Abstract Heavy metal (HM) contamination in agricultural soils may lead to plant uptake from roots to edible parts, posing risks to food safety. This study assessed the health risks from garlic consumption in Chiang Mai and Lampang, Thailand’s major garlic-producing provinces. Detected HM concentrations in soil followed the order: Pb > Cr > Cu >As > Cd, respectively. All measured levels were within acceptable limits. Heavy metal concentration in Chiang Mai soil was 7.08 ± 4.72 mg/kg (As); 0.84 ± 0.32 mg/kg (Cd); 19.25 ± 7.18 mg/kg (Cr); 14.64 ± 6.51 mg/kg (Cu); and 20.45 ± 6.70 mg/kg (Pb). By comparison, average concentrations in Lampang soil were 14.20 ± 2.81 mg/kg (As), 2.25 ± 0.24 mg/kg (Cd); 28.00 ± 3.87 mg/kg (Cr); 26.34 ± 2.33 mg/kg (Cu); and 28.62 ± 3.84 mg/kg (Pb). Elevated Pb levels in soils cultivated with garlic and other crops likely stemmed from agricultural chemical use, with Pb concentrations in garlic exceeding the Codex Alimentarius Commission (CODEX) guideline value.

Although contaminant levels in soil and garlic are low, prolonged intake may pose health risks. arget hazard quotient (THQ) and cancer risk (CR) were estimated for garlic consumption across three age groups: children, adults, and the elderly.  The total THQ values were highest in children (4.51E-02), indicating a low non-carcinogenic risk across all groups. As contributed most significantly to CR, with values ranging from 9.57E-07to 1.09E-06. The total cancer risk values in all age groups (1.36E-06 to 1.55E-06) were above the acceptable limit (10^-6), indicating potential health concerns.

Keywords: Soil contamination, Heavy metal contamination, Garlic, Target Hazard Quotient, Cancer Risk

Funding: This study was supported by Thammasat University Research Fund, Contract No TUFT 56/2565.

Citation: Nilkarnjanakul, W. and Srivieng, P. 2025. Potential health risk assessment of heavy metals via garlic consumption from cultivation areas in northern Thailand. Natural and Life Sciences Communications. 24(4): e2025064.

INTRODUCTION

The group of metals known as "heavy metals" is hazardous to living things and has a high density in comparison to water. Both anthropogenic activities and natural processes can be causes of heavy-metal contamination (Smiljanic et al., 2019). Many such activities have been found in mining, smelting, pesticides, personal care products, waste dump sites, industrial processes, etc. (Alengebawy et al., 2021; Galdames et al., 2022; Saha et al., 2022). As a result, heavy metals could be a contaminant in the environment and in the food chain, and can have adverse effects on ecosystems, livestock and human health. Heavy metals in soil and water supplies are potentially toxic to the liver, kidneys, brain, lungs and blood composition. Also, Parkinson's disease, Alzheimer's disease, mutations and cancer can all result from prolonged exposure to heavy-metal poisoning (Jaishankar et al., 2014; Engwa et al., 2019).

Due to growing concerns in food safety, agricultural experts now warn about heavy metal-contaminated soil containing biologically toxic metalloids (Nyiramigisha, 2021). A major source is mineral weathering, which releases contaminants depending on local geology (Alloway, 2013). Additional contamination may result from irrigation, fertilizers, and pesticides used during cultivation (Song et al., 2018). Phosphate fertilizers often contain arsenic (As) and cadmium (Cd), while chromium (Cr) and lead (Pb) are commonly found in general fertilizers and pesticides (Smiljanic et al., 2019). Furthermore, manure, sewage sludge, aerial deposition, and excessive phosphate fertilizer use contribute to Cd buildup in soil (Sana et al., 2024). Plants absorb these toxins through roots and distribute them throughout their tissues when grown in contaminated areas (Nyiramigisha, 2021). To counter metal toxicity, strategies like chelation, translocation, and compartmentalization have been developed (DalCorso et al., 2013).

Concerns about heavy metal poisoning and its effects on the environment and public health have been brought to light by recent studies conducted in northwest Iran.  According to studies, there are higher than recommended amounts of arsenic in water, soil, and agricultural products, and in some areas, the lifetime cancer risks were higher (Nemati-Mansour et al., 2022; 2024).  Significant contaminants from both geogenic and man-made sources, such as vehicle emissions and lake desiccation, was found to contribute to the widespread atmospheric deposition of toxic metals in Urmia, according to spatial analyses (Hosseinpoor et al., 2024a; 2024b).  These results highlight the need for frameworks for site-specific monitoring and risk assessment, which the work aims to implement in Thai agricultural zones.

Garlic is a staple herb in Thai cuisine and widely cultivated in Thailand, particularly in the Northern and Northeastern regions. Local varieties are grown in Si Sa Ket (76–90 days) and in Chiang Mai and Lampang (100–120 days) (Rungchareon, 2018). Major cultivation areas include Wiang Haeng District, Chiang Mai (11.49 km²), and Ngao District, Lampang (4.68 km²) (Office of Agricultural Economics, 2022), yielding about 8,054 tons annually (Lampang Provincial Agriculture and Cooperatives Office, 2020). Garlic, consumed mainly as root bulbs, is recognized for its detoxifying properties (Cha, 1987; Sahidur et al., 2023). However, it can accumulate heavy metals like Cd, Cu, and Pb from agricultural sources (Vannini et al., 2021). Market samples have shown measurable levels of Pb, Cu, Cd, and Cr (Yang et al., 2011).  Human health is significantly impacted by the transmission of heavy metals from soil to plants and their accumulation in the food chain (Ali and Khan, 2018). The ability of plants to accumulate heavy metals from soil into edible parts is frequently evaluated using the bioconcentration factor (BCF). A plant may be at possibility of passing contaminants up the food chain if its BCF value is high (Krivokapić, 2021). Thus, assessing BCF in garlic is important for comprehending how it affects exposure to harmful metals in humans.This study assesses heavy metal contamination (Pb, Cd, Cu, As) in garlic fields in Chiang Mai and Lampang to evaluate potential health risks.

Human activities, such as industrial processes, mining, and the use of agrochemicals, have contributed to the contamination of agricultural environments. These pollutants, especially heavy metals, can be absorbed by crops and subsequently enter the human food chain through consumption. Therefore, assessing the potential health risks associated with the intake of contaminated agricultural products is essential to ensure food safety and public health protection (Hlyan and Chanpiwat, 2022; Phadermrod and Srivieng, 2025). Human health risk assessment (HHRA) is an approach that has been suggested for evaluating the potential health risk of exposure to heavy metals in garlic by using Hazard Quotient (HQ) as non-carcinogenic risk, and Cancer Risk (CR) as carcinogenic risk (U.S. EPA, 1992). The main objective of this research was to assess the heavy metal concentrations in soil and garlic (Allium sativum L.) in the Provinces of Chiang Mai and Lampang and to estimate potential health risks related to garlic consumption.

MATERIAL AND METHODS

Study area

The study areas were in Piang Luang Sub-District of the Wiang Haeng District of Chiang Mai Province, and Ban Haeng Sub-District of the Ngao District of Lampang Province (Figure 1), where it can be seen that both areas are located in the Northern Region of Thailand. The study area of Wiang Haeng District, Chiang Mai Province was selected as the contaminated site. Since there has been ongoing illegal waste disposal for more than a decade in the vicinity of garlic farming areas, until the year 2022. Meanwhile, Ban Haeng Sub-District of the Ngao District of Lampang Province was studied to compare heavy metal concentration in soil and garlic as the control area.

Piang Luang covers an area of 151 km² (Tee Nee Wiang Haeng, 2024). It is characterized by a plateau and is located on a steep mountain range, with an altitude ranging from 750 meters to 1,200 meters above sea level. The study area has a tropical climate with summer, rainy and winter season. Rainfall was over 1,200 millimeters. Total number of rainy days was 123 days (Thai Meteorological Department; Chiang Mai, 2024, information as of January, 2024). Major water sources included the Mae Tang River, and Mueng Creek. Most cultivation activities were in rice (from April to September) and garlic (from October to March). Soil was characterized by two texture types: sandy and sticky. Also, farmers cultivated garlic combined with chili in some areas, including Ban Haeng Sub-District of the Ngao District of Lampang Province, which is located in the Northern Region of Thailand. The study area has a tropical climate including the summer, rainy, and winter seasons. Rainfall from January 2022 to February 2023 totaled 1,262.70 millimeters. Total number of rainy days was 123 days (Thai Meteorological Department; Lampang, 2024, information as of February 28, 2024). Major water sources included the Mae Haeng River, Mae Ngon Reservoir, Mae Mueang Reservoir and Huai Som Reservoir. The cultivation process involved the use of irrigation water and pond sources, along with the application of both chemical fertilizers and compost, as well as pesticides and fungicides during the growing season. Additionally, the rotational and relay cropping differed between the two provinces. In Chiang Mai Province, chili was commonly grown in rotation, whereas in Lampang Province, multiple cropping systems were practiced, including rice, field corn, and sun hemp. The characteristics of the cultivation areas were also influenced by topography, with contour cultivation observed in Chiang Mai and flat-land cultivation in Lampang. Both provinces were a similar harvesting period, with rotational crops typically grown from April to September, followed by garlic cultivation from October to March.

Figure 1. A study-area map showing the locations in the Wiang Haeng District of Chiang Mai (A) and Ngao District of Lampang (B).

Sample collection and analysis

All soil and garlic samples were collected from the northern part of Thailand during garlic harvest time in March, 2023. Thirty-one and twenty-seven soil samples in five locations were collected from Chiang-Mai Province and Lampang Province, respectively. The number of soil samples was drawn depending on the size of each location by systematic grid sampling (Stolbovoy et al., 2005), while latitude and longitudes were noted at each of the sampling points. The soil sample was collected from the surface down to a depth of 15 cm. by an aluminum hand shovel and kept in a polyethylene zipper bag; they were then stored in an ice box at 4°C. Soil samples were quartered and air-dried at room temperature and then ground and screened through a sieve of less than 2 mm. Soil texture was analyzed by the hydrometer method, while pH and electrical conductivity (EC) were determined by a pH meter (Schott Instruments Lab 850). Its organic matter (OM) was observed in accordance with the research of Walkley and Black (1934). The titration method and cation exchange capacity (CEC) were methodically implemented by the standard operating procedure using a CEC and 1N ammonium acetate with a pH 7.0. (FAO, 2022). Five garlic samples were sampled from the same soil-sampling locations in both provinces. They were collected as composite sampling in each location and preserved in the ice box. Both soil and garlic bulb samples were observed with their moisture contents by the room temperature on a dry weight basis.

Acid digestion of soil followed the U.S. EPA 3050B method (U.S.EPA, 1996), approximately 1 g of each prepared soil sample was weighed and transferred into a beaker. Then, 10 mL of 1:1 nitric acid (HNO₃) was added, and the beaker was covered with a watch glass. The samples were placed on a hot plate and gently heated to approximately 95°C for 15 minutes without boiling, under a fume hood. After that, 5 mL of concentrated HNO₃ was added, and the heating was continued for 30 minutes. If brown fumes appeared, an additional 5 mL of concentrated HNO₃ was added, followed by continued gentle heating without boiling for up to 2 hours or until the sample volume was reduced to approximately 5 mL. After cooling, 2 mL of milli-q water and 3 mL of 30% hydrogen peroxide (H₂O₂) were added. The mixture was reheated until the digested solution became light-colored or its volume was further reduced to about 5 mL. The digested solution was then filtered through Whatman No. 41 filter paper, and the filtrate was diluted to a final volume of 50 mL with milli-q water in a volumetric flask, and measured by the Agilent 5110 inductively coupled plasma - optical emission spectrometer (ICP-OES). An exception was made for As, for which hydride generation atomic absorption spectroscopy (PerkinElmer/PinAAcle 900F) was used.

Meanwhile, garlic samples were digested following the U.S. FDA (2022) protocol, approximately 2 g of the sample was weighed and transferred into a digestion vessel, followed by the addition of 8 mL of concentrated HNO₃ to wash down any residues adhering to the vessel walls. After allowing the initial reaction to proceed for 20 minutes, 1 mL of 30% H₂O₂ was added. The vessel was then sealed and gently shaken. microwave digester (Milestone ultraWAVE) was carried out with a program ramping to 180°C for 25 minutes. After digestion, the vessels were cooled to a temperature below 50°C. The digested solution was filtered through appropriate filter paper, and the final volume was adjusted to 25 mL with milli-q water. The concentrations of heavy metals were then performed using ICP-OES (Analytik Jena PlasmaQuant PQ 9000 Elite).

Quality control

The pH and EC meters were calibrated with buffer solutions at 4.0, 7.0 and 10.0 and a conductivity standard solution (1,413 μS/cm), respectively. All procedures were used the chemical reagents as the analytical grade. In case of heavy metal analysis, the utensils and equipment were cleaned and fixed with HNO₃ before being used. Milli-q water was used for the sample preparation. The limits of detection (LOD) of ICP-OES were 0.002 mg/L of Pb, 0.001 mg/L of Cd, Cu and Cr, and the HGAAS was 0.0003 mg/L of As for the heavy metals analysis in soil. The R² values of the calibration curve of these metals were higher than 0.995 and were within a working range of 0.1 – 5 mg/L of Cd, Cu, Cr and Pb, and 0.001 - 0.015 mg/L of As. Meanwhile, LOD of ICP-OES were 0.0017 mg/L for the heavy metals analysis in garlic sample. Their R² values were above 0.9999, except for As, which was 0.9998, and were within a working range of 0.005 – 0.5 mg/L. After calibration, the precision was re-checked for every running of 20 samples and using duplicate samples. Percent recovery range of heavy metals were in the acceptable between 90 and 110.

Statistical analysis

An independent t-test and Mann-Whitney U test were used to compare the heavy-metal concentrations in the soils and garlic in Chiang Mai Province with the corresponding concentrations in Lampang Province after testing for normal distribution as presented in Table S1. Statistical significance was considered to be at a P-value < 0.05, which was performed by PASW Statistics Base 18 for Windows. A geographic information system (QGIS) was used to generate the sampling points.

Bioconcentration factor (BCF)

The BCF is defined as the ability of plants to accumulate elements from the substrate (Mishra and Pandey, 2019). Hence, using Equation 1, the amount of heavy-metal transport from soil to plant was determined (Gebeyehu and Bayissa, 2020).

where Cplant is the heavy-metal contamination in edible plants (mg/kg), and Csoil is heavy-metal contamination in the soil (mg/kg). A plant could possibly accumulate the metals under consideration during evaluation whenever its BCF value is greater than 1.

Health risk assessment

A deterministic health risk assessment was conducted to estimate non-carcinogenic and carcinogenic risks from heavy metals in garlic, a commonly consumed dietary item, using fixed exposure parameters to calculate THQ and CR. The assessment considered three population groups: children (0–14 years), adults (15–59 years), and the elderly (60 years and above). The risk assessment methodology followed United States Environmental Protection Agency guidelines (U.S.EPA, 1989), including Target Hazard Quotient (THQ) and Cancer Risk (CR) calculations.

Calculation of daily intake

To evaluate the potential health risks from garlic consumption, the daily intake of heavy metals was first estimated. This was done by calculating the Average Daily Dose (ADD) for non-carcinogenic risk and the Lifetime Average Daily Dose (LADD) for carcinogenic risk, based on the United States Environmental Protection Agency (U.S.EPA, 1992) method.

The ADD and LADD were calculated using the following Equation 2 and 3:

Where:

• Cg = contaminant concentration in garlic (mg/kg)

• IR = ingestion rate of garlic (g/day; child = 0.48, adult = 2.08, elderly = 2.23) (National Bureau of Agricultural Commodity and Food Standards, 2016)

• EF = exposure frequency (days/year; 350)

• ED = exposure duration (years; child = 15, adult and elderly = 30)

• BW = body weight (kg; child = 12.8 (Lim et al., 2016), adult = 63.12, elderly = 62.67 (NSTDA, 2017))

• AT = averaging time for non-cancer effects (days; ED x 365 days)

• ATcancer = averaging time for cancer risk (days; 76 years (world bank, 2023) x 365 days)

Calculation of health risk assessment

The calculation of health risk assessment from garlic consumption involved the estimation of both non-carcinogenic and carcinogenic risks. The Target Hazard Quotient (THQ) was calculated to assess non-carcinogenic risk, while the Cancer Risk (CR) was used to estimate carcinogenic potential. This assessment helps identify whether long-term consumption of garlic contaminated with heavy metals may pose health hazards to consumers.

The THQ and CR were using the following Equation 4 and 5:

Where:

• ADD = Average Daily Dose (mg/kg-day)

• RfD = Reference Dose (mg/kg-day) see Table 1

• LADD = Lifetime Average Daily Dose (mg/kg-day)

• CSF = Cancer Slope Factor (per (mg/kg-day)) see Table 1

According to the United States Environmental Protection Agency (U.S.EPA, 1992), a CR value greater than 10^-6 (one in a million) indicates a potential carcinogenic risk, while A THQ value greater than 1 indicates potential non-carcinogenic health risk. This conservative threshold was applied in the present study to ensure that even low-level risks are adequately considered for public health protection.

Table 1. Reference dose (RfD) and cancer slope factor (CSF) for selected heavy metals.

RESULTS

Physico-chemical properties

The physico-chemical properties such as soil texture, pH, EC, OM and CEC were observed as presented in Table 2, where C denotes the sampling in Chiang Mai, and L denotes the sampling in Lampang. Nearly all of the sampling locations of Chiang Mai province contained sandy clay loam except for C4, which is clay loam classified according to the United States Department of Agriculture system. Most of the area presented weak acidic soil, ranging from 5.92 to 6.63. The EC value was between 65.7 and 92.52 μS/cm, while the average OM and CEC were 2.95% and 8.72 cmol/kg, respectively.

In the case of Lampang Province, the characteristics of the soil consisted of various soil textures, such as silty clay, clay loam and loam. The average pH value ranged from 6.67 to 7.78. The EC value varied from 34.34 to 100.48 μS/cm. The average OM and CEC were 2.3% and 14.80 cmol/kg, respectively. It could be explained that these sampling locations in both provinces were suitable for farming, since they were within the appropriate pH rang of 6 to 7.5 for plant cultivation, as recommended by the FAO, 2021. The content of OM and CEC, meanwhile, was useful as an indicator of soil fertility (Land Development Department, 2010).

Table 2. Physico-chemical properties of soil in Chiang Mai and Lampang.

Note: SCL = Sandy Clay Loam; CL = Clay loam; SiC = Silty clay; C = Clay

Heavy metals in soil and garlic

The heavy-metal concentration in the soil of Chiang Mai Province was 7.08 ± 4.72 of As, 0.84 ± 0.32 of Cd, 19.25 ± 7.18 of Cr, 14.64 ± 6.51 of Cu and 20.45 ± 6.70 of Pb in units of mg/kg. By comparison, the average concentration in soil of Lampang province was 14.20 ± 2.81 of As, 2.25 ± 0.24 of Cd, 28.00 ± 3.87 of Cr, 26.34 ± 2.33 of Cu and 28.62 ± 3.84 of Pb in unit of mg/kg. These metal concentrations were below the set standard of soil quality, as shown in Table 3. In contrast, all samples of soil from both provinces exceeded the background value of Cd in the soil, as set by the Department of Agriculture, 2002. Based on the result, the average concentration of these metals in the soil in Chiang Mai and Lampang Provinces was different (P-value < 0.001).

Table 3. Average concentration of heavy metals in the soil as observed in Chiang Mai, and Lampang (mg/kg).

Noted: ^aNotification of the National Environmental Board, Pollution Control Department, Ministry of Natural Resources and Environment, Thailand (Pollution Control Department, 2021)

^bCanadian soil quality guidelines for the protection of environmental and human health for agriculture (Canadian Council of Ministers of the Environment, 1997)

Table 4. Heavy-metals concentration in garlic in Chiang Mai and Lampang (mg/kg).

Noted   *Below the detection limit of 0.0017 mg/L

**Exceed the guideline value

^a Codex Alimentarius Commission, 2019

^b National Health and Family Planning Commission and the China Food and Drug Administration (CFDA, 2017)

^c Since Cu is the essential metal retaining the essential metal-ion equilibrium for a variety of cellular functions, the main among which is the central nervous system, so that the toxic standard for it has not been regulated.

As and Cr in garlic were very low in concentration in both provinces, as presented in Table 4. Meanwhile, the concentration of Cd was high, but did not exceed the guideline value of the Codex Alimentarius Commission 2019, except for the presence of Pb in some samples. The statistical analysis illustrated that the concentrations of heavy metals in the two provinces did not differ. The estimation of heavy metal uptake in plant tissue from the soil can be considered by use of the BCF as in Table 5. The transfer factors rose in the following order: Cr, Pb < As < Cd < Cu. However, the BCF of all metals was less than 1, which demonstrates a low risk of translocation.

Table 5. BCF of heavy metals, analyzed for the garlic samples.

Health risk assessment

Daily intake of metal contaminants

The estimated Average Daily Dose (ADD) and Lifetime Average Daily Dose (LADD) of five heavy metals (As, Cd, Cr, Cu, and Pb) through garlic consumption were calculated for three population groups: children, adults, and the elderly.

Among the studied metals, copper (Cu) exhibited the highest ADD and LADD values across all age groups, with the highest ADD observed in children in Chaing Mai (7.57E-05 mg/kg/day). Lead (Pb) and arsenic (As) showed moderate exposure levels, while cadmium (Cd) had the lowest ADD and LADD values in all population groups.

The differences between age groups were relatively small, with children generally showing slightly higher exposure values than adults and the elderly, due to lower body weight despite the same consumption rate. All exposure values (ADD and LADD) were in the magnitude of 10^-6 to 10^-5 mg/kg/day (Table S-3), which were further evaluated for non-cancer and cancer risk in the following analysis.

Non-carcinogenic and carcinogenic risk

The health risk assessment of garlic consumption from cultivation areas in Northern Thailand indicated that the Target Hazard Quotient (THQ) values for heavy metals, including arsenic (As), cadmium (Cd), chromium (Cr), and copper (Cu), were all below 1 across child, adult, and elderly population groups. The health risk assessment for crops cultivated in Chiang Mai Province revealed that the Target Hazard Quotient (THQ) ranged from 1.05E-02 to 1.20E-02 for As, 1.01E-02 to 1.15E-02 for Cd, 2.88E-03 to 3.28E-03 for Cr, and 2.22E-02 to 2.52E-02 for Cu. In Lampang Province, the Target Hazard Quotient (THQ) values for crop cultivation were consistent across all samples, measured at 6.07E-03 for As, 1.16E-02 for Cd, 2.50E-03 for Cr, and 2.26E-02 for Cu. These values indicate stable exposure levels that merit careful monitoring for long-term health implications. Based on the assessed THQ values, garlic consumption at these exposure levels is not expected to pose significant non-carcinogenic health risks (<1).

The Hazard Index results show that there were regional and age-group-specific differences in the non-carcinogenic health risks associated with crop consumption.  In comparison to Lampang, with reported HI values of 4.51E-02 (Children), 3.96E-02 (Adults), and 4.28E-02 (Elderly), Chiang Mai consistently displayed higher HI values across all age groups, including 5.20E-02 (Children), 4.57E-02 (Adults), and 4.93E-02 (Elderly). These findings illustrated the significance of focused risk management and ongoing monitoring in both regions, indicating that children were the most vulnerable demographic group even while exposure levels are still below the critical threshold of 1.

 However, the Cancer Risk (CR) values for arsenic and the total cancer risk in all population groups exceeded the recommended threshold of 10⁻⁶ set by the United States Environmental Protection Agency (U.S. EPA, 1992). In Chiang Mai Province ranged from 1.02E-06 to 1.94E-06 for As, 2.79E-07 to 5.29E-07 for Cr, and 4.28E-09 to 8.12E-09 for Pb, suggesting potential carcinogenic concerns, particularly with arsenic exposure. Meanwhile, The estimated cancer risk due to consumption of crops cultivated in Lampang Province ranged from 9.57E-07 to 1.03E-06 for As, 3.95E-07 to 4.26E-07 for Cr, and 1.50E-08 to 1.62E-08 for Pb, highlighting the importance of continual assessment, particularly with regard to arsenic's carcinogenic potential.

Notably, the total cancer risks in the Chiang Mai cultivation area were higher than those observed in the Lampang cultivation area. Arsenic was identified as the primary contributor to cancer risk, while lead (Pb) had the least impact. These findings highlight that although garlic consumption from these cultivation areas poses minimal non-carcinogenic risks, the carcinogenic risks—especially in children—exceed the recommended safety threshold (Figure 2) and underscore the need for ongoing monitoring and quality control of agricultural products.

The cancer risk values related to crop consumption between age groups and geographical areas. The risk in Chiang Mai were 1.30E-06 for children, 2.29E-06 for adults, and 2.47E-06 for the elderly. All of these values were higher above the acceptable threshold of 1.00E-06, especially for the elderly and adults. On the other hand, Lampang showed that were comparatively lower than Chiang Mai's, but still exceeded the guideline limit: 1.55E-06 for children, 1.37E-06 for adults, and 1.48E-06 for the elderly.

Figure 2. Comparative THQ and cancer risk (CR) of heavy metals in garlic consumption across age groups.

DISCUSSION

The levels of As, Cu, Cr and Pb in the soil of Chiang Mai and Lampang were found to be below the maximum standard. Although the presence of Cu is the highest, it is commonly present in natural soil and acts as an essential nutrient for plants. Similar to Akhter et al. (2022), the study discovered that while Cr and Pb were found to be lower in poultry waste, Cu appeared to be accumulating at a higher rate in soil and garlic. During the cultivation process, they started with using animal manures such as chicken or cattle manures to improve the soil quality. Chemical fertilizer was then mainly applied 15-15-15 which indicate the ratio of three essential nutrients; Nitrogen 15%, Phosphous 15%, and Potassium 15%, 2-3 times, and insecticides and fungicides were used twice. However, the amount of these application chemical products was different and the composition of organic manures depended on the characteristics of each source. It could have led to the different concentrations of heavy metals that were found in the two provinces. When comparing the control area (LP) and the contaminated area (CM), most heavy metal concentrations in LP appear to be higher. Although garlic cultivation in CM was located near an illegal dumpsite, the concentrations of heavy metals were lower than those in the control area. This can be explained primarily by differences in soil texture. The soil's ability to absorb and release heavy metals significantly influences their presence. Most samples collected from CM had a high sand content, while those from LP were predominantly clay, which tends to retain heavy metals more effectively. When considering the characteristics of topography, altitude plays an important role in influencing the distribution of heavy metals. Areas with slopes tend to greater metal mobilization due to enhanced surface runoff, whereas flat terrain has lower mobility potential, leading to a higher likelihood of heavy metal accumulation (Zhang et al., 2025).

Moreover, soil acidity-alkalinity, OM content, CEC, and microbial activity were the main soil-related variables. Organic and inorganic form, concentration, solubility, bioavailability and essential or non-essential heavy metals were some of the aspects associated to heavy metals (Rashid et al., 2023). When considered as soil for planting, Cd was at a high level in both provinces. The presence of Cd may have been caused by the phosphate fertilizer (Mehmood et al., 2009). The concentration depended on the percentage of phosphorous (P₂O₅) contained in the fertilizer (Suciu et al., 2022). Given that local farmers reported frequent use of chemical fertilizers and pesticides, along with proximity to areas with illegal waste dumping, these anthropogenic activities may have contributed to the elevated levels of certain metals. Similarly, Song et al. (2018) highlighted that metal contamination can result from irrigation water, as well as the use of fertilizers and pesticides during cultivation. The reports showed that the application of fertilizer led to a rise in the amounts of Cd, Pb and As in soils (Atafar et al., 2010). Additionally, the area in cultivation implemented a crop rotation program, switching seasonally to crops like corn, rice, chili, beans, etc., which may alter the soil's OM, chemical composition and microbial community structure (Wang and Xu, 2015; Soman et al., 2017). Another reason was the duration of garlic cultivation, as the species we studied required only 100 days to plant, suggesting that as culture timeframes increased, heavy-metal concentrations would also increase (Dong et al., 2021).

Plant, soil and metal variables could all have an impact on the way in which heavy metals interact with agricultural plants. Species, variety, genotype of crops, metabolic activities, abilities for intake, translocation and bioaccumulation were among the factors associated to plant variables. This relationship could explain the main route used by toxic contaminants to traverse the food chain, as well as the transport of heavy metals from soil to plants. In this study, the transferability between soil and garlic was assessed. Pb concentration in garlic was high which supported by Ata et al., 2013. The study by Phuengphai et al. (2022) reported that the concentrations of Pb, Cd, Cu, and Zn in shallots and garlic cultivated in Sisaket Province, Northeastern Thailand, were within permissible safety limits. The findings indicate that garlic cultivated in this region is considered safe for consumption. These results also support the assumption that despite potential variations in soil characteristics. High concentrations of arsenic (As), cadmium (Cd), and lead (Pb) were found in garlic samples purchased from major wholesale markets in the Kuala Lumpur and Selangor areas of Malaysia (Nordin and Selamat, 2013).

Although, the assessment of BCF was lower than 1, long-term accumulation of these toxic metals could pose a health concern to humans (Franke, 1996; Munishi et al., 2021). Furthermore, it clearly showed that garlic has a stronger bioaccumulation factor for Cu than it does for other metals. This conclusion was consistent with that of the tomato and cabbage sample reported by Gebeyehu and Bayissa (2020). The location in Chiang Mai has greater BCFs than the location in Lampang. The properties of the soil, particularly the OM (Xi et al., 2023) could have an impact on the transfer of heavy metals (HM) from the soil to plants (garlic). Previous studies have reported elevated levels of heavy metals in several regions of Thai agricultural soils, particularly in areas with intensive farming. These findings support concerns that long-term cultivation may contribute to heavy metal accumulation in the soil, which can subsequently be taken up by crops such as garlic (Zarcinas et al., 2004). However, both areas were cultivated and harvested during the same period, with a similar garlic growing duration of approximately 100–120 days. This likely resulted in comparable heavy metal uptake rates between the two provinces.

Based on the results of the health risk assessment (HRA), all Target Hazard Quotient (THQ) values for individual heavy metals were found to be below the acceptable threshold of 1, indicating no significant non-carcinogenic risk. However, the cancer risk (CR) value associated with arsenic (As) exceeded the recommended safety level of 10^-6, contributing substantially to the total CR value and leading to an overall carcinogenic risk that surpasses the acceptable limit. These findings are supported by Bhatti et al. (2013), who reported that various vegetables—particularly leafy types like spinach—accumulated high levels of arsenic when irrigated with contaminated water, resulting in cancer risk estimates exceeding the USEPA’s acceptable threshold of 10^-6. In contrast to Monboonpitak et al. (2018), who reported an acceptable cancer risk from inorganic arsenic in garlic using probabilistic assessment, our study found that the cancer risk (CR) exceeded the 10^-6 threshold, indicating an unacceptable risk. This discrepancy may be due to methodological differences, as their study used a probabilistic model (e.g., Monte Carlo simulation), while ours applied a deterministic approach based on worst-case scenario assumptions. In Noakhali, Bangladesh, a study by Islam et al. (2023) evaluated the health hazards associated with consuming turmeric, a root spice that is grown locally and sold in markets.  with similar results to our findings with garlic, they discovered an enhanced carcinogenic risk but an acceptable non-carcinogenic risk. The higher cancer risk observed in the Chiang Mai cultivation area compared to Lampang may be attributed to several environmental and agricultural factors. These include the illegal dumping of waste near cultivation zones, which potentially leads to soil and water contamination. Additionally, the predominant cultivation of chili in Chiang Mai involves intensive use of chemical fertilizers and pesticides, further contributing to elevated levels of hazardous substances in the agricultural environment.

Thus far, there are concerning levels of human health risk indices, and it has been found that garlic may accumulate heavy metals (Akhter et al., 2022; Mawari et al., 2022), all of which are related to the findings of our study. A study conducted in Pakistan (Rashid et al., 2023) investigated the health risks related to the heavy metals in garlic grown in contaminated soil with organic supplements derived from waste. The aforementioned study found similar results to the current study in terms of Pb and Cd levels, which were higher than recommended. The HHRA is dependent on ecological factors in its relationship to garlic, which include the use of fertilizer, pesticides, soil conditions, other synthetic compounds and multi-cropping, particularly in the study region where the farmers were also growing chilies. Furthermore, waste was illegally dumped in the research area without any technological management in the Chiang Mai area, which could have had the effect of transferring toxic elements into the food chain and impacting human health, as was found in China. Accordingly, the presence of illegally-dumped waste provided a greater risk of cancer for both adults and children (Du and Li, 2023). The results in terms of the effect of Cd on adults and children (Galdames et al., 2022) were likewise found to be at higher levels than the recommended the limit. It was noted that children were presented with greater danger to their health than were adults or the elderly, who may have been affected by factors related to their bodyweight. The health risk indices in Cd, Cr, Cu and Pb were lower than the recommended values, which were consistent with the Italian study (Vannini et al., 2022).

CONCLUSION AND RECOMMENDATION

The findings showed that the amount of heavy metals in the soil was below standard, except in the case of Cd, which was not suitable for agricultural soil. For garlic, Pb concentration was found to be higher than the guideline level set by the Codex Alimentarius Commission, while the presence of other metals was not excessive. In terms of the risk associated with garlic consumption, As exceeded the recommended threshold for cancer risk, indicating a potential health concern. However, the overall assessment suggests that garlic cultivars from both locations did not pose significant non-carcinogenic health risks. These results could assist in explaining the reason that even though heavy-metal concentrations in soil and garlic were below recommended levels. Based on the findings of this study, the following practical recommendations and future research are proposed to support environmental safety, protect public health, and promote sustainable agricultural practices:

• Water and soil quality monitoring: Irrigation water sources and agricultural soils should be regularly tested for heavy metal concentrations to prevent further contamination of soils and crop production systems.

• Controlled use of agrochemicals: This involves providing farmers with basic knowledge on the proper use of fertilizers, pesticides, and herbicides through practical guidelines, training programs, and technical consultations.

• Zoning: Enforce land-use regulations to restrict the cultivation of food crops in areas located near contamination area such as waste disposal sites, thereby reducing the risk of contamination.

• Alternative crop investigation: Explore the feasibility of cultivating alternative crops in the areas.

• Health risk assessment: It should be noted that while the results indicate a potential risk to human health, the prediction only applies to the area in general. Individual characteristics, such as age, health and lifestyle choices, may have different effects.

Although Enrichment Factors (EF) values were not calculated in this study, future research should incorporate EF analysis to quantify the extent of anthropogenic influence, especially in light of local reports indicating regular agrochemical use and suspected nearby pollution sources.

ACKNOWLEDGMENTS

The authors would like to express their sincere gratitude to the Piang Luang Subdistrict Administrative Organization and the staff of the Ngao District Agriculture Office for their valuable support. Special thanks are also extended to all garlic cultivators who generously contributed their time and effort to participate in this study. The authors gratefully acknowledge the guidance and insightful advice provided throughout the research by Asst. Prof. Pensri Watchalayann.

AUTHOR CONTRIBUTIONS

Wiyada Nilkarnjanakul prepared the core idea of the research and assisted in survey development, and conducted data collection and data analyses, and wrote the manuscript. Patsiri Srivieng prepared the core idea of the research, coordinated, conducted data collection and data analyses, and wrote the manuscript. All authors have read and approved of the final manuscript.

CONFLICT OF INTEREST

The authors declare that they hold no competing interests.

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