-burning stoves had higher levels of fluoride than water stored in outside wells. The same trend was observed for rice, peppers, tea, pickled vegetables, and smoked meat stored in coal-burning kitchens. Smoke dust in kitchens with coal stoves had a fluoride content 1 000 times higher than with wood-burning stoves. The use of fluorine-rich adobe in brick and tile manufacturing is another source for air pollution-type fluorosis. These results indicate that the conversion of insoluble fluorine to soluble fluoride during coal burning is the primary source of fluoride pollution that leads to endemic fluorosis.

The average total intake of fluoride in the region was significantly higher in the coal-burning areas than in the wood-burning areas, regardless of the method of measurement (i.e., methods of calculation in practical investigation or the method of weighing). In coal-burning areas, average total intake of fluoride was also determined, within 3.0 mg, by the method of weighing; results, however, accorded with the reports from Lianyuan County Hunan Province and Badong County Hubei Province, where levels were also were much lower than maximum safe fluoride value proposed by various scholars in China (i.e., 3.5 mg/d per person[5]).

Some questions remain for further study. Why does the level of fluoride intake found in the studies reported here lead to such a high incidence of fluorosis? Is there a relationship between increased sulfur dioxide levels in coal-burning kitchens and fluoride intake? What is the best method for determining the total fluoride intake per person?

References

[1.] Yixin C, Meiqi L, Zhaolong H, et al. An investigation of endemic fluorosis and research of its sources in Changing township, Pingxiang. Chinese J Epidemiol 1990;9:361-63. [2.] Zhaolong H, Xiaomao X, Jing Z, et al. Investigation of fluoride pollution in surroundings from minor brick and tile kilns. J Environ Health 1991; 8:117-19. [3.] Yuandong X, Meiqi L, Yixin C, et al. The character of geologic distribution of soil fluoride in Pingxiang. Comment on Environmental Studies in China 1991; 11:71-73. [4.] Yongquan L, Ziaomao X, Zhaodian W, et al. An evaluation on total fluoride intake by residents in Changing township, Pingxiang. Environment and Exploitation in Jiangxi 1991; 6:38-40. [5.] Shuzhang S, Xueqin C, Mouzhong L, et al. An evaluation of total fluoride intake and its routes of inhabitants in endemic areas of fluorosis induced by burning coal. Chinese J Epidemiol 1991; 10:211-14.

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