Introduction

Rice (Oryza sativa) is one of the world’s most widely consumed foods, particularly in Southeast Asia as it acts as an important source of energy, and nutrients and has a favorable taste (Rittirong and Saenboonruang 2018). Rice grain contains high content of carbohydrates, trace amounts of protein, and fat, including Omega-3 and Omega-6 fatty acids that help to regulate the blood pressure and inflammatory response in the cell membranes (Rittirong and Saenboonruang 2018). Southeast Asia is a significant rice producer and supplier to other countries worldwide, including Africa and the Middle East, representing approximately 26 and 40% of global rice production and exports. This region is also well-known for its high level of domestic consumption. For example, in Malaysia, each person consumes approximately 80 kg of rice per year, which accounts for roughly 26% of the total daily caloric intake and costs an estimated RM44 per month per household (Abdul Rahim 2016). In total, approximately 2.7 million metric tonnes of rice were consumed in Malaysia, of which about 67% was produced locally. The remainder was imported primarily from Thailand, Vietnam, and Pakistan (Accountant General of Malaysia 1990-2017).

The recorded increases in rice consumption and production are attributable to the rapidly expanding global population, the technological revolution, and the enhancement of rice nutrients in rice (Mohanty et al. 2013). Due to these factors, all the farmers in every country are urged to increase their production of rice either by improving the technologies and/or use of chemical fertilizers. The intensified application of agrochemicals, alongside wastewater irrigation, application of manures, rapid urbanization, industrialization, and mining activities are putting pressure on the environment and causing metal(loid)s pollution that has become a global concern (Singh et al. 2011; Arunakumara et al. 2013; Krishna et al. 2013; Hamilton et al. 2018; Gurtler et al. 2020). Iron (Fe), copper (Cu), selenium (Se), and cobalt (Co) are trace metals that are essential to both plants and animals (WHO 1996). These metals except for Se were collectively linked to anaemia in the human body. However, they can also exert toxicity in high doses. On the other hand, arsenic (As), chromium (Cr), and nickel (Ni) were classified as toxic metal(loid)s. Nickel and Cr were linked to symptoms like growth depression and diabetes respectively, but their essentiality is still in contention. Cadmium (Cd) was addressed as a soft acid chemical known for only its negative effects. Unlike organic pollutants, metals (loid)s are non-degradable in nature and persist in the soil and water bodies for a long time (Kabir et al. 2012; Lodenius 2013; Singh et al. 2018; Ali et al. 2019). Metal(loid)s can also bioaccumulate in crops that are planted on contaminated soil and pose serious health problems to consumers (Rittirong and Saenboonruang 2018; Sabir et al. 2019). Moreover, the addition of fertilizers and other agrochemicals that contain toxic metal(loid)s during cultivation can increase the amounts of metal(loid)s accumulated in the rice grain even if the rice grain is coming from non-contaminated lands (Ali et al. 2020).

The paddy plant requires a significant amount of water for its cultivation from seed germination until harvesting (Arunakumara et al. 2013). This cultivation method allows the rice grain to uptake and absorb the environmentally available metal(loid)s and bioaccumulate throughout its growth. Consumers who dispute their accessibility to safe rice consumption may become concerned about food safety and security issues related to metal pollution. Toxic metal exposure may be present in those who regularly and entirely depend on rice as their main food source (Solidum et al. 2012). Long-term consumption of an excessive amount of food containing toxic metals poses a threat to human health, including renal disease, respiratory issues, bone damage, cancer, and mutation (Rittirong and Saenboonruang 2018). Previous studies found that mercury (Hg), Cd, As, Cr, thallium (TI), and lead (Pb) were present in rice grain (Zazouli et al. 2010; Juen et al. 2014; Davis et al. 2017). They are also toxic and poisonous at low concentrations (Zoroddu et al. 2019).

Rice research is still inadequate in Malaysia. Most of the studies focused on the uptake of heavy metals in paddy-soil systems (Alrawiq et al. 2014; Payus et al. 2015; Zulkafflee et al. 2019; Zulkafflee et al. 2020; Sibuar et al. 2022; Zulkafflee et al. 2022). Existing studies on raw rice either focused on bioaccessibility or only analyzed a few types of metals (Omar et al. 2015a; Omar et al. 2015b; Abd Rashid et al. 2018; Gee et al. 2019). There is still a lack of baseline data on metal(loid)s contamination in various types of rice. As a result, this study is to determine the concentration of toxic and essential metal(loid)s (As, Cd, Fe, Se, Cu, Cr, Co, and Ni) in various types of rice samples (white, brown, basmati, fragrant, and glutinous rice) that are commonly consumed by Malaysians using the Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and to assess the potential human health risk. The findings of this study may shed light on the extent of existing metal(loid)s pollution for different types of rice sold commercially in Malaysia. The information would also be helpful to some relevant agencies in monitoring and evaluating food security and safety issues, notably, those involving metal contamination and the associated health risk of eating rice.

Material and Methods

Chemicals and reagents

All solutions were prepared with analytical reagent grade chemicals and ultrapure water (18 MΩ.cm) obtained by using a water purification system (arium® 611VF Ultrapure Water System, Sartorius Stedim, Germany) was used throughout the experimental process. Hydrogen peroxide (H2O2, 30%, Sigma Aldrich, Germany), and nitric acid (HNO3, ~65%, for analysis EMSURE® Reag. Ph Eur,ISO) were obtained from Merck KGaA, Darmstadt, Germany. For the calibration curve, the ICP-MS calibration standard, 10 μg/mL Multi-Element Calibration Standard 3 (PerkinElmer Pure Plus, USA) was used for the preparation of the standard solution. All stock solutions were stored in polyethylene bottles and kept in a fridge (4°C). The calibration solutions were prepared daily for analysis and the calibration curves for all examined metal(loid)s were established using these diluted standard solutions. For quality assurance, each batch of samples comprised a reagent blank and a standard reference material (SRM) to ensure the accuracy of the analysis. The SRM (1568b, rice flour) obtained from the National Institute of Standards and Technology (NIST), USA was subjected to the same analysis method employed for the rice samples to maintain consistency and comparability.

Sample collection and preparation

A total of 15 different rice samples, encompassing five types of rice (white, brown, basmati, glutinous, and fragrant rice), were purchased through random sampling, with each type of rice being represented by three different brands from local markets (Table 1). Firstly, the rice samples were washed with deionized water. Then, about 10 g of the sample was dried in the oven (OF-11E, JeioTech, Korea) at 65°C until constant weight. The dried rice samples were then crushed with a pestle and mortar, sieved through a 0.25 mm mesh, and placed in an acid-washed plastic zip-lock before acid digestion.

Table 1 Details of rice samples
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The samples were acid digested following USEPA Method 3050B as this method is suitable for environmentally available metal(loid)s like As, Cd, Fe, Se, Cu, Cr, Co, and Ni. Briefly, about 1.0 g rice sample was weighed and placed into a digestion tube. Then, 5 mL water and 5 mL concentrated nitric acid (HNO3) were added to the digestion tube (Juen et al. 2014). The digestion tube was heated without boiling around 95°C to 100°C on a block digester (Shimaden SRS12A) for 15 minutes and was covered with a crucible. After cooling for 2 to 3 minutes, 5 mL of concentrated HNO3 was added to the sample before heating for another 30 minutes. The procedure was repeated by adding 5 mL of HNO3 until the sample no longer emitted brown fumes. The sample was then heated for two hours at 95°C to 100°C without boiling before being allowed to cool, and 2 mL of water and 3 mL of 30% hydrogen peroxide (H2O2) were added. First, the beaker was covered and slowly heated. Then, a 1 mL aliquot of 30% H2O2 was added, and the heating process was repeated until the effervescence subsided. Next, the sample was heated for 2 hours at 95°C to 100°C without boiling. The digestion tube was covered with the crucible throughout the entire procedure to prevent metals from evaporating into the atmosphere. After that, the sample was cooled and filtered through a 0.45 μm syringe filter membrane into a 100 mL volumetric flask. Lastly, ultrapure water was added to the mark and the sample was analyzed by using Inductively Coupled Plasma-Mass Spectrometry, ICP-MS (ELAN DRC-e, Perkin Elmer, Shelton, CT, USA). The operational parameters applied are listed in Table 2.

Table 2 ICP-MS Operating Conditions
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Quality Assurance (QA) and Quality Control (QC)

Samples were collected in triplicate to estimate the variability resulting from the sampling activities. Blank and 10 μg/mL Multi-Element Calibration Standard 3 (PerkinElmer Pure Plus, USA) were used to calibrate the instrument. As a control, a blank solution was prepared and analyzed alongside the samples. The measurements were performed in triplicate (n=3). The instrument was calibrated daily, and the r2 value of the calibration curve was greater than 0. 999. The limit of detection (LOD) (Equation 1) and the limit of quantification (LOQ) (Equation 2) were calculated to determine the sensitivity of the instrument. LOD and LOQ are the concentration of the analyte corresponding to sample blank values plus three and ten standard deviations, respectively.

$$\mathrm{LOD}=\frac{3\times \mathrm{SDblank}}{\mathrm{b}}$$
(1)
$$\mathrm{LOQ}=\frac{10\times \mathrm{SDblank}}{\mathrm{b}}$$
(2)

where, SDblank stands for the standard deviation of blank samples and b is the slope of the calibration curve.

LOD for As, Cd, Fe, Se, Cu, Cr, Co, and Ni 0.000005, 0, 0.004057, 0.000132, 0.000001, 0.000023, 0, and 0.000003 μg/kg respectively; while LOQ for As, Cd, Fe, Se, Cu, Cr, Co and Ni obtained were 0.000015, 0, 0.013525, 0.000441, 0.000004, 0.000075, 0, and 0.000009 μg/kg respectively. The accuracy of the method was verified with standard reference materials (SRM) of rice flour (SRM 1568b) obtained from the National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA. The recovery (%) was calculated (Equation 3). The measured values, certified values, and recovery values of the metal(loid)s from SRM 1568b are presented in Table 3, and they showed an acceptable recovery percentage of the metal(loid)s in SRM 1568b.

$$\mathrm{Recovery}\ \left(\%\right)=\frac{\left(\mathrm{Actual}\ \mathrm{value}\right)}{\left(\mathrm{Certified}\ \mathrm{value}\right)}\times 100\%$$
(3)
Table 3 Recovery percentage for SRM 1568b (Rice Flour)
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Statistical analysis

The data of metalloid concentration in rice samples violated the normality assumption. Therefore, the non-parametric Kruskal-Wallis Test was executed to measure the variations among different rice types at the 5% level of significance. The statistical analysis was performed using SPSS version 28 (IBM, USA) software.

Estimation of potential human health risk

Estimated weekly intake

Approximately 80 kg of rice is consumed annually by a Malaysian weighing 64 kg on average (Anual et al. 2018). This results in a daily consumption of 0.219 kg and a weekly consumption of 1.534 kg of rice. To estimate the weekly intake of various metal(loid)s through rice consumption, a well-established method was employed (Kukusamude et al. 2021). The estimated weekly intake (EWI) was compared to the World Health Organization/Food and Agriculture Organization (FAO/WHO 1984; FAO/WHO 1989) expert committee's provisional tolerance weekly intake (PTWI). The EWI (μg kg-1 BW week-1) was calculated (Equation 4).

$$\mathrm{EWI}={\mathrm{C}}_{\mathrm{rice}}\times \frac{\mathrm{WC}}{\mathrm{BW}}$$
(4)

Where, Crice is the average metal content in rice (mg kg-1 dry weight), WC is the weekly rice consumption per person (g week-1) per capita for the Malaysian population (1534 g week-1), and BW is the average body weight (kg) of the Malaysian population.

Target hazard quotient (THQ)

The noncarcinogenic risks from the consumption of the studied rice samples by the Malaysian population were assessed based on the target hazard quotient (THQ) (Equation 5). The target hazard quotient (THQ) is defined as the ratio of exposure to the toxic element and the reference dose (RfD) which is the highest level at which no adverse health effects are expected. The reference dose is specific to the trace element being assessed. If the THQ is <1 then non-carcinogenic health effects are not expected. Meanwhile, if the THQ is >1 then there is a possibility that adverse health effects could be experienced. The THQ was estimated using the United States Environmental Protection Agency (USEPA) methodology based on the Region III risk-based concentration table. The RfD for As, Cd, Cr, and Ni is 3x10-4 mg/kg-day, 1x10-3 mg/kg-day, 3x10-3 mg/kg-day, and 2x10-2 mg/kg-day respectively (USEPA 2017).

$$\mathrm{THQ}=\frac{\mathrm{EF}\times \mathrm{ED}\times \mathrm{IR}\times \mathrm{C}}{\mathrm{BW}\times \mathrm{AT}\times \mathrm{RfD}}$$
(5)

where, EF is the exposure frequency (365 days/year), ED is the exposure duration for an adult (30 years), and IR is the rice ingestion rate (0.210 kg per person/day). C is the concentration of the element in rice (mg/kg), BW is the average body weight of the adult population (61.75 kg), AT is the averaging time (days) which is 30 years × 365 days for non-carcinogenic effects and 70 years × 365 days for carcinogenic effects and RfD is the reference RfD is the oral reference dose (mg/kg-day).

Hazard Index (HI)

The hazard index (HI) is the sum of the individual target hazard quotients of the metal(loid)s assessed for each rice sample. Generally, the HI assumes that the consumption of a particular food type would result in simultaneous exposure to several potentially toxic metal(loid)s. Even if individual THQs for the metal(loid)s in the food item are lower than unity, individually the cumulative effect of consumption may result in adverse health effects. If the HI is >1 there is the potential for adverse non-carcinogenic health effects. An HI value of 1 implies no health risk, while an HI value between 1 and 5 denotes a moderate health risk and a higher than 5 indicates a significant health risk. The HI was calculated as per Equation (6).

$$\mathrm{HI}=\sum {\mathrm{THQ}}_{\mathrm{As}}+\sum {\mathrm{THQ}}_{\mathrm{Cd}}+\sum {\mathrm{THQ}}_{\mathrm{Cr}}+\sum {\mathrm{THQ}}_{\mathrm{Ni}}$$
(6)

where, ∑THQAs is the average cumulative THQ for As, ∑THQCd is the average cumulative THQ for Cd, ∑THQCr is the average cumulative THQ for Cr and ∑THQNi is the average cumulative THQ for Ni.

Cancer risk

The carcinogenic risk of As was calculated using the Incremental Lifetime Cancer Risk (ILCR) as per Equation (7) (Sharafi et al. 2019). The lifetime duration used by EPA to characterize lifetime cancer risk is 70 years, consistent with the assumption that the risk of developing cancer continues even after exposure has stopped. It is also equivalent to the duration over which health effects are typically assessed in chronic studies of laboratory animals. Therefore, 70 years has remained the standard definition of “lifetime” even as human life expectancy has increased (USEPA 2011). Acceptable carcinogenic risk levels vary from 10-4 (the probability of acquiring cancer in a human lifetime is 1 in 10,000) to 10-6 (the risk of developing cancer over a human lifetime is 1 in 1 000 000) (USEPA 2001). The cancer slope factor (CSF) was obtained with the reference to USEPA Integrated Risk Information System (IRIS).

$$\mathrm{C}\mathrm{R}=\frac{\mathrm{EF}\times \mathrm{ED}\times \mathrm{IR}\times \mathrm{C}\times \mathrm{C}\mathrm{SF}}{\mathrm{BW}\times \mathrm{AT}\ }$$
(7)

where, EF is the exposure frequency (365 days/year), ED is the exposure duration for an adult (30 years), and IR is the rice ingestion rate (0.210 kg per person/day). C is the concentration of the element in rice (mg/kg), BW is the average body weight of the adult population (61.75 kg), AT is the averaging time (days) which is 70 years × 365 days for carcinogenic effects while CSF is the cancer slope factor (mg/kg-day)-1. For As, the oral CSF has been reported to be 1.5 mg/kg-day (USEPA 2017). For Cd, Cr and Ni it is 15, 0.50 and 0.91 mg/kg-day, respectively. In general, an ILCR value of less than 10−6 suggests negligible health risk, a value between 10−6 and 10−4 reveals moderate risk and a value more than 10−4 is unacceptable and indicates serious health hazards to humans (USEPA 2001).

Results and Discussion

Comparison of metal concentration in rice samples with the national and international maximum permissible limits

The mean metal(loid)s concentration in all studied rice samples was compared with the maximum permissible limits (MPLs) for human consumption set by a national and international organization. It is important to highlight that in accordance with CODEX General Standard for Contaminants and Toxins in Food and Feed, there is no allowable limit specified for Fe, Se, Cu, Cr, Co, and Ni other than the limits for As and Cd (Fig. 1). The mean concentrations of As in all rice samples are listed in Table 5. The As levels recorded in brown rice BR 3 were within the maximum permitted proportion in Malaysian Food Regulations Food 1985 (1.000 mg/kg). Contrastingly, the As concentrations in all rice samples except for the As concentration in brown rice (BR 1, BR 2 and BR 3) and white rice (WH 1 and WH 3) exceeded the maximum limit established by the CODEX General standard for contaminants and toxins in food and feed (0.300 mg/kg) (Fig. 1). For Cd concentration, the present study found that all types of rice and their respective brands were within the maximum limit provided by Malaysian Food Regulations Food 1985(0.400 mg/kg) and FAO/WHO Codex Alimentarius International Food Standard (0.400 mg/kg) (Fig. 1).

Fig. 1
figure 1

Distribution of (a) As and (b) Cd in different types of rice (brown, white, fragrant, basmati, and glutinous) compared with the permissible limit by Malaysian Food Regulations Food 1985 and FAO/WHO Codex Alimentarius Food Standard

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The correlation analysis of the selected metal(loid)s (As, Cd, Cu, Se, Cr, Co, Ni, and Fe) revealed significant relationships and potential associations (Table 4). Notably, a strong positive correlation was observed between As and Fe (r = 0.800, p < 0.01), suggesting a potential co-occurrence or affinity between these elements in the studied system. This finding could have important implications for understanding the transport, fate, and bioavailability of both As and Fe in the environment. Higher correlation coefficients between the metals indicated common sources, mutual dependence, and similar or nearly identical metal accumulation properties in rice (Kormoker et al. 2020).

Table 4 Spearman’s correlation of metal(loid)s in rice (n=45)
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Additionally, positive correlations were observed between As and Cu (r = 0.515, p < 0.05) as well as As and Cr (r = 0.757, p < 0.01). These findings suggest potential associations or shared chemical characteristics between As and these metals. In general, Cu in the crop soils is largely associated with the Fe–Mn oxide phase, followed by the organic/sulfide and exchangeable fractions which might have caused its dissociation in porewater under flooded conditions (Liao et al. 2016).

Conversely, negative correlations were detected between As and Cd (r = -0.277, p > 0.05) and As and Ni (r = -0.452, p >0.05). These negative correlations indicate potential repulsion or antagonistic effects between As and these metals. Both As and Cd has opposite biogeochemistry under the flooded circumstance of paddy fields with a low Eh and high pH. Soil Cd availability could be decreased by the precipitation of Cd2+ with OH− and S2− while soil As availability and toxicity could be increased by the competitive adsorption of OH− on the soil matrix, dissolution of Fe oxides-adsorbed As, and reduction of As (V) into As (III). This in turn affects the bioaccumulation and bioavailibilty of As and Cd in rice grains. (Tian et al. 2023).

Metal(loid)s concentration in various rice samples (brown, white, fragrant, basmati, and glutinous rice)

Arsenic

Based on the results, the concentration of As ranged from 0.470 to 1.010 mg/kg in brown rice, 0.270 to 0.420 mg/kg in white rice, 0.260 to 0.270 mg/kg in fragrant rice, 0.110 to 0.160 mg/kg in basmati rice, and 0.130 to 0.210 mg/kg in glutinous rice, respectively (Table 5). The overall mean As concentration was 0.352 ± 0.284 mg/kg with a range of 0.110-1.010 mg/kg. The As concentrations found in five different rice types were significantly different (p<0.05). There were statistical differences in the concentration of As between basmati rice and brown rice (p<0.05). In comparison to other types of rice, such as brown, white, fragrant, and glutinous, basmati rice was deemed to have a low As concentration (0.110 to 0.160 mg/kg) and can be regarded as suitable for consumption. The presence of a metallothionein-like protein composed of a sulfur-containing amino acid (cysteine) in basmati rice may have reduced the accumulation of As as sulfur plays an important role in the regulation of biosynthesis of thiol compounds that is necessary to alleviate As toxicity (Mahajan et al. 2018; Liu et al. 2022). Besides that, the mean concentration of As in white rice in the present study (0.363 ± 0.047 mg/kg) was higher than those reported in other studies from Argentina (0.230 ± 0.204 mg/kg) and Jamaica (0.200 ± 0.09 mg/kg) (Londonio et al. 2019; Antoine et al. 2012). The mean concentration of As in brown rice (0.823 ± 0.177 mg/kg) was five times higher than the brown rice sold in the Jamaican market (0.165 ± 0.056 mg/kg) and three times higher compared to the husked rice samples from Lebanon market (0.240 ± 0.110 mg/kg) but was comparable with brown rice samples collected from the state of Sao Paulo (0.660 mg/kg) (Akoury et al. 2023; Londonio et al. 2019; Mataveli et al. 2016). Generally, brown rice retains higher amounts of toxic metals due to its germ layer which remains intact as it does not undergo a polishing process (Mridha et al. 2022).

Table 5 Mean metal(loid)s concentration (mg/kg) in different types of rice (brown, white, fragrant, basmati, and glutinous) (n=45)
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Chronic exposure to As can trigger skin disorders, cardiovascular diseases, diabetes, and different types of cancers like skin, lung, kidney and bladder (Rehman et al. 2021). The As content in glutinous rice (0.173 ± 0.023 mg/kg) from this study was relatively lower compared to the mean concentration of different types of glutinous rice analyzed in Japan with Bob’s Red Mill Organic Sweet Rice (0.270 ± 0.040 mg/kg), Pacific Sweet Rice (0.300 ± 0.020 mg/kg), and Koda Sweet Rice (0.150 ± 0.050 mg/kg) (Sadiq et al. 2017). Similarly, another study also found lower As concentrations in two different glutinous rice, white (0.079 ± 0.010 mg/kg) and black (0.029 ± 0.020 mg/kg) (Parengam et al. 2010). However, no report has discussed the pathway for metal(loid)s accumulation in glutinous rice (Jo and Todorov 2019; Cano-Lamadrid et al. 2020).

Cadmium

In this study, the Cd concentration in brown rice ranged from (0.010-0.170 mg/kg) followed by glutinous rice (0.020 to 0.040 mg/kg), basmati rice (0.020-0.040 mg/kg), white rice (0.010-0.010 mg/kg), and fragrant rice (0.000- 0.010 mg/kg) (Table 5). The overall mean Cd concentration was 0.029 ± 0.041 mg/kg with a range of 0.000 to 0.170 mg/kg. The Cd concentrations found in five different rice types were significantly different (p < 0.05). Even though none of the rice varieties tested were above the maximum Cd concentration limit, it is still important to be cautious when eating rice because excessive Cd exposure can have harmful effects on the skeletal, urinary, reproductive, cardiovascular, central, and peripheral nervous systems, as well as the respiratory system. When Cd poisoning is present for a prolonged period, certain cases might result in mortality (Rafati Rahimzadeh et al. 2017). The range of Cd in the glutinous rice in the present study was lower compared to the Khaowong Kalasin sticky rice (0.003-0.092 mg/kg) (Kukusamude et al. 2021). On top of that, the concentration of Cd in white rice (<0.080 ± 0.040 mg/kg) from the Jamaican market was higher than in the present study (0.010 ± 0.000 mg/kg) while the brown rice samples (<0.082 ± 0.033 mg/kg) were slightly higher than the present study (0.073 ± 0.049 mg/kg). This may be since Jamaican soils have naturally elevated Cd concentrations. Similarly, white rice found in the Iran market also recorded high Cd concentrations in their domestic rice samples namely Tarom (0.340 ± 0.010 mg/kg), Kohmare (0.270 ± 0.050 mg/kg) and (Kamfirooz (0.410 ± 0.020 mg/kg) (Naseri et al. 2015). Rice samples from Lebanon (0.290 ± 0.130 mg/kg) and UAE (0.070 ± 0.040 mg/kg) had an overall mean Cd concentration higher than the present study (Akoury et al. 2023). The Cd concentration in fragrant and basmati rice samples was less reported in the literature. The existing studies reporting Cd concentration in these rice samples were of cooked rice which may have altered metal(loid)s concentration compared to raw rice samples as washing and cooking have been reported to change the metal(loid)s composition in rice (Aguilera-Velázquez et al. 2023; Praveena and Omar 2017; Omar et al. 2015a; Naito et al. 2015; Mihucz et al. 2010).

Nickel

Presently, Ni concentrations in brown rice, white rice, fragrant rice, basmati rice, and glutinous rice were in the ranges of 0.200-0.340 mg/kg, 0.010-0.530 mg/kg, 0.310-0.870 mg/kg, 0.380-0.430 mg/kg, and 0.350- 0.440 mg/kg, respectively. The Ni concentrations in all rice varieties examined in this study ranged from 0.010-0.870 mg/kg (Table 5) with an overall mean of 0.381 ± 0.183 mg/kg. There were no significant differences in average Ni concentrations in the five types of rice samples (p > 0.05) (Fig. 2). The range of Ni concentration in the present study was reportedly within the ranges in a previous study (0.120-1.260 mg/kg) (Kukusamude et al. 2021). Apart from that, rice samples from Iran also reported lower Ni levels that ranged from 0.720-0.790 mg/kg. A local study however, recorded higher concentration of Ni in the range of 1.850-2.680 mg/kg (Abd Rashid et al. 2018). Another study in China recorded Ni concentration in the range of 0.001-4.310 mg/kg in rice grains because of natural soil concentrations (He et al. 2019). Besides background concentration in agricultural soil, Ni may also accumulate in rice grains because of food processing equipment (Londonio et al. 2019). The absence of a clear tolerable limit for Ni in rice recommended by WHO is noteworthy. Considering the high Ni concentration reported in this study, understanding its interaction and accumulation in rice, and evaluating the health risks associated with it are therefore of utmost importance.

Fig. 2
figure 2

Mean distribution of metal(loid)s (mg/kg) (a) arsenic, (b) cadmium, (c) iron, (d) selenium, (e) copper, (f) chromium, (g) cobalt, and (h) nickel in different types of rice (brown, white, fragrant, basmati, and glutinous)

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Iron

At present, Fe concentrations were in the ranges of 88.870-121.030 mg/kg for brown rice, 13.970-56.120 mg/kg for white rice, 16.200-42.100 mg/kg for fragrant rice, 9.290-26.330 mg/kg for basmati rice and 6.330- 42.090 mg/kg for glutinous rice, respectively (Table 5). The mean Fe concentrations were 104.120 mg/kg, 39.800 mg/kg, 25.623 mg/kg, 16.000 mg/kg, and 21.300 mg/kg for brown, white, fragrant, basmati, and glutinous rice respectively. The overall mean concentrations for Fe in the rice samples were 41.369 ± 31.365 mg/kg with a range of 6.330-121.030 mg/kg. There were no significant differences in the average Fe concentrations in the five types of rice samples (p >0.05) (Fig. 2). The mean concentration of Fe in brown rice samples from the present study was higher (104.120 ± 9.321 mg/kg) when compared to brown rice samples from Qatar (23.300 ± 8.300 mg/kg), Jamaica (20.100 ± 7.770 mg/kg) and India (39.400 ± 17.600 mg/kg) (Shraim et al. 2022; Halder et al. 2020; Antoine et al. 2012). According to Jo and Todorov (2019), the loss of roughly 60% of the Fe during polishing along with the removal of the germ and bran accounts for the higher Fe content in brown rice compared to white rice. However, the mean Fe concentration in the present study was lower compared to those observed in rice grains from Pakistan (555 ± 1366 mg/kg) entirely because the concentration of Fe in soils was high (6623 ± 4958 mg/kg) which facilitated its accumulation (Natasha, and I., Niazi, N. K., Shahid, M., Ali, F., Masood ul Hasan, I., Rahman, M. M., Younas, F., Hussain, M. M., Mehmood, T., Shaheen, S. M., Naidu, R.,, and Rinklebe, J. 2022). Another important fact is that Fe shows higher mobility under reducing conditions under which rice is typically cultivated which further reiterates the high concentration of Fe in all the rice samples (Halder et al. 2020; Baruah et al. 2021).

Selenium

In the present study, Se concentrations in brown rice were in the ranges of 0.050-0.100 mg/kg, 0.020-0.100 mg/kg in white rice, 0.020-0.110 mg/kg in fragrant rice, 0.100-0.180 mg/kg in basmati rice and 0.020-0.050 mg/kg in glutinous rice, respectively (Table 5). The mean Se concentrations for brown rice, white rice, fragrant rice, basmati rice, and glutinous rice were 0.077 ± 0.014 mg/kg, 0.063 ± 0.023 mg/kg, 0.067 ± 0.026 mg/kg, 0.133 ± 0.024 mg/kg and 0.030 ± 0.010 mg/kg, respectively. The overall mean concentrations for Se in the rice samples were 0.074 ± 0.046 mg/kg with a range of 0.020-0.180 mg/kg. There was no significant difference in the average Se concentrations in the rice samples investigated in this study (p >0.05) (Fig. 2). The Se content in rice may be attributed to the quantity of Se present in the soil and water where it was grown or reared (Chang et al. 2019). While glutinous rice may be farmed in most Asian nations, including Vietnam and Thailand, basmati rice was grown and imported mostly from India and Pakistan (Omar et al. 2019; Mahajan et al. 2018; Sattaka et al. 2017). Therefore, because of soil and water contamination in Pakistan and India, the level of Se is higher in basmati rice than in glutinous rice in the present study (Daud et al. 2017). The overall mean Se concentrations in the present study are lower compared to the overall mean in rice grains from India (0.280 ± 0.200 mg/kg) and Qatar (0.260 ± 0.288 mg/kg) (Halder et al. 2020; Rowell et al. 2014). Alternatively, rice grains from Sri Lanka reported a mean Se concentration of 0.025 mg/kg which were lower compared to the present study (Diyabalanage et al. 2016). Singapore also recorded similar Se concentrations in the ranges reported by previous countries with a median of 0.232 and 0.091 mg/kg for polished and husked rice samples (Pedron et al. 2021). Meanwhile, Iran reported average Se concentrations of 0.108 ± 0.066 mg/kg and 0.131 ± 0.057 mg/kg in white and brown rice, respectively (Naseri et al. 2015).

Copper

The average concentrations of Cu in brown rice, white rice, fragrant rice, basmati rice, and glutinous rice were 16.200 ± 9.532 mg/kg, 3.557 ± 0.732 mg/kg, 4.767 ± 0.393 mg/kg, 4.037 ± 0.124 mg/kg and 3.977 ± 0.883 mg/kg, respectively (Table 5). The overall mean for Cu in the five types of rice samples is 6.507 ± 8.056 mg/kg. The range of Cu concentration across all rice types was between 2.800-35.220 mg/kg. There was no significant difference between Cu concentration and the investigated rice samples (p >0.05) (Fig. 2). The average Cu concentration in rice grains from Pakistan (21.000 ± 16.000 mg/kg) was higher while the average Cu concentration in rice grains from India (3.260 ± 0.920 mg/kg) was lower compared to the present study (Natasha, and I., Niazi, N. K., Shahid, M., Ali, F., Masood ul Hasan, I., Rahman, M. M., Younas, F., Hussain, M. M., Mehmood, T., Shaheen, S. M., Naidu, R.,, and Rinklebe, J. 2022; Halder et al. 2020). The average Cu concentration in brown rice samples from Qatar (3.110 ± 0.780 mg/kg) and Spain (3.610 mg/kg) were lower compared to the present study (Shraim et al. 2022; Cano-Lamadrid et al. 2020). The average Cu concentration of the glutinous rice in the present study is higher compared to the Kalasin sticky rice from Thailand (1.510 mg/kg) (Kukusamude et al. 2021) From the previous study by Govarethinam (2014), the Cu concentration in white rice (0.700 ± 0.100 mg/kg) and in brown rice (1.000 ± 0.100 mg/kg) was lower compared to the current study. The accumulation of Cu in rice grains is affected by the interaction between genotype, harvest season, and processing. Polishing causes a significant reduction in Cu concentration in rice grain which explains the higher Cu concentration in brown rice types compared to the other rice types analyzed in this study (Hensawang et al. 2020). It was estimated that approximately a 30% reduction in Cu concentration can be observed with the removal of the bran however Cu may also diffuse into the endosperm which may be the reason for the high concentration observed in basmati and fragrant rice (Yao et al. 2020; Oliveira et al. 2021). The Cu concentration in this study was higher than the Food and Agriculture Organization (FAO) threshold of 3 mg/kg for edible plants across all rice types, suggesting possible contamination.

Chromium

Based on the results, the concentration of Cr ranged from 1.630 to 2.770 mg/kg in brown rice, 1.640 to 2.790 mg/kg in white rice, 1.600 to 7.450 mg/kg in fragrant rice, 0.140 to 1.100 mg/kg in basmati rice, and 0.070 to 1.420 mg/kg in glutinous rice, respectively (Table 5). The total average Cr concentration was 1.909 ± 1.768 mg/kg with a range of 0.070-7.450 mg/kg. The Cr concentrations were significantly different in the five different rice types (p<0.05) (Fig. 2). Besides that, the mean concentration of Cr in white rice in the present study (2.160 ± 0.336 mg/kg) was higher than those reported in other studies from Iran (0.390 ± 0.030 mg/kg) and Jamaica (0.080 ± 0.045 mg/kg) (Naseri et al. 2015; Antoine et al. 2012). The mean concentration of Cr in brown rice (2.353 ± 0.363 mg/kg) was fifteen times higher than the brown rice sold in the Jamaican market (0.157 ± 0.136 mg/kg). The fragrant rice samples in this study have the highest mean Cr concentrations (3.613 ± 1.919 mg/kg) compared to brown rice. When comparing the overall mean of the rice samples with countries such as Lebanon (0.340 ± 0.130 mg/kg) and the United Arab Emirates (0.230 ± 0.110 mg/kg) the present study recorded the highest Cr concentration but lower than the Cr concentration in rice grains from Pakistan (8.000 ± 7.600 mg/kg). (Akoury et al. 2023; Natasha, and I., Niazi, N. K., Shahid, M., Ali, F., Masood ul Hasan, I., Rahman, M. M., Younas, F., Hussain, M. M., Mehmood, T., Shaheen, S. M., Naidu, R.,, and Rinklebe, J. 2022). The bioavailability of Cr in rice grains may be due to the mineral composition, pH, Fe, manganese oxides, and redox potentials of serpentine soils, as well as other soil characteristics in which the paddy is cultivated, even though the soil-to-rice transfer is low (Infante et al. 2021). Moreover, cross-contamination, poor barrier properties of packaging as well as poor storage conditions may further lead to Cr contamination post-harvesting (Akoury et al. 2023).

Chromium uptake, accumulation, and translocation depend on its speciation in rice grains. The maximum acceptable limits for Cr are 1.0 mg/kg (GB2762-2012 China National Food Safety Standard: Maximum Limit of Contaminants in Food). Although Cr in the form of Cr(III) was previously believed to be essential for human nutrition, recent studies have raised doubts about its necessity (EFSA 2010). It has been suggested that an ideal daily intake of 50 to 200 μg of Cr(III) supports various metabolic functions like protein, glucose, and fatty acid metabolism. However, the underlying mechanism behind these proposed roles of Cr(III) in metabolism is not yet fully understood or supported by sufficient evidence. On the other hand, hexavalent chromium (Cr(VI)) is significantly more toxic than Cr(III), exhibiting 10 to 100 times greater toxicity due to its potent oxidizing properties. Besides, Cr(VI) can cause severe health issues in humans, including skin eruptions, ulcers, respiratory problems, gastrointestinal disturbances, genetic mutations, liver damage, kidney failure, and lung cancer (Ertani et al. 2017; Ferreira et al. 2019; Ali et al. 2022).

Cobalt

The concentration of Co ranged from 0.020 to 0.030 mg/kg in brown rice, 0.010 to 0.050 mg/kg in white rice, 0.020 to 0.040 mg/kg in fragrant rice, 0.010 to 0.010 mg/kg in basmati rice, and 0.020 to 0.030 mg/kg in glutinous rice, respectively (Table 5). The overall mean Co concentration was 0.024 ± 0.012 mg/kg with a range of 0.010-0.050 mg/kg. The concentrations of Co found in five different rice types were not significantly different (p > 0.05) (Fig. 2). Besides that, the mean concentration of Co in white rice (0.027 ± 0.012 mg/kg) and brown rice (0.0279 ± 0.003 mg/kg) in the present study were lower than those reported in Jamaica for white (0.073 ± 0.132 mg/kg) and brown rice (0.081 ± 0.136 mg/kg) respectively (Antoine et al. 2012). In a separate study, the overall mean of Co concentrations in rice samples from India, Sri Lanka, Vietnam, and the USA were higher (0.047 ± 0.045 mg/kg) compared to the present study (Shraim et al. 2022). The mean Co concentrations in domestic rice from Iran were 0.340 ± 0.050 mg/kg, 0.130 ± 0.040 mg/kg, and 0.410 ± 0.050 mg/kg for Tarom, Kohmare and Kamfirooz (indica rice types) (Naseri et al. 2015) while the Co concentration of Khaowong Kalasin sticky rice from Thailand was 0.022 ± 0.011 mg/kg. It is not clear from the literature the definite Co concentration in food that is likely to pose a threat to human health. However, plant Co concentrations are typically less than 10 mg/kg and, often, less than 1 mg/kg (Collins and Kinsela 2010). Therefore, it is safe to say that all rice types are safe to be consumed in terms of Co.

Comparative Assessment of Metal(loid)s Profiles in Rice Grains from Different Nations

In many developing countries, both food and water are the primary sources of metal(loid)s exposure (WHO 2008). The global expansion of the food trade has raised significant concerns regarding the transfer of metal(loid)s through the food chain, posing risks to human health worldwide. This is particularly relevant as imported food can contain higher concentrations of metal(loid)s, exposing millions of people to such elevated levels, even in areas without local metal(loid)s contamination. Malaysia, being a culturally diverse nation with a substantial immigrant population from various countries, is no exception to this concern (Rahman et al. 2014).

Among the metal(loid)s analyzed in this study, the mean concentration (mg/kg) was highest for Fe (41.37) followed by Cu (6.51), Cr (1.91), Ni (0.38), As (0.35), Se (0.07), Cd (0.03) and Co (0.02) (Table 6). In general, the dominant presence of essential metal(loid)s, such as Fe and Cu, was commonly observed in rice grains across different countries, while Co and Se were found in smaller amounts with significant variations in their concentrations.

Table 6 Comparison of metal(loid)s concentration in rice grain from different studies around the world in mg/kg unless stated otherwise. Values are presented as mean with standard deviation.
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This study yielded similar trends concerning the concentrations of essential metal(loid)s. The observed Co level was comparable to that reported in various countries. Nevertheless, Iran (Naseri et al. 2015), Malaysia (Praveena and Omar 2017), Jamaica (Antoine et al. 2012), and Pakistan (Natasha, and I., Niazi, N. K., Shahid, M., Ali, F., Masood ul Hasan, I., Rahman, M. M., Younas, F., Hussain, M. M., Mehmood, T., Shaheen, S. M., Naidu, R.,, and Rinklebe, J. 2022), observed Co levels notably higher than the present findings.

Regarding Se levels, this study’s results aligned with those reported in rice grains from the USA (TatahMentan et al. 2023), South Korea (Lee et al. 2023), and the Kingdom of Saudi Arabia (Shraim 2017). However, higher Se concentrations were reported in Pakistan (Natasha, and I., Niazi, N. K., Shahid, M., Ali, F., Masood ul Hasan, I., Rahman, M. M., Younas, F., Hussain, M. M., Mehmood, T., Shaheen, S. M., Naidu, R.,, and Rinklebe, J. 2022) and India (Halder et al. 2020) for dehusked and unpolished rice, respectively, with the highest reported Se concentration reaching 0.28 mg/kg in India.

Comparing Cd concentrations, most studies reported slightly higher values than the prsent observations, except for Thailand (Kukusamude et al. 2021), the USA (TatahMentan et al. 2023), South Korea (Lee et al. 2023), Australia (Rahman et al. 2014), the Kingdom of Saudi Arabia (Shraim 2017), Myanmar (Myat Soe et al. 2023), and India (Halder et al. 2020), which showed Cd levels similar to this study’s findings.

Notably, the Cu (39 mg/kg) and Fe (348 mg/kg) contents in rice grain samples from Punjab, Pakistan (Natasha, and I., Niazi, N. K., Shahid, M., Ali, F., Masood ul Hasan, I., Rahman, M. M., Younas, F., Hussain, M. M., Mehmood, T., Shaheen, S. M., Naidu, R.,, and Rinklebe, J. 2022), exceeded their respective maximum allowable concentrations. In other studies, the mean Fe and Cu concentrations ranged from 1.50 to 104.12 mg/kg and 0.03 to 16.20 mg/kg, respectively.

Regarding As content, different countries reported values ranging from 0.05 to 0.66 mg/kg, surpassing the European Union's recommended maximum level of 0.30 mg/kg in rice grains. This was evident in the rice grain samples from West Bengal, India (Halder et al. 2020), and Punjab, Pakistan (Natasha, and I., Niazi, N. K., Shahid, M., Ali, F., Masood ul Hasan, I., Rahman, M. M., Younas, F., Hussain, M. M., Mehmood, T., Shaheen, S. M., Naidu, R.,, and Rinklebe, J. 2022).

Higher Cr (9.30 mg/kg) and Ni (5.60 mg/kg) concentrations were reported in rice grain samples from Punjab, Pakistan (Natasha, and I., Niazi, N. K., Shahid, M., Ali, F., Masood ul Hasan, I., Rahman, M. M., Younas, F., Hussain, M. M., Mehmood, T., Shaheen, S. M., Naidu, R.,, and Rinklebe, J. 2022), and Malaysia (Praveena and Omar 2017; Navaretnam et al. 2022), exceeding the FAO/WHO's maximum level of 1.00 mg/kg for Cr in rice grains. In other studies, Cr levels remained within the recommended maximum limit.

The concentrations of metal(loid)s in rice grains are influenced by several interconnected factors. Variations in metal(loid)s levels across different rice brands can be attributed to a multitude of variables, including the type of rice cultivated, the source of irrigation water, the type of fertilizer and pesticides employed, as well as the prevailing spraying conditions. Additionally, soil characteristics such as moisture content, pH levels, and redox potential play a crucial role in metal(loid)s accumulation. Geographical conditions and grain properties also contribute to the observed variations. These differences are indicative of the accumulation rate of toxicants and soil to rice transfer rates. Thus, suggesting that certain domestic versus international or non southern versus southern regions had higher accumulation rates metal(loid)s than other regions (TatahMentan et al. 2023; Shariatifar et al. 2020).

The Health Risk Assessment

Estimated weekly intake

The calculated estimated weekly intake (EWI) for metal(loid)s exposure through rice consumption in five different rice types (brown, white, fragrant, basmati, and glutinous rice) and the values for provisional tolerance weekly intake (PTWI) recommended by FAO/WHO expert committee summarized (Table 7). For Cr and Ni, the PTWI values were set up at 23.3 and 35 μg/kg BW (Naseri et al. 2015). The EWI for Cr intake in rice was in the following order fragrant rice > brown rice > white rice > glutinous rice and basmati rice. The highest EWI for Cr was in fragrant rice, FR 1 (178.458 μg/kg BW) while the lowest was in glutinous rice, GL 3 (1.724 μg/kg BW). The calculated EWI of rice consumption for Cr was much higher than the PTWI for all rice samples except for basmati rice (BA 1 and BA 3) and glutinous rice (GL 3). The highest EWI (20.750 μg/kg BW) of Ni was observed in fragrant rice, FR 1. The average EWI values of Ni for brown rice (6.770 μg/kg BW), white rice (6.622 μg/kg BW), fragrant rice (13.297 μg/kg BW), basmati rice (9.543 μg/kg BW) and glutinous rice (9.311 μg/kg BW) were lower than the recommended PTWI (35 μg/kg BW). Due to the long half-life of Cd, the PTWI of 7 μg/kg BW for Cd was withdrawn. Thus, a provisional tolerable monthly intake (PTMI) of 25 μg/kg BW was established. The estimated monthly intake (EMI) for Cd is computed and shown in parentheses (Table 7). The EMI values of Cd are in the range of 0.542 μg/kg BW (white rice WH 1) to 16.505 μg/kg BW (brown rice BR 1) which is equivalent to 46 times and one-fold lower respectively than the established PTMI indicating that Cd concentration in all rice samples is safe for consumption.

Table 7 EWI and PTWI of metal(loid)s exposure through rice consumption in five different rice types (brown, white, fragrant, basmati, and glutinous rice) for the Malaysian population
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Although there is no established PTWI for Co, the MTDI is 100 μg/kg BW per day or 700 μg/kg BW in a week. The EWI values of Co for consumption of all rice types (0.156 -1.170 μg/kg BW) were extremely lower than the maximum tolerable weekly intake. A recent study from Thailand (0.160-2.270 μg/kg BW) also had similar observations (Kukusamude et al. 2021). This implies that the consumption of all rice types investigated in this study is safe in terms of Co. The EWI values of Cu for the consumption of rice types were in the range of 67.024 μg/kg BW (white rice WH 1) to 844.194 μg/kg BW (brown rice BR 1). Among all the rice types across different brands, brown rice BR 1 recorded the highest Cu concentration, almost 0.6 times higher than the recommended PTWI of 500 μg/kg BW. The PTWI for Fe (5600 μg/kg BW) is derived from the provisional maximum tolerable daily intake (PMTDI) established at 0.8 μg/kg BW (Codex Alimentarius Commission 2018). The EWI values of Fe through consumption of all rice types across different brands (151.791- 2900.971 μg/kg BW) were lower than the PTWI. Since the benchmark dose level (BMDL0.5) value for inorganic As was in the same range as the PTWI value, the previous PTWI value of 15 μg/kg BW (2.1 μg/kg BW per day) was no longer health protective (JECFA 2011). However, in this study, the EWIs calculated for As were lower in almost all the rice samples except for brown rice BR 2 (23.619 μg/kg BW) and brown rice BR 3 (24.157 μg/kg BW). This suggests that the consumption of brown rice investigated in this study may cause adverse non-carcinogenic health risks if consumed for a prolonged period. Nevertheless, the extent of As toxicity is highly dependent on its chemical forms, thus more accurate conclusions can be made only if a speciation analysis is conducted for the concerned rice samples. The average EWIs for Se in brown rice, white rice, fragrant rice, basmati rice, and glutinous rice were 1.913 μg/kg BW, 1.526 μg/kg BW, 1.651 μg/kg BW, 3.255 μg/kg BW and 0.743 μg/kg BW, respectively. There is currently no established PTWI for Se. The calculated EWI was compared with the Recommended Nutritional Intake (RNI) of the Ministry of Health Malaysia for Se. These values were lower than the recommended weekly intakes considering the RNI for this metal(loid). A safe and adequate range for Se intake for adults varies from 24 to 32 μg/day (168– 224 μg/week) (MOH 2017). Overall, most types of rice samples in this study have controlled essential (Cr, Fe, Se, Co, and Cu) and toxic (As, Cd, and Ni) metal(loid)s concentrations.

Non-carcinogenic risk

The THQ and HI are used generally to determine the potential non-carcinogenic risk to humans from metal(loid)s. In this study, both THQ and HI for As, Cr, Ni, and Cd of adult individuals through rice consumption of different varieties were determined. In all rice varieties, the THQ value of Ni (0.002–0.147) was below 1, strongly suggesting that the rice consumers are safe from the non-carcinogenic effects of these metals (Fig. 3). However, the THQ for As (1.285 – 11.425), Cr (0.082 –8.440), and Cd (0.192 – 5.854) were above 1, indicating that the rice consumers of the studied rice samples have a high risk of As, Cr and Cd induced non-carcinogenic health hazard. The highest THQ for As (0.510), Cr (0.001), and Cd (0.470) in a previous study on similar type of marketed rice samples reported lower THQ values (Praveena and Omar 2017).

Fig. 3
figure 3

The target hazard quotient (THQ) values for toxic metal(loid)s (As, Cr, Cd, and Ni) from white, brown, fragrant, basmati, and glutinous rice ingestion by adult individuals in Malaysia

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The HI indicates the total non-carcinogenic health hazard to individuals from multiple toxic metal(loid)s. It emerged that the HI values of the metal(loid)s were above the lower risk threshold value (1) for all rice samples while BR 1, BR 2, BR 3, WH 1, WH 3, FR 1 and FR 2 exceeded the upper HI risk threshold value of 5 (Fig. 4). Regarding the studied toxic trace elements, on an average As alone contributed 63% to the overall HI, indicating that As possesses the major non-carcinogenic health risk to rice consumers in this study’s chosen locations.

Fig. 4
figure 4

Hazard index (HI) values for toxic metal(loid)s (As, Cr, Cd, and Ni) from white, brown, fragrant, basmati, and glutinous rice ingestion by adult individuals in Malaysia

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Carcinogenic risk

Arsenic, Cd, Cr, and Ni are classified as group 1 carcinogens by the International Agency for Research on Cancer (IARC) (IARC 2012). The overall CR values were in the order of Cr (1.4 ×10-3)> As (7.7 ×10- 4)> Cd (6.90 ×10- 4)> Ni (5.00 ×10-4). In all rice samples, the CR values for As, Cd, Cr (except GL 3) and Ni (except WH 1) were greater than 10-4 and ranged between 2.48 × 10-4 to 2.20 × 10-3, 1.24× 10-4 to 3.76 × 10-3 and 5.24× 10-5 to 5.43 × 10-3 (Fig. 5). Islam et al. (2017) reported a cancer risk value of 0.54 × 10-3 to 2.12 × 10-3 for As from rice consumed at different places in Bangladesh, and it was similar in relation to this study. The CR values for Cr (10-6) from rice consumption in Dhaka and Satkhira districts from the same study revealed that the carcinogenic risk from Cr exposure is negligible in these two locations. The CR value varies with the average daily intake of the element, exposure duration, and lifetime of the individuals (Shariatifar et al. 2020). A decline in the ADI, exposure duration, and increase in the lifetime will decrease the CR and vice versa.

Fig. 5
figure 5

The cancer risk (CR) values for toxic metal(loid)s (a. As, b. Cd, c. Cr and d. Ni) from white, brown, fragrant, basmati, and glutinous rice ingestion by adult individuals in Malaysia

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Considering the significant variations and uncertainties found among the maximum levels reported in existing literature, it is crucial to address these discrepancies to ensure a reliable and robust assessment. To achieve this, it is recommended to adopt the maximum levels utilized in previous studies conducted in Malaysia. It should be noted that different countries, such as China, may have different maximum limits due to higher levels of elemental soil contamination. Based on the findings of this study, a strong advocacy is made for the standardization of maximum limits for trace elements in rice, taking into account the diverse rice varieties and their varying capacities to accumulate elements. This standardization effort will not only improve clarity but also facilitate consistent monitoring practices by researchers and relevant regulatory bodies. By establishing uniform standards, the scientific community can effectively evaluate and manage potential risks associated with trace elements in rice, ensuring food safety and protecting the well-being of consumers.

Furthermore, the metal(loid) concentrations in various types of rice, measured in mg/kg on a dry weight basis, displayed the following sequence: iron (Fe) > copper (Cu) > chromium (Cr) > nickel (Ni) > arsenic (As) > selenium (Se) > cadmium (Cd) > cobalt (Co). It was observed that approximately one-third of the rice samples exceeded the recommended limits for arsenic (As), while none surpassed the limits for cadmium (Cd) as specified by the FAO/WHO. This study highlights that rice consumption can be a significant route of exposure to toxic metal(loid)s, potentially leading to both noncarcinogenic and carcinogenic health issues for individuals consuming rice on a daily basis. The noncarcinogenic health risk primarily stemmed from arsenic (As), contributing to 63% of the hazard index, followed by chromium (Cr) (34%), cadmium (Cd) (2%), and nickel (Ni) (1%). Notably, the carcinogenic risk associated with As, Cr, Cd, and Ni was found to be high (>10-4) for adults. The cancer risk (CR) for each element was 5 to 8 times higher than the upper limit set for environmental carcinogens (<10-4).

These study findings, in conjunction with the need for standardization of maximum limits, provide valuable insights into the pollution status of metal(loid)s in different rice varieties. This information can assist relevant authorities in addressing concerns related to food safety and security while promoting a harmonized approach to trace element regulation. By implementing the recommendations and encouraging collective action, comprehensive guidelines for regulating trace elements in rice can be established, ultimately safeguarding public health and enhancing the overall understanding of metal(loid) contamination in rice.

Conclusion

This study revealed the presence of metal(loid)s, including As, Cd, Fe, Se, Cu, Cr, Co, and Ni, in different types of rice. Notably, the concentration of Cd in all rice types adhered to the permitted limits set by Malaysian Food Regulation 1985 and FAO/WHO Codex Alimentarius International Food Standards. However, the As levels in most white rice and all tested rice samples, except for brown rice, exceeded the maximum limit established by FAO/WHO for rice (0.300 mg/kg). Furthermore, toxic metal(loid)s such as As, Ni, Cd, and Cr were found in higher concentrations compared to essential metal(loid)s like Cu, Fe, Co, and Se in the examined rice samples. Considering the FAO/WHO’s weekly tolerable intakes and the recommended nutrient intake by the Ministry of Health, Malaysia, it is advisable to consume brown rice in moderation due to its high concentration of toxic metal(loid)s (As, Cr, and Ni). However, the study highlighted that brown rice, white rice, fragrant rice, basmati rice, and glutinous rice could fulfil the everyday requirement of essential metal(loid)s. The risk assessment indicated that non-carcinogenic effects were primarily associated with As exposure, contributing 63% to the total hazard index. Carcinogenic risks were higher with As, Cr, and Cd, while Ni demonstrated a moderate carcinogenic risk associated with rice consumption. These findings emphasize that rice grain can be contaminated not only with As but also with other toxic metal(loid)s, posing health implications for rice consumers. To mitigate this risk, it is advisable for the general population to diversify their diet by consuming different types of rice, as the maximum allowable rice consumption may vary depending on the specific type. The findings of this research can be utilized by relevant authorities for future monitoring and food regulations, particularly concerning domestic and imported rice. Nonetheless, it is crucial to underscore the significance of conducting comprehensive studies with larger sample sizes to yield more robust and scientifically valid conclusions. Additionally, the Malaysian Food Regulation could consider using this initial data to propose, revise, and establish permissible limits for other metals that may pose health risks when consumed, specifically in rice, rather than solely focusing on As and Cd. Future studies are encouraged to encompass all significant metal(loid)s present in food to ensure the presence of adequate nutrients that meet individuals' dietary requirements and prevent malnutrition.