Munir et al. 2022 — Heavy metal contamination of natural foods: a review (Sustainability)

This open-access narrative review in MDPI Sustainability surveys the literature on heavy metal contamination in plant-based foods (cereals, root and leafy vegetables, fruits), focusing on the five metals the authors treat as the most relevant for dietary toxicity — arsenic, cadmium, lead, mercury, and chromium — with additional sections on nickel and iron. It is structured in three blocks: a per-metal toxicology block (mechanisms of action, target organs, carcinogenicity), a per-crop occurrence block reporting concentration ranges cited from prior studies for rice, wheat, potato, tomato, lettuce, cabbage, carrot, avocado, orange, pawpaw, and pineapple, and a biomarkers block covering internal-dose, biologically-active-dose, and susceptibility biomarkers. The authors retrieved literature from Science Direct, PubMed, Google Scholar, and the Directory of Open Access Journals; no systematic-review protocol (PRISMA, defined inclusion/exclusion criteria, risk-of-bias assessment) is reported. The paper makes no primary measurements and presents no new occurrence data — every concentration value in the body is paraphrased from a cited secondary source.

Key numbers

All numerical values below are reported by Munir et al. as paraphrases of cited prior studies, not as primary measurements. Concentrations are in mg/kg fresh weight unless otherwise noted; Munir et al. do not specify wet vs dry weight for most figures.

Toxicology table (Table 1, ref [28])

The authors tabulate organ toxicity and disrupted-macromolecule mechanisms for the five metals they treat as central:

• Mercury (Hg): CNS injury, renal dysfunction, GI ulceration, hepatotoxicity; mechanisms include aquaporin mRNA reduction, glutathione peroxidase inhibition, increased c-fos expression, ROS production, enzyme inhibition, thiol binding (GSH conjugation).
• Lead (Pb): CNS injury, hematological changes (anemia), pulmonary dysfunction, GI colic, liver damage, reduced pulmonary function, cardiovascular dysfunction; mechanisms include enhanced IL-1β/TNF-α/IL-6 in CNS, increased serum ET-1/NO/EPO, inactivation of δ-ALAD and ferrochelatase (heme biosynthesis inhibition), and reduced GSH/SOD/CAT/GPx.
• Chromium (Cr): kidney dysfunction, GI disorders, dermal disease, increased cancers of bladder, kidney, lung, larynx, testes, bone, thyroid; mechanisms include DNA damage, genomic instability, oxidative stress and ROS generation.
• Cadmium (Cd): degenerative bone disease, kidney dysfunction, liver damage, lung injury, GI disorders, metabolic syndrome with Zn/Cu interactions, cancer; mechanisms include miRNA dysregulation, apoptosis, ER stress, Cd-MT renal absorption, dysregulation of Ca/Zn/Fe homeostasis, low serum PTH, ROS, altered phosphorylation cascades.
• Arsenic (As): cardiovascular dysfunction, CNS injury, skin and hair changes, liver damage, GI discomfort; mechanisms include neurotransmitter homeostasis alteration, oxidative phosphorylation uncoupling (ATP inhibition), capillary endothelial damage, thiol binding.

Cereals

• Rice (Bangladesh, n = 71 irrigated- and rain-fed-season samples; cited from ref [121]): irrigated-season vs rain-fed-season mean ± SD — arsenic 0.153 ± 0.112 vs 0.140 ± 0.080 mg/kg; cadmium 0.073 ± 0.069 vs 0.038 ± 0.032 mg/kg; lead 0.264 ± 0.125 vs 0.147 ± 0.077 mg/kg; chromium 1.208 ± 0.913 vs 0.986 ± 0.796 mg/kg. Munir et al. paraphrase the source’s conclusion that groundwater contamination drives the elevated arsenic and that industrial-effluent contamination drives the elevated cadmium, lead, and chromium.
• Rice exposure (cited from ref [122]): for Bangladeshi adults consuming 400 g rice/day at 60 kg body weight, estimated daily intake ranges are As 18.6–214 μg, Cd 2.6–119 μg, Pb 25.0–241 μg, Cr 59.0–1846 μg; rice alone is reported to contribute up to 46%, 57%, 50%, and 60% of the maximum tolerable daily intake (MTDI) for As, Cd, Pb, and Cr respectively. (Speciation is not stated; the values are total-element exposures, not iAs or Cr-VI.)
• Wheat (cited from ref [124], multiple soils/water sources): ranges — Cd 0.011–0.039, Pb 0.166–0.209, As 0.005–1.113, Ni 0.015–2.060, Cu 0.089–4.625, Zn 0.111–3.169, Cr 0.013–1.018, Mn 0.100–4.467 mg/kg.

Vegetables (root, fruit, and leafy)

Values below are cited by Munir et al. from refs [127] (potato) and [128] (Bempah et al. 2011, Ghanaian market basket survey of fruits and vegetables); ranges are mg/kg.

• Potato: Fe 48.87–72.64; Cu 3.07–5.43; Zn 13.80–18.88; Mn 6.93–13.06; Pb 0.51–0.77; Ni 2.02–3.55; Cd 0.08–0.32. Accumulation order reported as Fe > Zn > Mn > Cu > Ni > Pb > Cd.
• Tomato: Pb 0.14–0.28; Cd 0.004–0.06; Cu 1.64–2.86; Zn 6.46–8.42; Cr 0.02–0.08.
• Lettuce: Pb 0.48–0.63; Cd 0.02–0.12; Cu 1.12–2.44; Zn 8.09–13.6; Cr 0.09–0.17.
• Cabbage: Pb 0.22–0.53; Cd 0.006–0.08; Cu 1.84–3.40; Zn 6.86–14.3; Cr 0.03–0.13.
• Carrot: Pb 0.12–0.23; Cd 0.01–0.05; Cu 1.14–2.33; Zn 8.22–10.6; Cr 0.06–1.22.

Fruits

All four fruit subsections cite ref [131] (Ihesinachi & Eresiya 2014, Rivers State, Nigeria, single-farm survey, Kaani-Bori). Single point values, not ranges; basis not specified in the review. mg/kg:

• Avocado pear: Cd 0.15; Cu 3.10; Zn 8.87; Fe 28.60; Pb 1.69; Ni 3.34; Mn 1.31; Co 1.62.
• Orange: Cd 0.10; Cu 0.23; Zn 7.22; Fe 19.0; Pb 5.80; Ni 2.99; Mn 1.09; Co 1.67.
• Pawpaw (papaya): Cd 0.22; Cu 5.29; Zn 7.31; Fe 29.60; Pb 5.87; Ni 5.57; Mn 1.03; Co 3.56.
• Pineapple: Cd 0.08; Cu 0.64; Zn 6.78; Fe 25.70; Pb 4.52; Ni 1.16; Mn 2.60; Co 1.43.

The orange and pawpaw Pb values (5.80 and 5.87 mg/kg) are an order of magnitude above the EU ML for fruits (0.10 mg/kg, Reg. 2023/915) and well above lead concentrations reported in mainstream fruit-survey literature; values flow from a single Nigerian farm-site survey to a single review citation and should be checked against the primary [131] before any downstream use.

Other framing claims

• Munir et al. note that Pb, Cd, As, Cr, and Hg are listed among the top twenty toxic chemicals by the US EPA (citing refs [24,25]).
• Co, Cu, Ni, Fe, Mn, Zn, Mo, and Se are framed as essential at low concentrations but capable of causing serious complications above a “safe limit” (no numerical limit is given in this review).
• The review does not present any Codex Alimentarius ML table, FAO/WHO Codex CXS 193-1995 numbers, or any other harmonized regulatory limits. Earlier wiki text claiming this review contains a Codex ML summary was incorrect — see Verification notes.

Methods (brief)

Non-systematic narrative literature review. No PRISMA flow, no documented inclusion/exclusion criteria, no risk-of-bias assessment, no quantitative synthesis. Sources retrieved from Science Direct, PubMed, Google Scholar, and DOAJ. Published 24 December 2021 in Sustainability 14(1), 161 (received 2 November 2021; accepted 16 December 2021). 141 cited references, primarily 2000–2021 vintage. Funding: KU Research Professor Program, Konkuk University, Seoul. Authors declare no conflict of interest. Evidence tier B because this is a non-systematic peer-reviewed secondary review with all concentration data derived from cited secondary studies; the review’s primary value is its consolidated toxicology table for the five core metals.

Implications

Certification: The paper is suitable as a secondary citation when articulating the toxicology basis for the HMTc analyte panel (Pb, Cd, tAs/iAs, tHg/MeHg, Cr-VI). It is not suitable as a source of occurrence values for threshold-setting — the concentration ranges in §3 are tertiary citations of primary studies (refs [121], [122], [124], [127], [128], [131]), and any HMTc workbench use of these ranges should go directly to those primary sources, several of which are themselves single-site or single-country surveys with limited geographic generalizability.

Courses: The toxicology summary (Table 1, §2.1–2.5) is well-organized and pedagogically usable as an introductory framing for course modules on heavy metal mechanisms and target-organ effects. The crop occurrence block is more uneven and should be paired with newer systematic syntheses (HBBF, Consumer Reports, FDA Closer to Zero) when used in teaching materials.

App: Not a source of contamination_profile values for ingredient pages. Values cited here are too thin and too geographically narrow to populate typical_ppb / p95_ppb fields for any of the named ingredients. The cited primary studies, if ingested separately, would feed the synthesis layer.

Verification notes

• 2026-05-18 (Claude session, merge-enhance): Earlier version of this page reported FAO/WHO Codex maximum levels in plant foods (Pb in cereal grains 0.2 mg/kg, root/tubers 0.1, fruity vegetables 0.05, leafy vegetables 0.3; Cd cereal grains 0.1, rice 0.4, leafy vegetables 0.2; Codex CXS 193-1995). Re-read of full PDF (20 pages) confirms the review does not present a Codex ML table and does not cite Codex CXS 193-1995. The fabricated Codex numbers have been removed and replaced with the review’s actual quantitative content (rice / wheat / vegetables / fruits concentration ranges from refs [121]–[131]).
• 2026-05-18 (audit subagent flagged; verified against PDF — applied): First-pass transcription of the pawpaw subsection (§3.3.3) dropped one of the eight values listed in the published table. The PDF actually states “0.22, 05.29, 07.31, 29.60, 05.87, 05.57, 01.03, and 3.56 mg/kg, respectively” for cadmium, copper, zinc, iron, lead, nickel, manganese, and cobalt — i.e. eight values for eight named metals, with no omission. Audit subagent (2026-05-18) caught the dropped 05.57 and the resulting downstream errors (the wiki was assigning 1.03 to Ni and marking Mn as missing, when 5.57 belongs to Ni and 1.03 belongs to Mn). Corrected to Ni 5.57, Mn 1.03; the “value missing” claim has been removed because no value is actually missing.
• 2026-05-18 (Claude session): The orange (5.80 mg/kg) and pawpaw (5.87 mg/kg) Pb values originate in a single Nigerian Rivers-State farm-site survey (ref [131]) and are roughly 50× the EU fruit ML. They are reproduced here as the review states them, but flagged because they would substantially distort any aggregate fruit-Pb profile if treated as representative.
• 2026-05-18 (Claude session): Evidence tier left at B (peer-reviewed non-systematic review with consolidated toxicology table and tertiary concentration data); raw_handle corrected from the legacy papers-cube value to the current PCMF_… convention; metals and ingredients frontmatter expanded to reflect the review’s actual coverage (Ni, Cu, Zn, Fe, Mn, Co added; potato, tomato, lettuce, cabbage, carrot, avocado, oranges, cereals, leafy-vegetables added as ingredient links). Pineapple and pawpaw are mentioned in body but not declared as ingredient links because no ingredients/pineapple or ingredients/papaya page exists; freq-1 mention from a single review does not meet the 5-paper threshold for ingredient-page creation.

Wiki pages updated on ingest

• lead
• cadmium
• chromium
• mercury-total
• arsenic-total
• nickel
• rice
• wheat
• vegetables
• fruits

Page history

The five most recent substantive edits to this page. The full version history lives in git; when DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.