Arsenic in seaweed—Forms, concentration and dietary exposure

Overview

Rose and colleagues (UK Food Standards Agency with Defra Central Science Laboratory) measured total and inorganic arsenic in 31 dried seaweed products covering five varieties on UK retail sale, analysing each as sold and, where preparation was advised, after soaking/cooking and in the soaking water. Total arsenic ranged 18–124 mg/kg across all samples, but inorganic arsenic was detectable (LOD 0.3 mg/kg) only in the nine hijiki samples, at 67–96 mg/kg as sold (68–73% of total arsenic); every arame, wakame, kombu, and nori sample held inorganic arsenic below 0.3 mg/kg (<2% of total). Preparation reduced but did not eliminate hijiki’s inorganic arsenic (mean 77 mg/kg as sold falling to a 67% inorganic fraction after soaking), contradicting producer claims that soaking removes it. On the strength of this survey the FSA issued formal consumer advice to avoid eating hijiki seaweed.

Key numbers

Limits of detection: total arsenic (As) 0.02 mg/kg (solids) / 0.003 mg/kg (water); inorganic As 0.3 mg/kg (solids) / 0.01 mg/kg (water). All values mass-balance corrected for reagent blank and spike recovery.

Overall: total arsenic 18–124 mg/kg across all 31 samples; inorganic arsenic detected only in hijiki (67–96 mg/kg as sold); all other varieties <0.3 mg/kg inorganic As (the method LOD, equating to <2% of total As).

Per-variety total arsenic, as sold (mg/kg; individual samples and mean):

Hijiki — full per-sample arsenic across the three basis states (Table 1; mg/kg; ”%” = inorganic fraction of total As):

Additional source-reported facts:

• Inorganic arsenic was found only in hijiki; the as-sold inorganic fraction was 68–73% of total As, falling to 61–73% after preparation (mean 71% → 67%). Soaking transfers arsenic to the water but does not remove the inorganic fraction from the seaweed, contradicting producer claims.
• Risk assessment: a 25 g hijiki portion at the maximum inorganic arsenic concentration (the paper states 22.7 mg/kg on the as-consumed basis) gives 0.57 mg inorganic arsenic intake — a 30- to 50-fold increase over typical UK dietary inorganic-arsenic exposure per the risk-assessment section (the paper’s conclusion states “around 30 times”; equivalent to roughly one to two months’ worth of dietary inorganic arsenic in a single portion).
• The method’s inorganic-arsenic back-extraction co-extracts monomethylarsonic acid (MMA) but not arsenobetaine or arsenosugars.
• Regulatory context: UK Arsenic in Food Regulations (SI 1959 no. 831) set a general 1 mg/kg total-As limit but explicitly exempt fish and edible seaweed; no EU-wide arsenic-in-food limit existed at the time. JECFA PTWI for arsenic was 15 µg/kg bw/week (1989); COT (2003) recommended inorganic-arsenic exposure be kept ALARP.

Methods (brief)

Thirty-one samples (five varieties) were collected at random from UK retail outlets (London area, Oct–Dec 2003), all imported and purchased dried. Total arsenic was determined by ICP-MS after microwave-assisted closed-vessel nitric-acid digestion (Multiwave, Perkin-Elmer), using the published 1997/1999 UK Total Diet Study method with rhodium internal standard. Inorganic arsenic was determined by the non-chromatographic Muñoz et al. (2000) method: HCl dissolution converting inorganic arsenic to As(III), conversion to a covalent halide, chloroform extraction, back-extraction into dilute HCl, and ICP-MS at 10 000 resolution. Nori was analysed as sold only (ready-to-eat); the other four varieties were analysed both as sold and after the package-directed preparation, with the soaking water analysed gravimetrically to permit a mass-balance. The total-arsenic method is ISO 17025 (UKAS) accredited; no certified reference materials exist for inorganic arsenic, so QC used in-house reference materials and recovery from fortified tissue (acceptance 80–120%), with satisfactory contemporaneous proficiency-test results. Values are dry weight (“as sold”) or wet weight (“prepared”/water) as labelled.

Implications

• Certification (HMTc): a direct-evidence occurrence-and-speciation source for the Category 6 seaweed-kelp-foods row (iAs/total arsenic (tAs) platform). It independently reproduces the central structure of the category — inorganic arsenic is a hijiki-specific hazard (67–96 mg/kg as sold) while every other common edible seaweed sits below the 0.3 mg/kg detection limit — directly supporting the within-row hijiki/Hizikia species sub-split flagged in the Category 6 Step 0 lock. The paired as-sold (dry) versus prepared (wet) data are relevant to the row’s native-basis choice: certification works on the as-placed-on-market dried basis, where hijiki inorganic arsenic is highest.
• Courses: a teachable regulatory case study (survey → ALARP risk assessment → FSA consumer advisory) for brand QA and regulatory-affairs audiences.
• App: contributes to the seaweed ingredient contamination_profile for total and inorganic arsenic, with the species-specific caveat that the inorganic fraction is concentrated in hijiki.

Update history

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