Heavy metals in Onions

This page draws on evidence from UK Food Standards Agency, FDA, and Codex Alimentarius, together with 10 peer-reviewed studies. The complete citation list is at the bottom of the page.

Overview

FSA/Fera measured this ingredient or non-infant-specific food composite in Table 6 of the FS102048 survey. Exact concentration values remain in progress until Table 6 is parsed into structured ingredient rows with less-than and semi-quantitative flags preserved. Survey of metals in commercial infant foods, infant formula and non-infant specific foods

Why this commodity accumulates heavy metals

Onions (Allium cepa) are bulb vegetables grown underground, but their metal accumulation pattern differs substantially from root vegetables such as carrots and potatoes. Onion bulbs are modified leaf bases rather than storage roots, and they lack some of the active metal-sequestration mechanisms that concentrate cadmium (Cd) and lead (Pb) in root tissues of other vegetables. Cadmium uptake in onions follows the general soil-to-plant pathway via root absorption, but the transfer factor from soil to bulb is lower than in dedicated root vegetables; onion bulbs do not appear to be hyperaccumulators of Cd under typical agricultural conditions. Lead accumulates in onion bulb tissue at low concentrations because Pb transfer from soil to above-ground plant parts is inefficient, and the bulb’s protected position within the soil and outer scales provides some barrier from surface Pb deposition. Nickel is detectable in onion tissue; the FDA TDS FY2018-FY2020 data FY2018-FY2020 TDS Elements Analytical Results show nickel (Ni) in the 0 to 100 ppb range (max 100 ppb) in raw mature onion (n=27). Sulfur compounds distinctive to Allium vegetables may influence metal speciation and binding in tissue, though specific mechanistic data on this interaction are limited in the current corpus. Total arsenic is detectable at low concentrations (median approximately 4 ppb in FDA TDS data), with inorganic speciation expected to represent a portion of that total; the level is substantially below arsenic concentrations in rice or seafood.

Heavy metal contamination profile

This table summarizes the available occurrence evidence for each metal. A data gap means the literature was reviewed for this commodity and metal but no usable occurrence data was found. Key sources link to contributing studies.

Synthesis basis and censoring treatment

The FDA Total Diet Study reported lead below its 4 ppb reporting limit in 26 of 27 raw-onion composites with a single detection at 7.4 ppb, and total mercury and total chromium below their 1 ppb and 50 ppb reporting limits in all 27 composites (FDA 2022). The former [0,0] lead value was a left-censored non-detect. Cleaner low-limit datasets do detect onion lead: the long-term UK Nafferton field trials reported onion-bulb lead of 13.4 to 14.5 ppb fresh weight (Rempelos et al. 2023), and the German BfR MEAL total diet study reported a vegetable-group lead mean near 14 ppb against a 2 ppb quantification limit (Fechner et al. 2022). The New Zealand survey found allium lead below the detection limit (Dearing et al. 2025).

The synthesized central ranges are lead 0 to 12 ppb, total mercury 0 to 1 ppb, and total chromium 0 to 25 ppb, all fresh weight. Lead is set at medium confidence because four consistent commercial fresh-weight datasets bracket a low but non-zero value. Total mercury is a fully censored bound. Total chromium reflects the sub-limit detections in the Jiaozuo survey (Wu 2024) that the 50 ppb FDA reporting limit cannot resolve. Elevated hotspot values are excluded from the central estimate.

Routing

This ingredient is listed in the ingredient index.

Contamination Profile State

Per-analyte state — populated, in progress, or declared data gap — is carried authoritatively in the machine-readable contamination_profile frontmatter and the contamination-profile table above. Ingredient-level values belong here; finished-product values belong on the relevant product-category page.

FDA TDS FY2018-FY2020 Evidence

FDA’s FY2018-FY2020 Total Diet Study dataset includes this page’s routed matrix as TDS Food 128, “Onion, mature, raw.” The normalized row-level data is stored in data/evidence/fda_tds_fy2018_2020_element_results_samples.csv, with per-food/per-analyte summaries in data/evidence/fda_tds_fy2018_2020_summary_by_food_analyte.csv. Concentrations are retained as FDA reported them, with reporting limits preserved separately; reported zeroes are not rewritten as <LOD without a source-specific rule. FY2018-FY2020 TDS Elements Analytical Results

FDA TDS FY2018–FY2020 source observations

FDA measured the prepared foods named below. Each row describes that food and preparation, not every form of this ingredient. Values are µg/kg (ppb) on the FDA sample basis. ND means not detected; it is not a measured zero. Reporting limits can vary between composites. FY2018-FY2020 TDS Elements Analytical Results

Ranges by source, region, and variety

The FDA TDS FY2018-FY2020 dataset FY2018-FY2020 TDS Elements Analytical Results for raw mature onion (n=27) shows Cd ranging from 2.3 to 26 ppb and Ni from 0 to 100 ppb, providing a US retail baseline. The Determination of heavy metals in selected vegetables from markets in Tamale Metropolis, Ghana survey measured Cd, Pb, chromium (Cr), and Ni in spring onion from three markets in Tamale, Ghana (n=15 samples), offering a West African occurrence context that may reflect higher soil contamination than managed European or North American growing regions. Geographic variation in Cd is primarily driven by soil Cd levels and pH; regions with history of phosphate fertilizer application on onion-growing soils may show elevated Cd relative to low-contamination baseline soils. Onion variety (yellow, white, red, spring onion, shallot) may show modest differences in metal concentrations, but cross-variety comparison data are not yet available in the current corpus to quantify this dimension.

Processing effects

Removal of dry outer scales during commercial processing and home preparation eliminates the outer tissue that may carry surface Pb deposition from atmospheric particulates. Cooking onions by boiling, sautéing, or roasting does not materially reduce total Cd or Pb concentrations on a dry weight basis; moisture loss during cooking concentrates analytes per unit of wet weight. Dehydrated or powdered onion (produced by hot-air drying or spray-drying) will show higher ppb values than fresh onion on a wet weight basis in proportion to the degree of moisture removal. Pickling in acidic brine may leach some metals into the brine, but quantitative data on this effect for onions are not available in the current corpus.

Ingredient-derivative risk

Dehydrated onion flakes, onion powder, and granulated onion are the main derivatives with materially different metal profiles relative to fresh onion, due to moisture removal concentrating all analytes. Onion oil and oleoresin (flavoring extracts) carry negligible metals because the extraction process is selective for volatile and lipophilic compounds rather than metal-containing hydrophilic fractions. These derivatives appear in ingredient lists of many processed foods and seasoning blends; their Cd and Pb concentrations on a wet-weight-equivalent basis should be compared on a moisture-adjusted rather than per-gram basis against regulatory limits for the fresh commodity.

Mitigation options

Sourcing levers

Preferring onions from growing regions with documented low soil Cd and appropriate soil pH management (higher pH reduces Cd bioavailability) is the primary upstream control. For dehydrated onion applications, requesting occurrence data from suppliers on a dry-weight basis with moisture content specified enables proper comparison against regulatory limits.

Agronomic levers

No quantified data on this lever in the current corpus; section will be expanded when relevant evidence is ingested.

Processing levers

Removal of outer dry scales before processing eliminates the highest-exposure tissue layer. For dehydrated products, expressing analytical results on a fresh-equivalent basis with moisture content documented prevents overestimation of risk relative to the fresh commodity.

Formulation levers

No quantified data on this lever in the current corpus; section will be expanded when relevant evidence is ingested.

Testing and QC levers

No quantified data on this lever in the current corpus; section will be expanded when relevant evidence is ingested.

Packaging and storage levers

No quantified data on this lever in the current corpus; section will be expanded when relevant evidence is ingested.

Regulatory limits that apply

European Union Regulation (EU) 2023/915 EU Regulation 2023/915 maximum levels for contaminants in food sets maximum levels of 0.10 mg/kg for Pb and 0.050 mg/kg for Cd in fresh vegetables on a wet weight basis, applicable to onions. The Codex General Standard for Contaminants and Toxins in Food and Feed (CXS 193-1995) General Standard for Contaminants and Toxins in Food and Feed (CXS 193-1995) sets Cd and Pb maximum levels for vegetables that include onion. No specific inorganic arsenic (iAs) or total mercury (tHg) regulatory limit applies to fresh onion under EU or Codex frameworks. See EU Regulation 2023/915 maximum levels for contaminants in food and Codex Alimentarius — Maximum Levels for Cadmium in Food for applicable regulatory reference pages.

Interpretation of source evidence

Source observations and population estimates are different. A non-detect supplies a reporting-limit bound for the tested composites. It does not show that this ingredient contains zero metal. Where a previous profile lacked matching source, species or basis support, its generic concentration has been withdrawn; the available source evidence is kept below.

uranium (U). FDA source observations are non-detects at the reporting limits below. They do not estimate a zero population concentration; other literature remains separate context.

References

Works cited in this page’s text, in first-appearance order. See Sources for this page’s source inventory. Each title links to a source record with bibliographic details, reported findings, and document provenance.

1. Survey of metals in commercial infant foods, infant formula and non-infant specific foodsFood Standards Agency / Fera Science Ltd · UK Food Standards Agency report FS102048 · 2016 · www.food.gov.ukGovernment
2. FY2018-FY2020 TDS Elements Analytical ResultsU.S. Food and Drug Administration · FDA Total Diet Study · 2022 · www.fda.govDataset
3. General Standard for Contaminants and Toxins in Food and Feed (CXS 193-1995)Codex Alimentarius Commission · Codex Alimentarius (Joint FAO/WHO Food Standards Programme) · 1995 · www.fao.orgGovernment
4. Effect of Climatic Conditions, and Agronomic Practices Used in Organic and Conventional Crop Production on Yield and Nutritional Composition Parameters in Potato, Cabbage, Lettuce and Onion; Results from the Long-Term NFSC-TrialsLeonidas Rempelos, Marcin Baranski, Enas Khalid Sufar, Jenny Gilroy, Peter Shotton, Halima Leifert, et al. · Agronomy · 2023 · doi.org/10.3390/agronomy13051225Peer-reviewed
5. Results of the BfR MEAL Study: In Germany, mercury is mostly contained in fish and seafood while cadmium, lead, and nickel are present in a broad spectrum of foodsFechner C, Hackethal C, Höpfner T, Dietrich J, Bloch D, Lindtner O, et al. · Food Chemistry: X · 2022 · doi.org/10.1016/j.fochx.2022.100326Peer-reviewed
6. Assessment of Heavy Metals in Organic and Non-Organic Vegetables Post Severe Tropical Cyclone Gabrielle: A cross-sectional comparative analysisDearing C, Ye Z, and Robertshaw G · F1000Research · 2025 · doi.org/10.12688/f1000research.175538.1Peer-reviewed
7. Contamination of Heavy Metal(Loid)S in Cereals, Vegetables, and Legumes Purchased from Local Markets of Jiaozuo, China and The Associated Health Risk AssessmentWu Z · International Journal of Natural Resources and Environmental Studies, 2(1): 180-200 · 2024 · doi.org/10.62051/10.62051/ijnres.v2n1.21Peer-reviewed
8. Determination of heavy metals in selected vegetables from markets in Tamale Metropolis, GhanaAmetepey ST, Cobbina SJ, Akpabey FJ, Duwiejua AB, and Atule AA · International Journal of Food Contamination · 2018 · doi.org/10.1186/s40550-018-0067-0Peer-reviewed
9. Application of principal component analysis in the pollution assessment with heavy metals of vegetable food chain in the old mining areasIosif Gergen and Monica Harmanescu · Chemistry Central Journal6:156 · 2012 · doi.org/10.1186/1752-153x-6-156Peer-reviewed

Sources

Source records associated with this topic. Inclusion does not establish that every finding applies to this product or ingredient. References above identify works cited in the text.

Update history

No substantive edit history is available in this build. The full commit record is available in git.

Additional geographic and remediation evidence

Iosif Gergen, 2012. Gergen and Harmanescu analyze metal contamination patterns across two historic Romanian mining areas and a reference area. Their PCA study reproduces vegetable concentrations and male hazard quotients from the earlier Harmanescu 2011 study; these remain linked secondary observations rather than additional independent samples. Supports legacy mining and tissue/region differences. Parsley root and leaf remain distinct, as do carrot root and leaf. The fresh-matter food table is retained with cohort linkage so that a reanalysis cannot inflate the evidence pool. Source THQ rankings are model results, not clinical outcomes.