This study was an in-vitro study conducted on soil, grain and water samples of 15 villages of Jalgaon district in Maharashtra, India. The study was carried out at Department of Public Health Dentistry, ACPM Dental College, Dhule, Maharashtra, India. Permission to conduct the study was obtained from Institutional Ethical Clearance Committee of ACPM Dental College and Hospital, Dhule. Jalgaon district consists of 15 talukas/tehsils and one village from each taluka was chosen randomly for the present study. From each selected village, a sample each of soil grain and water was collected.

Collection and Analysis of Soil: Soil samples were collected from 15 different croplands which were used for the cultivation of Jowar crop. Soil samples were collected from immediate adjacent areas of the roots of the crop within a depth range of 1.0 to 30.0 cm. The surface of each soil sample was checked to confirm that it was free of stocks, remains of plants or other debris. The soil samples were obtained by digging vertically and collecting from distinct horizons. The samples were transported to the laboratory in plastic bags. The soil samples were finely grinded and sieved with mesh (size less than 2 mm) and the homogenized soils were sub sampled for digestion. About 50 g of each soil sample was digested with aqua regia. The digested sample was cooled and was filtered using Whatman No.1 filter paper into a flask [13,14]. After filtration, fluoride concentration was analyzed by SPADNS spectrophotometric method.

Collection and Analysis of Grains: Jowar grains were harvested from the Jowar plant from the selected farm itself. Care was taken that no other parts of plant were present. The samples were kept in plastic bags and transported to the laboratory. Samples were washed with distilled water, dried in an oven at 105°C for 24 hour and then crushed into a powder so as to pass through a 40 mesh sieve. About 0.5 g each of the powdered sample was transferred into a 150 ml nickel crucible and moistened with a small amount of de-ionized water. A 6 ml of 16.8 N NaOH was added and crucible was placed in muffle furnace and slowly raising the temperature to 600°C for half an hour, followed by dissolving the residue by heating with water on a hot plate. After the treatment samples were removed, allowed to cool and then 10 ml of distilled water was added to the samples with stirring to adjust the pH to 8–9. The sample solution was transferred to a 100 ml plastic volumetric flask, made upto volume with distilled water and filtered through a Whatman No. 40 filter paper [15]. The filtrate was used for analysis of fluoride using SPADNS method.

Collection and Analysis of Water: One liter of well water which was used for cultivation of the crops was collected and transported to the laboratory. Water was collected in pre-cleaned sterilized polythene bottles as per the standard protocol. Water samples were analyzed for fluoride content using SPADNS method.

All the samples were precoded from 1 to 15 and not disclosed to the technician and the statistician so as to eliminate any potential bias.

Collected data was complied, tabulated and analyzed using SPSS 17 software. Mean difference in fluoride levels in between the groups were analyzed using ANOVA and Post hoc Tukey test. Linear regression method was applied to analyse the association of the fluoride content of grain with water and soil.

The results of the present study showed that mean fluoride level of soil was 2.631 ppm, that of water was 1.637 ppm and for grains was 1.435 ppm [Table/Fig-1]. There was statistically highly significant (p<0.01) difference of mean fluoride levels in between the three groups [Table/Fig-1]. Fluoride concentration showed significant difference in all comparison except between water vs grain [Table/Fig-2].

Fluoride levels in all the three groups showed significant association with each other. Maximum association was seen among fluoride level of soil and grain (r-value = 0.8) [Table/Fig-3]. Univariate linear regression analysis demonstrated a strong association (β=0.984) of fluoride level between soil and water. The strength of association of fluoride level between soil and grain decreased when adjusted for the fluoride level in water (β= 0.350). Similarly, the strength of association of fluoride level between water and grain decreased severely when adjusted for the fluoride level in soil (β=0.319) [Table/Fig-4].

Endemic fluorosis has remained a major public health problem throughout the world, especially in India [16]. Absolute treatment is difficult, hence prevention and control of fluorosis is important, thus changing the water source, reducing the fluoride concentration of drinking water and decreasing the consumption of fluoride rich food are the main strategies that can effectively diminish the incidence of fluorosis. It is the total amount of Fluoride absorbed in a human body that needs to be considered i.e., the sum of fluoride intake from water, food, and air. Therefore, determining dietary fluoride intake is also useful and important for estimating the retention of fluoride in man [17]. This provided an impetus with the aim to estimate the absorption of fluoride to edible grains from soil and water.

In the present study, the mean fluoride level of water used for irrigation was found to be 1.63 ppm and is above the permissible limit. The amount of fluoride concentration in water differed from place to place; this may be attributed to factors such as hydrological condition, land form, rainfall, evaporation and also adsorption and leaching of fluoride in soil [18]. The fluoride level of soil was found to be 2.631 ppm. This is in accordance with the study conducted by Abida Begum 2012 in which the fluoride concentration of black soil obtained from five agricultural locations across Mysore district was in the range of 2.3-4 mg/l [14]. In a study conducted by Wang et al., it was found that the ability of black soil to absorb fluoride was higher than other soils [18]. This may be a probable reason for high amount of fluoride in soil tested in present study. The mean fluoride concentration in the Jowar grain was found to be 1.435 ppm. The results of the present study could not be compared with other studies as to our best knowledge, till date, no previous research has been done to assess the fluoride level in jowar grains. The amount of fluoride present in Jowar grains may be due to uptake from the soil or from water used for irrigation and it may also be attributed to presence of molybdenum in Jowar grain which helps in retention of fluoride ions [12]. Fluoride levels in all the three groups showed significant association with each other with maximum association being found in fluoride levels of soil and grain (r=0.8).

Soil fluoride level was found to be higher than grain and water fluoride level. This may be attributed to presence of naturally occurring fluoride in the soil in the form of minerals as well as release of fluoride in the soil from anthropogenic activities such as application of fertilizers and aluminum smelting industrial emissions [13]. While the uptake of fluoride by plant is dependent on a number of factors i.e., soil pH, soil type and presence of other elements [18,19]. From the results, it can be seen that grain derives almost equal fluoride content from both soil and water (adjusted regression analysis in [Table/Fig-4]) and it is also clear that fluoride content of the grains cultivated in fluorosis prone area (having high amount of fluoride in soil and water) is on the higher side (1.435 ppm).

Thus, fluoride accumulation in the people residing in fluorosis prone area can be attributed to consumption of grain containing high amount of fluoride in addition to fluoride contaminated drinking water (above permissible limits). Gupta S et al., conducted a study to determine fluoride intake through meal, vegetables and drinking water [17]. The results showed that fluoride uptake accounted for 26%, 30% and 44% of the total fluoride amount, respectively for meal, vegetables and drinking water. Thus, it can be seen that substantial amount of fluoride was ingested through meals and vegetables (56%) apart from drinking water.

The use of fluoride containing water for irrigation for crops that tend to accumulate fluoride should be reduced as much as possible in order to reduce the risk of human exposure to fluoride. However, in areas where only fluoride contaminated irrigation water is available then it is advisable to grow crops with relatively low capabilities to enrich fluoride or crops having high fluoride levels can be transported for consumption in those regions which are having deficient levels of fluoride in drinking water. Also, the farmers should analyse for the fluoride levels in the soils before using fertilizers which could prevent additional source of fluoride to the soil.

Further studies should be conducted with large sample size for comparison of the fluoride trend in the district as well as nearby places. Research should focus on evaluating the effect of temperature, type and amount of fertilizers used, etc., on the fluoride content of jowar grains.

Various factors such as environmental conditions, type and amount of fertilizers used, amount of water used to cultivate the crops, pH of soil, etc., which may affect the fluoride absorption were not taken into consideration during this study due to financial and time constraints.

From the present study, we can conclude that water and soil fluoride levels have a great influence on fluoride concentration in grain. Use of fluoridated water for irrigation should be minimized for the crops that tend to accumulate fluoride which could result in less human exposure to excess fluoride. Authorities should take into consideration the fluoride content of jowar grains before selling it in the market.