Trace Metals in Rice Grains and Their Associated Health Risks from Conventional and Non-Conventional Rice Growing Areas in Punjab-Pakistan

Abstract

Rice (Oryza sativa L.) is cultivated and consumed worldwide, but the contamination of rice grains with trace metals (TMs) could cause adverse impacts on human health. The aims of this study were to determine the concentrations of TMs in different rice varieties available for sale in local markets and to determine whether consumers are likely to be at risk via the consumption of these rice cultivars. For this purpose, samples of rice grains were collected from 12 rice growing districts (administrative units) in Punjab, Pakistan. These districts were further classified based on rice growing methods due to specific soil type. In conventional districts, the puddling method was used, while direct seeding was used for rice cultivation in non-conventional districts. The samples were collected and analyzed for the determination of essential (Cu, Fe, Zn, and Mn) and non-essential (Cd, Ni, and Pb) TMs using an atomic absorption spectrophotometer (AAS). The results showed that the maximum respective concentrations of Cd, Ni, and Pb (0.54, 0.05, 1.10 mg kg⁻¹) were found in rice grains in conventional areas, whereas values of 0.47, 0.20, and 1.20 mg kg⁻¹ were found in non-conventional rice growing areas. The maximum concentrations of essential TMs (Cu, Fe, Mn, and Zn) were 4.54, 66.01, 4.82, and 21.51 mg kg⁻¹ in conventional areas and 3.76, 74.11, 5.66, 19.63 mg kg⁻¹ in non-conventional areas. In the conventional rice growing areas, Fe and Zn concentrations exceeded the permissible limits in the 27 and 7% samples, respectively. In the non-conventional rice areas, the concentrations of Cu, Fe, and Mn exceeded the permissible limits in the 15, 26, and 3% samples, respectively, while its Zn concentration was found within the permissible limits. The estimated weekly intake (EWI) and maximum tolerable dietary intake (MTDI) values for all studied metals were found within the permissible values set by WHO, except for Fe, in both sampled areas. It was concluded that no health risks were associated by utilizing the rice grains. However, the mean values of TMs were found considerably higher in collected rice samples from non-conventional areas than the conventional areas. Therefore, the concentrations of TMs should be monitored properly.

1. Introduction

Trace metals (TMs) pollution has become a serious problem worldwide. There is a continuous buildup of TMs in agricultural soils through the application of solid waste and low quality irrigation water [1,2]. The continued use of raw or partially treated wastewater for growing fodder and cereals has resulted in the contamination of food crops, and thus their entry into the food chain [3,4]. The TMs in soils could contaminate the environment and damage human health through various exposure pathways, including direct ingestion, dermal contact, and inhalation [5]. TMs concentration in edible parts of several crops such as spinach, clover, grape vines, shrubs, barley, and wheat have been reported by various reports [6,7,8]; thus, the risk to humans of excessive TMs in edible parts of food crops should be a matter of concern. The crops absorb these metals from contaminated soils through their roots or from surface-deposited particles [9]. As TMs in crops enter the human body through the food chain and present health risks to humans, food consumption is also an important exposure pathway [10]. Vitamin C, Fe, and other nutrients stored in the body are significantly decreased if humans consume food contaminated by these TMs, leading to a decline in immunity, a deterioration of human function, and disabilities associated with malnutrition [9,11].

As rice is fast growing crop and produces large biomasses, several studies have indicated that rice grown in TM-contaminated soils have higher concentrations of metals than those grown in un-contaminated soils [12,13]. In un-contaminated soils, a major pathway of soil contamination is through the atmospheric deposition of TMs from point sources such as mining, smelting, and industrial activities [14]. Other sources of agricultural soil contamination include agricultural inputs such as fertilizers, pesticides, sewage sludge, organic manures, and composts [15,16]. Therefore, some eco-friendly alternatives could be applied for clean production and sustainable cultivation. The sustainable production of quality food for mankind is a big challenge of the era. Biofertilizers are being used to not only improve the yield but also improve the nutrition status of crop plants. Scientists are using nitrogen-fixing bacteria along with phosphorus-fixing bacteria and getting good results [17]. The use of biofertilizers and compost instead of chemical fertilizers improves soil health and decreases TM contamination in soil, improving plant quality [18]. The balanced use of fertilizer is very effective technique in producing crops and fulfilling their deficiencies [19]. Rice is a major staple food in many countries, particularly in Asian countries such as Bangladesh, India, Thailand, China, Pakistan, and Vietnam, where soil and groundwater pollution with high levels of TMs have been reported [20,21,22,23]. Increased levels of TMs in agricultural soils and their uptake in rice, vegetables, and other food crops have become a real health issue in this region [24]. A significant number of studies have focused on studying TM concentrations in Pakistani, Bangladeshi, Chinese, and Indian food items including vegetables [25,26,27,28]. The work of Wasim et al. [29] assessed the TM concentrations in rice, but sampling was performed only from one site, i.e., Karachi, Pakistan. Additionally, this study did not involve different rice growing districts and comparisons between conventional and non-conventional rice growing areas from Punjab, Pakistan. The conventional rice growing areas of Punjab are Sheikhupura, Gujranwala, Hafizabad, Sialkot, Narowal, and Gujranwala; rice is grown using the puddling method because of the specific soil type (clay loam), as clay loam soil puddling is easy due to the water retention capacity of soil. However, soil puddling enhances the possibility of TM availability to the plants, as the metals could be solubilized in the water and converted into the more available forms to the plants [30], i.e., zinc sulphide (ZnS) can be converted to cadmium sulphide (CdS) with the exchange of their ions during this practice. The non-conventional rice growing areas are Bahawalnagar, Chiniot, Jhang, Toba Tek Singh, Shorkot, and Mandi Bahauddin; rice is usually grown by using the direct seeded method because the soil is sandy loam. However, soil puddling is difficult in sandy loam because water retention and anaerobic conditions are not possible, moreover, TMs are leached down in these types of soils, thereby reducing the risks of TM contamination. Thus, there is a possibility that conventional rice growing areas could have more TM concentrations than the non-conventional growing areas. This study is based on the same hypothesis, i.e., TMs could behave differently due to the use of conventional and non-conventional rice growing techniques. Previous studies have not reported on this aspect along with the potential health risks to humans. Therefore, the objectives of the present study were to: (a) determine the concentrations of TMs in different rice varieties available for sale in local markets, produced from different conventional and non-conventional rice growing areas of Punjab, Pakistan; and (b) whether consumers are likely to be at risk of TM exposure via the consumption of these rice cultivars.

2. Materials and Methods

2.1. Sampling

This experiment was conducted to find out the concentrations of different essential (Cu, Fe, Mn, and Zn) and non-essential (Cd, Ni, and Pb) TMs in different types of crop (rice) varieties available in markets from conventional and non-conventional rice growing areas of Punjab, Pakistan. Fifty-five (55) rice samples (in triplicate) from conventional and non-conventional rice growing districts (6 conventional: Sheikhupura, Gujranwala, Hafizabad, Sialkot, Narowal, and Gujranwala; 6 non-conventional: Bahawalnagar, Chiniot, Jhang, Toba Tek Singh, Shorkot, and Mandi Bahauddin) of Punjab, Pakistan were collected during the month of January 2015 (Figure 1,

Table 1. Coordinates of sampling sites.

Sr. No.	Sampling Location	Longitude	Latitude
1	Chiniot	31.710762	72.992123
2	Bahawalnagar	29.613738	73.137526
3	Jhang	30.843993	71.850886
4	Shorkot	30.844500	72.085357
5	Toba Tek Singh	30.970966	72.479624
6	Faisalabad	31.413924	73.074134
7	Narowal	32.096795	74.862740
8	Gujranwala,	31.982594	74.130777
9	Mandi Bahauddin	32.597835	73.476281
10	Hafizabad	32.052014	73.659907
11	Sialkot	32.386376	74.408410
12	Sheikhupura	31.715192	73.982211

).

The collected rice varieties were preferred to be cultivated by the farmers of the respective sampling districts. The collected samples details (sampling district, rice variety, and sample ID) are given in Supplementary Table S1. The collected samples were stored at 4 °C prior to further processing. All samples were washed with tap water (three times) followed by deionized (DI) water twice. The samples were then dried in open air for 24 h and in an oven at 65 ± 5 °C for 48 h. Then, the samples were homogenized by grinding with a ceramic mortar and pestle and processed for metals analysis.

2.2. Sample Preparation and Analysis

One gram of ground grain sample was taken in a 25 mL conical flask and kept overnight by adding 10 mL of digestion mixture (2:1 ratio of HNO₃ and HClO₄, respectively). The next day, samples were digested by placing on a hot plate at a temperature of 250 ± 5 °C until a clear solution was obtained. Following digestion, tubes were removed, and the samples were diluted to 20 mL with DI water after cooling. Prior to analysis, all samples were filtered through 0.45 µm filter paper (Whatman No. 41). A flame AAS (Solaar S-100, CiSA) was used for the determination of Cd, Cu, Fe, Mn, Ni, Pb, and Zn in rice samples (AOAC, 1990). Elemental concentrations in rice were determined on a dry weight basis.

2.3. Quality Control

The blank reagents and standard reference materials such as Batch 1701-3, BCR no. 150 and Fluka Kamica, Busch Switzerland were used to verify the accuracy and precision of the digestion procedure and subsequent analyses. Each sample batch was analyzed in triplicate under the standard operating conditions within the confidence limit of 95%. The validity of the method was further ascertained by cross method checks, spiked recovery, and replicate analysis.

2.4. Health Risk Assessment

The potential human health risk assessment was conducted by considering the following parameters according to Onsanit et al. [31]. The estimated weekly intake (EWI) and provisional tolerance weekly intake (PTWI) were jointly established by FAO/WHO [32]. The EWI (mg kg⁻¹ body weight/week) was calculated using the following equation:

$$\mathrm{E}\mathrm{W}\mathrm{I}=\mathrm{C}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{e}\times \frac{\mathrm{W}\mathrm{C}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{e}}{\mathrm{B}\mathrm{W}}$$

where C rice = average trace metal concentrations in rice (mg kg⁻¹ dry weight), WCrice = weekly rice consumption (g week⁻¹) per capita (18 kg/capita/year or ≈50 g per capita per day × 7) (PACRA, 2020), and BW = average body weight (kg) of the Pakistani population (72 kg). The values are expressed in mg week⁻¹ person⁻¹. The calculated EWI values were compared with levels for typical daily exposure and provisional tolerable weekly intake (PTWI) set by the Codex committee on food additives and contaminants of the joint FAO/WHO food standards program for TM.

2.5. Statistical Analysis

XLSTAT 2018 and Origin 2018 were used for the descriptive statistics and further analysis. Data presented as means ± SD. The Pearson correlation heatmap with dendrograms was developed using Origin v2018 (Origin Lab Corp., Northampton, MA, USA).

3. Results

The mean and range of TM concentrations in rice grains from different conventional and non-conventional growing areas are presented in Supplementary Information (SI) Table S1. All the permissible limits set by WHO/FAO are presented in

Table 2. Permissible limits of trace metals in rice grains.

Metals	Minimum Value		Maximum Value		Limits (mg kg−1)	References
	CA *	NCA **	CA *	NCA **
Cd	0	0.10	0.54	0.47	0.2	WHO/FAO [29]
Pb	0.12	0.30	1.10	1.20	0.2
Ni	0	0.01	0.05	0.20	1.0
Fe	25.43	31.11	66.01	74.11	15–50
Cu	1.13	1.01	4.54	3.76	4–15
Zn	12.61	12.74	21.51	19.63	20
Mn	1.05	1.13	4.82	5.66	05

* Conventional Rice Growing Areas; ** Non-Convention Rice Growing Areas.

for reference and comparison to our data presented in this paper. The weekly intake rates of essential and non-essential metals are presented in

Table 3. EWI rate of essential and non-essential metals.

Rice Growing Area	Estimated Weekly Intakes (EWI)
	Essential Metals				Non-Essential Metals
	Cu	Fe	Mn	Zn	Cd	Ni	Pb
Non-conventional	18.27	360.25	27.51	95.42	2.28	0.97	5.83
Conventional	22	321	23.43	105	2.65	0.24	5.34
EWI by WHO	70 *	315 *	77 *	280 *	7	35	25

EWI: estimated weekly intake for toxic elements, based on WHO guidelines. * There is no PTWI set for Cu, Fe, Mn, and Zn, but it is used as MTDI in mg kg⁻¹ body weight in a week).

.

3.1. Trace Metal Concentration in Rice Grains from Non-Conventional Growing Areas

3.1.1. Essential Metals

There were six non-conventional rice growing areas (Bahawalnagar, Chiniot, Jhang, Shorkot, Toba Tek Singh and Faisalabad), from which we collected rice samples. From non-conventional rice-producing areas, the highest Cu, Fe, Mn, and Zn concentrations in the rice samples were recorded having values of 3.76, 74.11, 5.66, and 19.63 mg kg⁻¹, respectively. The average Cu concentrations were 2.16, 1.57, 1.79, 2.16, 2.93, and 3.25 mg kg⁻¹ in Bahawalnagar, Chiniot, Jhang, Shorkot, Toba Tek Singh, and Faisalabad, respectively. Iron (56.38, 40.45, 38.58, 33.90, 36.65, and 50.10 mg kg⁻¹), Mn (2.00, 2.46, 3.32, 2.41, 3.54, and 3.84 mg kg⁻¹), and Zn (16.75, 15.34, 16.72, 16.66, 17.54, and 16.35 mg kg⁻¹) concentrations were recorded, respectively (Figure 2).

3.1.2. Non-Essential Metals

From non-conventional rice-producing areas, the highest Cd, Ni, and Pb concentrations in collected rice samples were recorded having values of 0.47, 0.20, and 1.20 mg kg⁻¹, respectively. The average Cd concentrations were recorded as 0.23, 0.20, 0.31, 0.28, 0.31, and 0.24 mg kg⁻¹ in Bahawalnagar, Chiniot, Jhang, Shorkot, Toba Tek Singh, and Faisalabad, respectively. Average Ni (0.08, 0.06, 0.02, 0.04, 0.01, and 0.01 mg kg⁻¹) and Pb (0.59, 0.88, 0.84, 0.98, 0.92, and 0.87 mg kg⁻¹) concentrations were found in the same order of the rice growing areas, respectively (Figure 3).

3.2. Trace Metal Concentration in Rice Grains from Conventional Rice Growing Areas

3.2.1. Essential Metals

From conventional rice-producing areas, the highest Cu, Fe, Mn, and Zn concentrations in collected rice samples were recorded as 4.54, 66.01, 4.82, and 21.51 mg kg⁻¹, respectively. The average concentrations of Cu were found as 2.12, 2.89, 2.74, 1.97, 3.62, and 2.21 mg kg⁻¹ in Narowal, Gujranwala, Hafizabad, Mandi Bahauddin, Sialkot, and Sheikhupura, respectively. Iron (42.75, 48.31, 34.00, 44.70, 45.19 and 45.81 mg kg⁻¹), Mn (2.53, 2.89, 1.99, 2.76, 4.37, and 3.89 mg kg⁻¹), and Zn (18.69, 14.64, 14.74, 16.51, 15.87 and 16.59 mg kg⁻¹) concentrations were found in the same order of the conventional rice growing areas, respectively (Figure 4).

3.2.2. Non-Essential Metals

From conventional rice-producing areas, the highest Cd, Ni and Pb concentrations in the collected rice samples were recorded as 0.54, 0.05, and 1.10 mg kg⁻¹, respectively. The average concentrations of Cd were found as 0.37, 0.31, 0.09, 0.29, 0.30, and 0.36 mg kg⁻¹ in Narowal, Gujranwala, Hafizabad, Mandi Bahauddin, Sialkot, and Sheikhupura, respectively. Nickel (0.02, 0.01, 0.01, 0.02, 0.01, and 0.02 mg kg⁻¹) and Pb (0.85, 0.82, 0.80, 0.66, 0.78 and 0.81 mg kg⁻¹) concentrations were found in the same order of the conventional rice growing areas, respectively (Figure 5).

3.3. Relation between the Toxic Metals

Table 4. Correlation matrix of metals in rice grains from conventional and non-conventional growing areas.

Metals	Essential Metals				Non-Essential Metals
	Cu	Fe	Mn	Zn	Cd	Ni	Pb
Metal concentrations in rice grains from non-conventional growing areas
Cu	1.00
Fe	0.30	1.00
Mn	0.14	−0.02	1.00
Zn	0.00	0.05	0.00	1.00
Cd	0.34 *	0.08	0.11	0.22 **	1.00
Ni	−0.15	0.47 *	−0.22	0.40 *	−0.21	1.00
Pb	0.23	−0.28	0.30	−0.21	0.35 *	−0.42	1.00
Metal concentrations in rice grains from conventional growing areas
Cu	1.00
Fe	−0.10	1.00
Mn	−0.01	−0.11	1.00
Zn	−0.42	−0.20	0.14 **	1.00
Cd	−0.11	0.27 *	0.23 *	0.18 *	1.00
Ni	−0.18	−0.02	−0.10	0.09	0.30 *	1.00
Pb	0.01	−0.03	−0.19	0.12	−0.03	−0.38	1.00

* Values with single asterisk are statistically significant at p < 0.00; ** values with double asterisk are statistically significant at p < 0.005.

shows the Pearson correlation (R) between essential and non-essential toxic metals found in rice grains from conventional and non-conventional rice growing areas. At both places, the TMs displayed varied responses. In non-conventional areas, Cd had a significant positive effect with Zn (R² = 0.22, p < 0.01) and Cu (R² = 0.34, p < 0.01), whereas Ni had positive relation with Fe (R² = 0.47, p < 0.05), as well as Cd with Pb (R² = 0.35, p < 0.05). In the conventional areas, although the R² showed slight effect, the Zn had a significant positive effect on Mn (R² = 0.14, p < 0.01), Cd had positive relationship with Fe (R² = 0.27, p < 0.01), Mn (R² = 0.23, p < 0.01) and Zn (R² = 0.18, p < 0.01), while Ni also had positive relation with Cd (R² = 0.30, p < 0.01).

3.4. Principal Component Analysis and Heatmap

The first three components of the PCA accumulated 61.7% of the total variance (Figure 6). The first principal component explained 24.4% of the variance and reflected the negative coordination with Pb and positive with Ni. The second principal component explained 19.0% of the variance and reflected a covariation of Zn and Cd with loading factors of 0.75 and 0.67, respectively. The third principal component explained 18.3% of the variance and was linked to Fe and Cu (loading factor 0.78 and 0.56). The biplot shows the overlapping of the confidence eclipse for conventional and non-conventional rice growing areas that means the differences in metal concentrations, with respect to areas, were non-significant. The points near the lines originating from the center depicted higher values as compared to distant points. The heat map (Figure 7) gives the clearer grouping of metals on the basis of the Pearson’s correlation. It can be seen from Figure 5 that Pb and Cu; Mn, Cd, and Zn; and Ni and Fe were grouped together in three clusters.

4. Discussion

4.1. Sources of Metals in Rice Growing Areas of Pakistan

Agricultural soil contaminations with TMs have gradually increased in the last decade due to urban sprawl, population increase, and industrialization [33,34]. It is considered a major global environmental issue because of their persistence and associated risks to both the biotic and abiotic factors of the environment. Trace metals such as Cd, Cu, Fe, Mn, Ni, Pb, and Zn are naturally present in soil through rocks weathering and volcanic eruptions, but their concentrations may be locally elevated as a result of nearby anthropogenic activities such as mining, resource extraction, industrial and vehicular emissions, etc. [35,36]. Additionally, soil preparation, crop growing, and management practices also greatly influence the TM concentration in rice growing areas. In conventional rice growing areas, puddling is done before the rice nursery transplanting. Puddling could increase the bioavailability of TMs to rice crop by enhancing their solubilization [30]. Higher concentrations of TMs in agricultural soils are responsible for their transfer from soil to food crops and grains, leading into long-term risks to the ecosystem. Vegetables, grains, and other essential foods may have a range of toxic and essential trace metals [37,38].

Additionally, TM pollution in rice growing areas is attributed to adjacent industrial sites and their effluent’s use to irrigate agricultural soils in Pakistan, as indicated by Nawaz et al. [39], Azam et al. [40], and Qi et al. [41]. In addition to the use of industrial wastewater for irrigation, some farmers are over-using fertilizers and pesticides recommended for crop growth [42,43]. Phosphatic fertilizers are a rich source of Cd, and their repetitive application results in the magnification of Cd in soil and the edible parts of plants [44,45]. Moreover, submerged conditions during rice growth and development stages result in more mobility of TMs in soil solutions and their translocation to plants’ vegetative and edible parts [46,47].

4.2. Non-Essential Metals in Rice

Cadmium is a metallic element that occurs naturally at low levels in the environment. Food, rather than air or water, represents the major source of Cd exposure [48,49]. The highest Cd concentration in the rice grains was found as 0.54 mg kg⁻¹ (Supplementary Table S1). Cd concentrations in all the rice samples in the present study ranged from 0 to 0.47 mg kg⁻¹. The Cd concentration ranged from 0 to 0.54 in conventional areas, while it was 0.10–0.47 mg kg⁻¹ in non-conventional sites. Thus, the mean Cd concentrations were considerably higher in non-conventional areas as compared to conventional areas. The Cd concentrations in rice samples from non-conventional areas were found higher than the WHO limits in Bahawalnagar and Chiniot (33%), Jhang (100%), 75% of the samples from Shorkot, and 100% samples from Toba Tek Singh and Faisalabad, respectively. These results are supported by the previous work of Dehghani and Mosaferi [50], who found higher Cd and Pb concentrations in imported Pakistani rice grains available in Irani markets and Cd, Cr, Ni, Fe, and Pb in Shakargarh Tehsil, District Sialkot, Pakistan [51]. This is probably due to fact that non-conventional areas receive relatively more contaminated irrigational water as compared to conventional areas. The conventional areas fall in upper Punjab, Pakistan, which has more slopes compared to the non-conventional areas (higher than sea level, due to which the water flow is high, minimizing the chances of water contamination) [52]. Secondly, conventional areas are not densely populated and are mainly agricultural lands [53], thus the risks of water contamination from sewage effluents are rare. In contradiction, the rice in non-conventional areas is mostly grown in the peri-urban areas and regularly receives sewage water for use in irrigation. The maximum concentrations of Cd in rice from conventional areas were found to be higher in 75% samples from Narowal, 100% from Gujranwala, 25% from Hafizabad, and 100% in Mandi Bahauddin, Sialkot, and Sheikhupura. Khan et al. [33] reported similar results of TM (Cd, Pb and Ni) contaminations in the Khyber Pakhtunkhwa Province of Pakistan. Nickel concentrations in rice grains from conventional areas of Punjab, Pakistan were 0–0.05 and 0.01–0.05 mg kg⁻¹ in non-conventional areas. All the samples collected from conventional and non-conventional areas remained within the permissible limits (1.0 mg kg⁻¹) of the WHO. Lead ranged from 0.12 to 1.10 and 0.30 to 1.20 mg kg⁻¹ in conventional and non-conventional areas, respectively. All the samples from non-conventional areas exceeded Pb concentration limits in rice prescribed by WHO (except one sample from Mandi Bahauddin), indicating a significantly higher Pb pollution level in the Punjab. The higher Pb levels could be due to the aerial deposition of Pb from vehicular emissions and their subsequent entry to rice grains through plant tissues and mixing in soil solution and uptake [54].

4.3. Essential Metals in Rice

Copper concentration in rice grains was found highest (4.54 mg kg⁻¹) in conventional areas (Figure 4). A previous study by Xu et al. [55] reported a mean Cu concentration in Chinese rice of up to 18 mg kg⁻¹, which is considerably higher than that found in the present study. Mean Mn concentrations in the conventional and non-conventional areas were 43.57 and 44.03 mg kg⁻¹, respectively. It was notable that Mn concentrations in the rice samples exceeded permissible limits. Roychowdhury [56] reported a mean Mn concentration of 9.9 mg kg⁻¹ in rice in West Bengal, India. The mean Zn concentrations in rice in the present study did not differ significantly for different areas. The highest mean Zn concentration (16.42 mg kg⁻¹) in rice grains was found in the conventional area, which was comparable to that in the non-conventional areas’ rice grains 16.41 mg kg⁻¹.

4.4. Health Risk Assessment

As peer reviewed data on all the parameters used for the health risk assessment of rice in this study are not available, we could not estimate the health risks associated with the consumption of essential metals (Cu, Fe, Mn, and Zn), but we calculated the health risks associated with consumption of non-essential metals (Cd, Pb, and Ni) through comparing PTWI values devised by the FAO/WHO. The same procedures were used by Naseri et al. [57] and Ghoreishy et al. [58] in Iran. Based on the results, it was recorded that the EWI of the samples from both areas fell within the permissible limits, except for Fe, whose EWI limits exceeded the MTDI in the conventional as well as non-conventional areas (Table 3).

5. Conclusions

In this study, most of the mean metal concentrations (Cd, Pb, Fe, Zn, and Mn) were found to be substantially higher in conventional rice growing areas, followed by non-conventional areas. Specifically, the mean Cd concentrations were considerably higher in rice grains collected from non-conventional areas compared to conventional rice growing areas. The Cd, Pb, and Ni concentrations in rice samples from non-conventional areas were found higher than the WHO limits in the 70, 100, and 0% collected samples, respectively. In conventional areas, Cd, Pb, and Ni concentrations were found higher than WHO limits in the 77, 99, and 0% samples, respectively. A health risk assessment indicated that all the EWI and/or MTDI values were found within the permissible values set by the WHO due to less rice consumption by Pakistani people, except for Fe in both areas.