Preliminary Assessment of Toxic Metals in Cookware, Moulding Materials and Soils from Manufacturing Sites in Southwest Nigeria, with Evaluation of Cookware Leaching Potential

doi:10.21203/rs.3.rs-7536704/v1

Informally manufactured cookware is often associated with elevated levels of toxic metals due to poor quality control and unregulated use of moulding materials. In this study, a preliminary assessment of toxic metal levels in cookware, associated moulding materials, and soils collected from informal cookware manufacturing sites in Saki, southwest Nigeria, with evaluation of cookware leaching potential, was conducted. Samples were acid-digested and analyzed for Mn, Pb,Cr, Cd and Ni using flame atomic absorption spectrophotometry. Cookware leaching potential was evaluated by boiling tap water in pots for 1, 2, and 3 hours, followed by metal analysis of the leachates. Metal concentrations (mg/kg) in cookware samples ranged as follows: Mn (94.0–1,064), Pb (18.4–439), Cr (10.4–60.2), Cd (0.300–5.45), and Ni (33.3–578). Moulding materials showed: Mn (79.2–185,388), Pb (8.35–1,004), Cr (5.78–58.5), Cd (<0.001–11.7), and Ni (0.0250–48.5). Soil samples contained: Mn (257–1,306), Pb (10.7–177), Cr (30.0–95.0), Cd (0.0250–3.88), and Ni (9.90–58.0). Metal concentrations in cookware generally exceeded accepted safety limits. Manganese was particularly predominant in soil and moulding materials, while Pb and Cd were present at levels of concern. However, metal concentrations in all leachate samples were below detection limits, indicating minimal immediate risk of metal release under normal cooking conditions. Despite the negligible leaching observed, the high total metal concentrations in cookware highlights potential long-term risks to public health. These findings underscore the need for regulatory oversight, improved manufacturing practices, and sustainable quality control measures in informal cookware manufacturing to prevent toxic metal exposure and contamination.

Toxic metals

cookware

informal manufacturing

environmental contamination

leaching potential

Cookware is essential household equipment used for food processing and preparation, encompassing various cooking containers such as saucepans and frying pans that facilitate heat transfer during food preparation (Imura and Johnson, 2011). Internationally, metals and alloys are widely employed in cookware manufacture due to their excellent thermal conductivity and resistance to temperature changes (Koo et al. 2020). Industrial cookware production commonly utilized metals including Fe, Al, Sn, Cr, Ni and Cu, either individually or in combination to form alloys such as stainless steel, aluminium alloy, brass and bronze, etc. However, toxic metals may be introduced as impurities in the manufacturing process (Conti, 2007).

In many developing countries, including Nigeria, informal cookware manufacturing has emerged as a significant economic activity involving the recycling of various metallic waste materials. This process, known as casting, involves the recycling of various metallic wastes such as engine and car parts, old utensils, scrap iron, and electronic wastes for making cookware (Kuhangana et al. 2024; Street et al. 2020; Weindenhamer et al. 2014). While this informal recycling represents a form of local entrepreneurship, it may lead also to substantial contamination of the environment, with potential health implications for cookware makers (Street et al. 2020). The composition of collected waste metallic materials is typically unknown, and the produced cookware often lacks proper finishing treatments such as anodization, which require specialized electrochemical equipment under controlled conditions that are generally unavailable in informal manufacturing setups (Kuhangana et al. 2024).

The leaching of metals from cookware into foods represents a significant pathway for human exposure to both essential and toxic metals. Various factors influence metal migration, including cooking duration, pH levels, food type, and surface treatment (Fatunsin et al. 2022). While certain metals such as such as Fe, Ni, Cr and Zn are essential for human health, prolonged and excessive intake above recommended levels can result in adverse health issues. More concerning is the potential leaching of non-essential toxic metals such as Al, Cd, Pb, etc. from cookware, which poses significant public health risks due to their lack of biological function and potential for bioaccumulation (Fatunsin et al. 2022). Previous studies have suggested possible links between aluminium cookware use and Pb absorption (Binkhorst et al. 2025; Sultan et al. 2023; Street et al. 2020; Swaddiwudhipong et al. 2013), with reports indicating that the quantity of metals leached from aluminium cookware increased with the age of the cookware (Ollor et al. 2022), temperature of cooking, and cookware age (Habimaama et al. 2022; Weidenhamer et al. 2017; Mohammad et al. 2014; Odularu et al. 2013; Dabonne et al. 2010). However, some studies have shown that immediate migration level of metals such as Al, Fe and Mn into boiled water using informally produced aluminium cookware were within the WHO limits for drinking water (Adelabu and Campbell, 2020). pH has been identified as a critical factor influencing leaching, with low pH conditions causing greatest metal migration into deionized water (Fatunsin et al. 2022).

Environmental contamination at cookware manufacturing sites presents additional public health concerns. Toxic metals can be released into surrounding soils during production processes and through improper disposal of manufacturing byproducts. Soil contamination with toxic metals can affect local ecosystems and lead to groundwater pollution, impacting both human health and the environment. Previous research has documented elevated Pb levels in surface dust at cookware foundries, exceeding acceptable safety limits (Kuhangana et al. 2024). Despite these concerns, limited data exists on the extent of environmental contamination at informal cookware manufacturing sites, particularly in developing countries where such activities are prevalent. Given the widespread use of locally manufactured cookware in Nigeria and the potential health risks associated with toxic metal contamination, a comprehensive assessment of metal levels in cookware and associated manufacturing environments is crucial. The aim of this study was to conduct a preliminary evaluation of selected metal levels in cookware, moulding materials, and soils from informal cookware manufacturing sites in Saki Southwest Nigeria, and to evaluate leaching potential of the manufactured cookware. This study addresses the current knowledge gap regarding human exposure to metals from cookware and environmental contamination at informal manufacturing sites.

2.1 Sampling and sample collection

Four iron cookware samples were obtained from different artisanal manufacturers within Saki West Local Government Area of Oyo State, Nigeria. The materials used in moulding cookware such as clay and sandy soil (utilized for shaping purposes), charcoal (to facilitate the melting process of the scrap metals), ash (to achieve shiny silvery colour) and batteries (to eliminate impurities from the melted metals) used in moulding the cookware (Fig. 1) were collected from one of the locations of the finished products. A total of 16 soil samples comprising eight topsoil (0–15 cm) and eight subsoil (15–30 cm) were collected from eight iron cookware manufacturing sites, while one background soil sample was collected from uncontaminated sites within the same geographical area. The soil sampling locations and their corresponding GPS coordinates are shown in Table 1.

Table 1

Sampling location in Saki West Local Government and GPS
S/N	Location	GPS
1	Oke Oro	8⁰40'01.6"N 3⁰23'41.9"E
3	Marikas	8⁰41'26.0"N 3⁰24'31.6"E
3	Muslim Hospital	8⁰40'37.2"N 3⁰'24'27.8"E
4	Ayetoro	8⁰39'40.9"N 3⁰23'38.5"E
5	Oke Dio	8⁰39'39.4"N 3⁰23'40.5"E
6	Ajegunle	8⁰39'33.4"N 3⁰23'50.5"E
7	Apinite	8⁰39'32.5"N 3⁰23'45.6"E
8	Ero Omo	8⁰41'14.1"N 3⁰24'37.2"E
9	Arafat (background)	8⁰41'31.0"N 3⁰24'40.7"E

2.2 Sample preparation

Small portions (approximately 2 g) were clipped from each cookware sample using handheld stainless-steel pliers, weighted to know the exact weight and placed in separate plastic containers and labeled properly. Collected soil samples and moulding materials were air-dried for one week, followed by grinding using an agate mortar, and sieved through a 2 mm mesh. The processed samples were then packed into plastic containers and stored in a dry place for subsequent analyses.

2.3 Sample digestion and analysis

Exactly 2.00 g of each sample (cookware, moulding materials and soil) was separately weighed into 50 mL polyethylene digestion bottles using an analytical balance. Then, 20 mL of aqua regia was added, and the mixture was heated in a water bath on a hot plate in a fume cupboard for 2 hours. After cooling, the mixture was filtered using Whatman No. 1 filter paper into a 50 mL volumetric flask, which was then filled to the mark with distilled water. A sample blank was also prepared. Digested samples were analysed for their heavy metals content using Flame atomic absorption spectrophotometer (Buck Scientific Model 210, United States) for Mn, Cr, Cd and Ni.

2.4 Metals’ leaching potential of cookware

To assess the leaching potential of locally manufactured cookware of different sizes, 500 mL of distilled water was added to each cookware and boiled for 1 h, 2h and 3h, respectively. The water was then allowed to cool and made up to 100 mL in a volumetric flask using distilled water. The choice of boiling time was informed by the general average time of boiling water in typical households in Nigeria. Control samples (not boiled with any cookware) were prepared using distilled and tap water. The boiled and control water samples were analyzed for five toxic metals (Mn, Cr, Cd, Ni and Pb) using a Buck Scientific Atomic absorption spectrophotometer (Model 210, United States).

2.5 Quality control and statistical analysis

Analytical grade reagents were used throughout the analysis. Reagent blanks were employed in all analyses to detect reagent impurities and environmental contaminants. Plastic bags were used for collection of soil samples to avoid potential contamination from metal-made containers. Glass and plastic ware were soaked in 10% nitric acid for 24 hours, then rinsed with distilled water and dried at room temperature before use to remove any adsorbed metal of interest. Instruments were calibrated, and other tools and work surfaces were meticulously cleaned to prevent cross-contamination during grinding. Samples were analysed in duplicates. One-way analysis of variance (ANOVA) was performed across all metals to determine if there was significant differences among the metals concentrations, while Turkey’s HSD test was used to perform pair-wise comparisons between each pair of metal for each location.

3.1 Metal concentrations

The average total metal concentrations in cookware are presented in Table 2, while all leachable metals were below detection limit and are therefore not reported in any table. The total metal concentrations (mg/kg) in cookware samples ranged as follows: Mn (94.0–1,064), Pb (18.4–439), Cr (10.4–60.2), Cd (0.300–5.45), and Ni (33.3–578). Moulding materials showed: Mn (79.2–185,388), Pb (8.35–1,004), Cr (5.78–58.5), Cd (< 0.001–11.7), and Ni (0.0250–48.5). Soil samples contained: Mn (257–1,306), Pb (10.7–177), Cr (30.0–95.0), Cd (0.0250–3.88), and Ni (9.90–58.0). Notably, concentrations of all metals in leachate samples were below the detection limit of the analytical equipment.

Table 2

Concentration of metals in cookware in this study
Sample Location	Concentration (mg/kg)
Sample Location	Mn	Cr	Cd	Pb	Ni
1	1,064^a	62.2^c	5.45^d	439^b	578^b
2	172^a	10.4^c	0.625^d	49.1^b	49.0^b
3	170^a	21.9^b	0.300^d	18.4^c	33.3^b
4	94^a	13.4^d	0.675^e	35.3^c	63.6^b
Mean	375	26.9	1.70	136	181
SD	461	23.9	2.50	201	265

Values with different superscript across a row differ significantly

Metal concentrations in cookware materials were significantly different (F_calc >F_crit), with Mn recording the highest concentrations among all the studied metals across all sampling locations. Manganese concentrations were significantly greater (p < 0.05) than other studied metals, with standard deviations exceeding mean concentration, indicating substantial variability in Mn levels across the four locations. The presence of Mn in cookware likely originated from steel scrap used during the casting process and primary cell batteries, which manufacturers reportedly use to remove impurities from cookware. Additionally, steel naturally contains small amounts of Mn that can transfer to cookware during melting and casting processes. The Mn concentration observed in this study are comparable to, but lower than, those reported by previous researchers who characterized total Mn levels in aluminium cookware using XRF analysis (Fellows et al. 2024; Sultan et al. 2023). When expressed as mg/kg of initial cookware weight, the results of this study are relatively higher than those reported for Mn leached into foods cooked with stainless steel and alloy pots (Elemo et al. 2021), and water leached into aluminium cookware (Habimaama et al. 2022; Adelabu and Campbell 2020). Although Mn is an essential trace metal serving as a cofactor for important enzymes in different metabolic pathways, excessive exposure, especially through ingestion, can cause neurodegenerative damage and lung toxicity (Crossgroove and Zheng, 2004). However, unlike other metals, regulatory limits for Mn in food are not commonly specified.

Chromium concentrations in cookware samples ranged from 10.4 to 165 mg/kg. Although Cr is essential for human health and assists insulin function, its toxicity is mainly associated with the hexavalent form (Cr⁶⁺). Comparing these values to the FAO/WHO acceptable limit for chromium in food (2.3 mg/kg) suggests potential concern regarding food contamination when using this cookware (FAO/WHO 2001). However, the absence of detectable Cr in tap water leachates aligns with previous reports for Cr leached into water boiled in locally made alloy pots (Habimaama et al. 2022; Elemo et al. 2021), and aluminium and steel cookware (Sultan et al. 2023) suggesting minimal leaching of Cr under normal conditions.

Cadmium concentrations ranged from 0.30 to 5.45 mg/kg in cookware samples, exceeding the FAO/WHO acceptable limit for cadmium in food (0.20 mg/kg) and raising concerns about potential food contamination (FAO/WHO 2001). Despite elevated total concentrations, no Cd leaching into tap water was observed after boiling for up to 3 hours. These leaching results are lower than those reported for metals leached from both new and old cookware (Sultan et al. 2023) and metals leached into rice cooked with new cookware (Ojezele et al. 2016). Chronic Cd exposure is associated with irreversible renal dysfunction and heightened excretion patterns (Bernard 2004).

Lead concentrations in cookware samples ranged from 18.4 to 439 mg/kg. These values greatly exceed levels found in foods cooked with stainless steel pots (Elemo et al. 2021) and rice cooked in newly purchased cookware (Ojezele et al. 2016). A related study reported that the total Pb concentration in several aluminium pots ranged from 4 ppm to 16,000 ppm, with an average of 4 ppm (Binkhorst et al. 2025). Although Pb was not detected in boiled water leachates, these concentrations far exceed the FAO/WHO allowable limit for Pb in food (0.3 mg/kg), raising concerns about potential food contamination, and particularly under acidic conditions. Binkhorst et al. (2025) demonstrated significant leaching of Pb from aluminium cooking pots under acidic conditions. Lead’s bioaccumulation potential and its association with behaviourial and cognitive disorders, especially in children, makes this finding particularly concerning.

The generally elevated total metal concentrations observed in this study relative to literature values, likely reflect the acidic sample preparation conditions employed, representing a “worst-case scenario” for metal extraction. Metals are known to leach more readily under acidic conditions compared to neutral pH environments typical of boiling water or normal food preparation, as confirmed by leaching results of this study. Previous studies have documented enhanced metal leaching into foods prepared under acidic conditions (Ali et al. 2021; Exley 2013).

The non-anodized nature of the cookware samples in this study likely contributed to greater leaching potential of total metals. This observation is supported by previous research demonstrating significant metal leaching in new non-anodized aluminium cookware compared to old non-anodized aluminium cookware, with old anodized cookware showing greater leaching susceptibility than new anodized cookware (Sultan et al. 2013). The absence of detectable metal leaching under neutral conditions in this study suggests that immediate health risks may be limited under normal cooking conditions, though, a high total metals content remains a concern for long-term exposure, particularly under acidic cooking conditions.

3.2 Toxic metal levels in moulding materials used in manufacturing cookware

Table 3 presents the toxic metal concentrations in materials used for cookware manufacturing. The average metal concentrations followed the pattern: Mn > Pb > Cr > Ni > Cd across all moulding materials. Manganese consistently exhibited the highest concentrations among all studied metals in manufacturing materials, with an exceptionally high concentration of 185,388 mg/kg detected in battery materials. The extreme Mn level in batteries used in cookware manufacturing from the study area likely accounts for the elevated Mn concentrations observed in finished cookware products.

The elevated levels of Pb and Cd in battery materials are consistent with their roles as natural components in battery manufacturing processes. Lead-acid batteries typically contain substantial amounts of Pb, while Cd is commonly found in rechargeable batteries. The incorporation of these battery materials into informal cookware manufacturing process represent a significant source of toxic metal contamination in the final products.

The presence of elevated metal concentrations in cookware manufacturing materials, especially Mn, Cd and Pb, poses considerable occupational health risks for artisanal workers involved in informal cookware manufacture. Such workers face potential metal exposure through several pathways, including inhalation of dust particles, dermal contact with contaminated materials, accidental ingestion of these materials in the course of production. These findingd highlight the need for improved safety protocols and protective equipment in informal manufacturing settings to minimize exposure to toxic metals.

Table 3

Total concentration (mg/kg) of toxic metals in moulding materials used in cookware manufacturing
Moulding material	Concentration (mg/kg)
Moulding material	Mn	Cr	Cd	Pb	Ni
Clay & sand	240	37.6	< 0.001	13.8	14.8
Clay	434	58.5	< 0.001	28.1	36.2
Sand	165	42	< 0.001	8.35	8.88
Ash	79.2	27.0	< 0.001	15.3	4.38
Charcoal	107	5.8	< 0.001	6.58	0.025
Battery	185,388	24.1	11.5	1,004	48.5

< 0.001 - below limit of detection by the measuring equipment

3.3 Toxic metal concentrations in soils of cookware manufacture sites

The concentration of toxic metals in soils from cookware manufacturing sites are presented in Fig. 2. Limited existing research on metal contamination at cookware manufacturing sites restricted direct comparison with previous studies, highlighting the novelty and importance of this investigation.

Manganese concentrations consistently decreased with soil depth across all sampling locations (Fig. 2a), ranging from 257 mg/kg to 1,305 mg/kg. The lowest concentration observed in topsoil from the background site, while the greatest concentration was observed in topsoil of Apinite. This depth-related pattern suggests surface accumulation of Mn, likely arising from deposition of Mn-containing particles, and limited vertical migration through the soil profile.

Chromium concentrations varied from 29.8 mg/kg to 94.8 mg/kg (Fig. 2b). When compared to the NESREA maximum allowable limit of 200 mg/kg for Cr (NESREA 2009), all soil samples, including those from the background site, remained within acceptable limits. This suggests that Cr contamination from cookware manufacturing activities has not reached critical levels in the study area, possibly due to the relatively low mobility of Cr in soil environments.

Cadmium concentrations ranged from 0.025 mg/kg to 3.88 mg/kg, with non-detectable levels in two subsoil samples-one topsoil sample and the background site. Comparison with the NESREA (2009) maximum allowable limit of 3 mg/kg revealed that most samples, including background sites, remained within acceptable limits, with exception of subsoil at Ayetoro, which recorded 3.88 mg/kg. This localized high concentration suggests specific Cd-releasing processes in this location.

The Pb concentration ranged from 15 mg/kg to 188 mg/kg across the studied studied. There was no regular pattern in soil-Pb profile, with 50% of the soil profiles showing decreased concentration with depth, including the background samples. Similar soil Pb leaching profile have been previously reported in soils (Iniaghe and Adie 2018). Comparison with the NESREA (2009) maximum permissible limit of 164 mg/kg showed that most soil samples, including background sites, remained within acceptable limits, except for subsoils at Apinite. These findings are consistent with previous research by Kuhangana et al. (2024) who reported Pb level up to 347 mg/kg in surface dust at cookware foundries, which were higher than acceptable limits.

Nickel concentrations showed considerable spatial variations, with concentrations ranging from 23.8 mg/kg in subsoil at Ero Omo to 68.5 mg/kg in topsoil at Marikas. Comparison with NESREA maximum allowable limits revealed that about 18% of studied soil samples exceeded acceptable limits. Sites with elevated Ni concentrations included both topsoil and subsoils samples from Marikas, Mushin hospital, and Apinite, as well as topsoil from Oke Oro. This pattern suggests that Ni contamination is more widespread than other metals, possibly due to greater mobility in soil environments.

3.4 Correlation Studies

The correlations between metals in cookware, moulding materials used in cookware manufacture and soils within cookware manufacturing sites are shown in Table 4. A strong positive correlation between Mn and Cd, Mn and Cr, Mn and Pb, and Mn and Ni in cookware was observed, with correlation values of r = + 0.99, + 0.97, + 0.99 and + 0.99, respectively. In moulding materials used in cookware manufacture, strong positive correlations were observed between Mn and Pb, and Mn and Ni, with correlation values of r = + 0.99 and + 0.75, respectively; and a weak negative correlation between Mn and Cr (-0.22). However, the correlation between manganese and cadmium could not be calculated due to the absence of cadmium in most materials. In soils, there was a weak positive correlation between Mn and Cr (+ 0.29), and a strong positive correlation between Mn and Cd, Mn and Pb, and Mn and Ni, with correlation values of r = + 0.74, + 0,72, + 0.83, respectively. The positive correlation between Mn and the other metals suggests that they originate from the same source. Conversely, the negative correlation value suggests that the contamination of these metals arises from distinct sources.

Table 4

Correlation values between manganese and other metals in iron cookware, soil, and moulding materials
Correlation values	Mn & Cr	Mn & Cd	Mn & Pb	Mn & Ni
Iron cookware	+ 0.97968	+ 0.99164	+ 0.99493	+ 0.99277
Soil samples	+ 0.29626	+ 0.74615	+ 0.72746	+ 0.83866
Materials	-0.22749	-	+ 0.99984	+ 0.75599

This study has shown the presence of bioavailable toxic metals (Mn, Cd, Ni, Pb, and Cr) in cookware, moulding materials used in cookware manufacture and soils within cookware informal cookware manufacturing sites in southwest Nigeria. Among the detected metals, Mn was the predominant metal, particularly in materials derived from used batteries. The presence of Pb and Cd, also associated with batteries, raise some level of concern due to their toxicities. Although, comparing the metal concentrations in cookware with regulatory limits indicated potential risks, especially considering the possibility of metal leaching from non-anodized cookware, leaching tests simulating boiling conditions (up to 3 hours) showed that metal levels were below detection limits. This suggests minimal instant migration of metals during single-use scenarios. However, the high total metals concentration in cookware suggests that long-term use, especially with acidic foods or repeated use, could lead to cumulative leaching and pose significant health risks. Soil samples from the vicinity of cookware manufacturing sites also showed elevated metal concentrations, indicating environmental contamination and potential occupational exposure for workers and surrounding communities.

To limit the risks associated with these findings, regulatory oversight and implementation of standardized safety testings are essential. There is the need for safer alternative materials, especially excluding battery component, to be used in cookware manufacture. Furthermore, there is the need for public health education and awareness campaigns targeted at informal cookware manufacturers to promote sourcing of safer alternatives and handling. Biomonitoring studies comparing the blood metal levels of individuals involved in informal scrap metal are encouraged to evaluate occupational exposure and guide appropriate health interventions.

Data availability statement

All data generated or analysed during the study are included in the manuscript.

Declaration of competing interest

The authors declare that no conflict of interest exist before, during and after the study.

Informed consent statement

Informed consent was obtained from local cookware moulders prior to obtaining samples of cookware, moulding materials and soils within the vicinity.

Ethics approval and consent to participate

Not applicable

Consent for publication

Not applicable

Availability of data and materials

All data generated or analysed during the current study are included in this article.

Competing interests

The authors declare that they have no competing interests.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authors’ contributions

ZAA: Project administration, Resources, Data curation, Writing - original draft. GUA: Conceptualization, Supervision, Writing - review and editing. POI: Statistical analysis, Writing - review and editing. All authors read and approved the final manuscript.

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No competing interests reported.

Preliminary Assessment of Toxic Metals in Cookware, Moulding Materials and Soils from Manufacturing Sites in Southwest Nigeria, with Evaluation of Cookware Leaching Potential

Status:

Version 1

Abstract

Figures

1 Introduction

2 Methods/Experimental

2.1 Sampling and sample collection

2.2 Sample preparation

2.3 Sample digestion and analysis

2.4 Metals’ leaching potential of cookware

2.5 Quality control and statistical analysis

3 Results and Discussion

3.1 Metal concentrations

3.2 Toxic metal levels in moulding materials used in manufacturing cookware

3.3 Toxic metal concentrations in soils of cookware manufacture sites

3.4 Correlation Studies

4 Conclusion

Declarations

References

Additional Declarations

Status:

Version 1