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 Table of Contents  
SYSTEMATIC REVIEW
Year : 2016  |  Volume : 7  |  Issue : 3  |  Page : 81-87

Fluoride concentration of drinking water and dental fluorosis: A systematic review and meta-analysis in Iran


1 Department of Environmental Health Engineering, School of Public Health, Tehran, Iran
2 Department of Environmental Health Engineering, School of Public Health; Center for Solid Waste Research, Institute for Environmental Research (IER), Tehran, Iran
3 Department of Epidemiology and Biostatistics, School of Public Health, Tehran, Iran
4 Dental Research Center, Dentistry Research Institute, Tehran University of Medical Sciences, Tehran, Iran

Date of Web Publication14-Sep-2016

Correspondence Address:
Amir Hossein Mahvi
Department of Environmental Health Engineering, School of Public Health, Center for Solid Waste Research, Institute for Environmental Research (IER), Tehran University of Medical Sciences; Tehran
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2155-8213.190482

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  Abstract 

Introduction: Recently, a number of studies have investigated the impact of fluoride concentration of drinking water on dental fluorosis. These Studies should be reviewed to provide a new outlook on the analysis of the causes and effects of dental fluorosis in specific regions. The objective of this study was to systematically review the fluoride concentration of drinking water and investigate its relation to the frequency of dental fluorosis in Iran. Materials and Methods: Dean's index was used to classify data, and a meta-analysis was conducted to obtain summary measure with 95% confidence interval (CI). In this regard, Stata/SE 11.1 was employed for data analysis based on random effect models for reporting the results. In this systematic review, Scientific Information Database (SID) and IranMedex databases were searched and studies were included based on specific criteria. Data validity was assessed using the strengthening the reporting of observational studies in epidemiology (STROBE) checklist adapted for cross-sectional study designs. Furthermore, a series of predefined keywords were used, and the combination of these keywords were considered using operators. The inconsistency was examined using the χ2 test at a significance level of 10%. In addition, heterogeneity was quantified across studies using the І2 statistic. The difference between study variance was analyzed based on τ2 statistic. Results: In the age group of 6-18 years old based on the fluoride level in drinking water and exposure time, there was significant heterogeneity among the studies in all subgroups for determining the frequency of dental fluorosis and assessing the effect of other variables. Conclusions: The variables, water fluoride exposure time, and any exposure to fluoride are considered as confounding factors. Analyzing the subgroups and examining the heterogeneity showed that the results of the studies in all subgroups cannot be pooled.

Keywords: Dental fluorosis, fluoride, Iran, water


How to cite this article:
Goodarzi F, Mahvi AH, Hosseini M, Nodehi RN, Kharazifard MJ, Parvizishad M. Fluoride concentration of drinking water and dental fluorosis: A systematic review and meta-analysis in Iran. Dent Hypotheses 2016;7:81-7

How to cite this URL:
Goodarzi F, Mahvi AH, Hosseini M, Nodehi RN, Kharazifard MJ, Parvizishad M. Fluoride concentration of drinking water and dental fluorosis: A systematic review and meta-analysis in Iran. Dent Hypotheses [serial online] 2016 [cited 2019 May 23];7:81-7. Available from: http://www.dentalhypotheses.com/text.asp?2016/7/3/81/190482


  Introduction Top


Deficit of fluoride is often reported as one of the significant factors associated with dental fluorosis. Recently, a number of studies have been carried out to explain the impact of water fluoridation on dental problems. [1] In a study using data from the nationally representative 2004-2006 Australian National Survey of Adult Oral Health, the authors investigated the effects of fluoridating drinking water on dental fluorosis. The results of this study revealed that fluorosis-incidence impacts of water fluoridation were at least as serious in adults born before extensive fluoridation as after it. [2]

Broffitt et al. assessed the frequency of fluorosis as well as risks for young individuals at ages 9 and 13. The findings indicated that the frequency of dental fluorosis was low except for the first molar occlusal surfaces. [3] Another cross-sectional study was carried out in Vadodara district, India, where 6 out of 261 villages with high fluoride level and 5 out of 1490 ones with normal fluoride level in drinking water were adopted. The results revealed that the risk of dental fluorosis was higher in areas with more fluoride content in drinking water. However, there was lesser degree of dental fluorosis in the same area. [4]

Four systematic reviews have been conducted in this field so far. The first study was conducted in 1989. In this study, the effectiveness of water fluoridation in decreasing dental fluorosis was reviewed based on surveys conducted in the last decade in United States, Australia, Britain, Canada, Ireland, and New Zealand. [5] In another systematic review, McDonagh et al. reviewed the effectiveness of fluoridation of drinking water. A comprehensive search was conducted in 25 electronic databases, and consequently 214 studies were included. The evidence of a beneficial reduction in fluorosis should be considered together with the increased incidence of dental fluorosis. There was no clear evidence of other potential adverse effects. [6] Griffin et al. reviewed the effectiveness of fluoride in preventing fluorosis in adults. The objective of this meta-analysis was to examine the effectiveness of water fluoridation among adults. They used a random effects model to estimate the effect size of fluoride (absolute difference in annual fluorosis increment or relative risk ratio) for all individuals aged more than 20 years. [7]

Rugg-Gunn and Loc-Do assessed the efficacy of fluoridation of water resources in the prevention of dental fluorosis, with emphasis on the results of studies published worldwide since 1990. They discussed different aspects of the design and reporting of these studies in comparison to those published before 1990. Studies published worldwide reporting the effect of water fluoridation in terms of the dmf/DMF fluorosis index between 1990 and 2010 were examined. [1]

Altogether, the lack of complete analysis is considered as one of the main shortcomings of prior similar systematic reviews. It should be mentioned that one review in this field was conducted by Azami-Aghdash in Iran in 2013. The study illustrated that, despite the low amount of fluoride in drinking water, the prevalence of dental fluorosis was high. In this study, the average amount of fluoride in drinking water and the average prevalence of fluorosis was measured separately. [11] In addition, Azami-Aghdash did not consider all of the effective confounding factors for grouping and analysis.

Regarding the abovementioned limitations, conducting a similar systematic review was needed to address the shortcomings and including the recently published articles. Hence, the current review was conducted.


  Subjects and Methods Top


Search strategy

The databases of Scientific Information Databases (SID) (www.sid.ir) and IranMedex (www.iranmedex.ir) were used in this research. Further searches were carried out through Google Scholar and bibliographies of the included studies. All studies gathered from databases were considered in the current review irrespective of language. The time scope covered the starting date of the databases to December 2014. Further, specific keywords which were used for performing searches mainly include:

SID database: 6 studies were detected for the keyword "florosis," and 13 studies were detected for the keyword "fluorosis."

IranMedex database: 23 studies were detected for the keyword "fluorosis."

Inclusion criteria

The articles having the following inclusion criteria were eligible for abstraction

  • Being a primary study (not a review study)
  • Studies about humans
  • Being related directly to fluoride in drinking water supplies and dental fluorosis
  • At least one group of individuals were studied
  • Reported measurable outcomes (i.e., Dean's index) in a group accompanying the amount of fluoride in its drinking water supply.


Data extraction and assessment of study validity

The inclusion criteria were examined by at least 2 reviewers. The reviewers independently extracted data from studies and assessed the validity. Furthermore, a third reviewer checked the results. The validity was assessed based on the STROBE checklist adapted for cross-sectional study designs. On the checklist, a complete score (1 score) was given to high quality items. In addition, a score of 0.5 and 0 were, respectively, given to moderate quality and low quality items. Eventually, the scores of all questions were summed up, and the overall quality score was determined for the study as low, moderate, or high quality.

Outcome measures

Dean's index was used to classify dental fluorosis. Dental fluorosis was defined in this review as any level other than normal and questionable level based on Dean's index. Moreover, a meta-analysis was conducted to obtain summary measure with 95% confidence interval (CI). Stata/SE 11.1 (StataCorp LP, College Station, TX 77845, USA) was employed for data analysis using random effect models for reporting the results. [8]

Heterogeneity

The inconsistency was examined using the χ2 test at the significance level of 10%. In addition, heterogeneity was quantified across studies using І2 statistic. [9] The difference between-study variance was analyzed based on τ2 statistic. [10]

Analysis

Measures of effect were plotted (95% CI) when data was in the right format. The heterogeneity was assessed based on the visual examination of plots and χ2 statistics. Meta-regression was performed for significant heterogeneity. In addition, random effects models were used for combining the results, and meta-regression was applied for identifying the impact of study characteristics on outcome for explaining any heterogeneity between studies. [12] Data analysis was conducted by software Stata/SE 11.1(StataCorp LP, College Station, TX 77845, USA).

Data categorization and analysis of subgroups were carried out to decrease the impact of confounder factors such as consumption of fluoride-containing supplements that can affect the relation of the fluoride level in drinking water and dental fluorosis. According to dentists and epidemiologists, age was considered as the most significant variable in dental fluorosis. Therefore, categorization was first based on age, and second based on the fluoride level in drinking water. Other factors such as exposure time to fluoride in drinking water and any exposure to fluoride can also affect the prevalence of dental fluorosis, however, they were not considered because of no mention in most of the studies. The categorization of variables was conducted as below:

  • Age (year): Less than 6, 6-18, and more than 18
  • Amount of fluoride in drinking water (PPM): Less than 0.7, 0.7-1.2, 1.3-2, and more than 2
  • Any exposure to fluoride in supplements, diet, air, etc: Yes, No, and Not reported
  • Exposure time to fluoride in drinking water: Lifetime exposure, exposure in 6-8 years in early life, and not reported.


Finally, two groups were identified. It should be mentioned that all individuals were 6-18 years old because the other two age groups were excluded from this review for mentioned reasons. In the current study, the risk of bias for the prevalence of fluorosis across studies was unjustifiable. Therefore, there was no assumption of bias.


  Results Top


Four articles met the inclusion criteria and were then included. Hence, these articles that included 12 study areas were reviewed. In reviewing these studies, data categorization and analysis of subgroups were carried out to decrease the impact of confounder factors which affect the relation of fluoride level in drinking water and dental fluorosis. All included articles were cross-sectional in design and were of evidence level B (low quality). The risk of bias for the prevalence of fluorosis across these studies was unjustifiable. A diagram showing the stages of systematic review is presented in [Figure 1].
Figure 1: Process of inclusion of studies for review and analysis

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In this review, categorization of groups into subgroups was conducted based on fluoride level in drinking water, the exposure time to fluoride in drinking water, and any exposure to fluoride in supplements, diet, air, etc., or quality of studies for the age group of 6-18 years old. Group 1 and Group 2 were categorized in 12 subgroups.

According to [Table 1], group 1 was categorized into subgroups based on the fluoride level in drinking water and the exposure time for the age group of 6-18 years old. There was significant heterogeneity (P < 0.001) among the studies in all subgroups. Therefore, the result of the studies in the subgroup 2 cannot be pooled. It should be mentioned that, in the same exposure time, the prevalence of dental fluorosis increased with fluoride level.
Table 1: Categorizing group 1 into subgroups based on fluoride level in drinking water and the exposure time for the age group of 6-18 years old

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As illustrated in [Table 2], group 2 was categorized into subgroups based on the fluoride level in drinking water and any exposure to fluoride for the age group of 6-18 years old. There were not enough studies to be pooled. It should be added that the studies in which exposure time or any exposure to fluoride in supplements, diet, air, etc., was not reported, were not considered in this review.
Table 2: Categorizing group 2 into subgroups based on fluoride level in drinking water and any exposure to fluoride in supplements, diet, air, etc., for the age group of 6-18 years old

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Fluoride level in water at less than 0.7 ppm and lifetime exposure to this fluoride level (subgroup 1), significant heterogeneity (P < 0.001) was found among the studies in this subgroup. Hence, the pooled estimate of dental fluorosis prevalence could not be reported [Figure 2].
Figure 2: Forest plot of the dental fluorosis prevalence for individuals with lifetime exposure to fluoride level in drinking water less than 0.7 ppm (subgroup 1)

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The amount of fluoride in water at 0.7-1.2 ppm and lifetime exposure to this level (subgroup 4), there was significant heterogeneity (P < 0.001) among the studies in this subgroup. Therefore, the estimates of dental fluorosis prevalence could not be pooled [Figure 3].
Figure 3: Forest plot of the dental fluorosis prevalence for individuals with lifetime exposure to fluoride level in drinking water 0.7-1.2 ppm (subgroup 4)

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Considering the forest plot shown in [Figure 4], the amount of fluoride in water at 1.3-2 ppm and lifetime exposure to this level (subgroup 7), there was significant heterogeneity (P < 0.001) among the studies in this subgroup. Therefore, the estimates of dental fluorosis prevalence could not be pooled.
Figure 4: Forest plot of the dental fluorosis prevalence for individuals with lifetime exposure to fluoride level in drinking water 1.3-2 ppm (subgroup 7)

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Meta-regression

Meta-regression showed that exposure time to fluoride in drinking water and any exposure to fluoride in supplements, diet, air, etc., had a significant association with the difference in the prevalence of dental fluorosis. These variables were the sources of heterogeneity. The studies must be grouped and subgrouped based on these variables and the analysis must be performed according to the groups. The results of meta-regression are presented in [Table 3].
Table 3: The results of meta-regression for exploring the source of heterogeneity in studies*

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Fluorosis prevalence in relation to water fluoride level in Iranian studies

A number of studies have been conducted in different regions of Iran. In these studies, surveys were performed in Tehran, Bastak, Larestan, Ghouchan, Bushahr, Mahmoud Abad, Titkanlou, and Dayer town including cities as well as rural areas to explore the prevalence of dental fluorosis with respect to fluoride level in drinking water. The results of these studies are illustrated in [Figure 5].
Figure 5: Fluoride level and prevalence of dental fluorosis in studies conducted in Iran

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  Discussion Top


The objective of this study was to conduct a systematic review regarding fluoride level of drinking water and prevalence of dental fluorosis in Iran. The effect of other variables on the frequency of dental fluorosis was examined. These variables include water fluoride exposure time and any exposure to fluoride in supplements, diet, air, etc., Furthermore, temperature was considered as a confounding factor, which few included studies had considered. Hence, the categorization could not be performed based on this factor, and consequently, it was not considered in the analysis. Investigating the heterogeneity and analyzing the subgroups revealed that there was significant heterogeneity among studies in all subgroups, and there were not enough studies to be pooled. Analyzing the subgroups and examining the heterogeneity showed that the results of the studies in all subgroups cannot be pooled.

In the current study, the following factors were considered for grouping:

  • Age
  • Fluoride level in drinking water (p.p.m)
  • Water fluoride exposure time
  • Any exposure to fluoride in supplements, diet, air, etc.


The prevalence of dental fluorosis was assessed based on all the confounding factors. The number of included studies considerably decreased because all of the above factors were included. Most of the similar systematic reviews have mentioned at most two of the abovementioned confounding factors. Furthermore, the significance of heterogeneity in all subgroups were due to not considering all of these confounding factors simultaneously.

A number of studies have been conducted in different regions of Iran to explore the prevalence of dental fluorosis in relation to fluoride level in drinking water. The results revealed that the relation between the prevalence of dental fluorosis and fluoride level in drinking water varies from one place to another.

The main limitation of the current systematic review is the fact that the number of pooled studies was substantially decreased due to not including the studies in which important confounding factors such as exposure time to water fluoride or any exposure to fluoride in supplements, diet, air, etc., were not reported.


  Conclusion Top


The sources for the current systematic review are SID and IranMedex. The findings are presented as an overview of the previous researches on water fluoride concentration and prevalence of dental fluorosis with emphasis on different regions of Iran. A close look at similar systematic reviews illustrated that most of these reviews have only mentioned 1 or 2 confounding factors. Whereas, in current review, all the confounding factors were considered. In fact, the number of included studies in this review considerably decreased because of consideration to all the detected confounders. Altogether, the current study provided a more extensive overview of the literature regarding fluoride concentration of drinking water and prevalence of dental fluorosis in Iran. The most obvious weakness of the prior systematic reviews regarding fluoride concentration of drinking water and prevalence of dental fluorosis was the lack of analysis and the limited number of factors. The proposition of authors for further research would be an in-depth investigation on the geographical distribution of dental fluorosis prevalence in different regions considering all confounding factors to provide a wide epidemiological outlook on water fluoridation and frequency of dental fluorosis.

Acknowledgment

The authors would like to appreciate the support of Tehran University of Medical Sciences.

Financial support and sponsorship

This study is supported by Tehran University of Medical Sciences.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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Rugg-Gunn AJ, Do L. Effectiveness of water fluoridation in fluorosis prevention. Community Dent Oral Epidemiol 2012;40:55-64.  Back to cited text no. 1
    
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Broffitt B, Levy SM, Warren J, Cavanaugh JE. Factors associated with surface-level fluorosis incidence in children aged 9 to 13: The Iowa Fluoride Study. J Public Health Dent 2013;73:304-10.  Back to cited text no. 3
    
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Kotecha PV, Patel SV, Bhalani KD, Shah D, Shah VS, Mehta KG. Prevalence of dental fluorosis & dental fluorosis in association with high levels of drinking water fluoride content in a district of Gujarat, India. Indian J Med Res 2012;135:873-7.  Back to cited text no. 4
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Azami-Aghdash S, Ghojazadeh M, Azar F, Naghavi-Behzad M, Mahmoudi M, Jamali Z. Fluoride concentration of drinking waters and prevalence of fluorosis in iran: A systematic review. J Dent Res Dent Clin Dent Prospects 2013;7:1-7.  Back to cited text no. 11
    
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]


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