Hahn et al. 2022 — Chemical contaminants in edible seaweeds of the Salish Sea
Hahn et al. measured concentrations of heavy metals (cadmium, mercury, lead, arsenic, chromium, nickel, and others) as well as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) in three commercially harvested edible brown seaweeds collected at 43 sites across the Salish Sea (coastal Washington State, USA and British Columbia, Canada). Cadmium exceeded international regulatory limits for human consumption at every single collection site, and total arsenic concentrations were elevated across all three species. The authors conclude that current international standards for Cd and tAs in seaweed may be inadequate to protect frequent consumers of locally harvested seaweed, particularly given that inorganic arsenic is typically a small fraction of total arsenic in seaweed matrices.
Key numbers
All values are reported on a dry weight (DW) basis in mg/kg unless noted.
Fucus distichus (rockweed; N=38 samples from 38 sites):
- Cd: mean 2.52 mg/kg DW (range 1.15–4.25)
- tHg: mean 0.06 mg/kg DW (range <LOQ–0.21)
- Pb: mean 0.66 mg/kg DW (range <LOQ–13.2); 3 sites exceeded international limits
- tAs: mean 26.8 mg/kg DW (range 16.4–37.0)
- Cr: mean 3.21 mg/kg DW
- Ni: mean 4.84 mg/kg DW
Fucus spiralis (spiral wrack; N=3 samples):
- Cd: mean 2.03 mg/kg DW (range 1.73–2.34)
- tAs: mean 19.1 mg/kg DW (range 18.7–19.7)
Nereocystis luetkeana (bull kelp; N=17 samples from 12 sites):
- Cd: mean 5.69 mg/kg DW (range 2.48–7.91); highest Cd levels of the three species
- tHg: mean 0.12 mg/kg DW (range <LOQ–0.25)
- Pb: mean 0.37 mg/kg DW (range <LOQ–1.51)
- tAs: mean 72.5 mg/kg DW (range 56.7–98.9); highest tAs of the three species
Cadmium: All 58 samples at all 43 sites exceeded the most permissive international limit identified by the authors (France: 0.5 mg/kg fresh weight, equivalent to approximately 2.5–3 mg/kg DW at typical moisture content). Multiple national and regional limits for Cd in seaweed range from 0.5–3.0 mg/kg depending on jurisdiction and wet/dry weight basis; the majority were exceeded.
Lead: Three Fucus distichus samples (all from a single site near industrial activity) exceeded international Pb limits for seaweed; maximum 13.2 mg/kg DW.
Total arsenic: Very high concentrations in all species; however, the authors note that arsenic in brown seaweeds is predominantly organic (arsenosugars), with inorganic arsenic (iAs) typically comprising 1–5% of tAs. The elevated tAs figures therefore do not directly translate to iAs risk. Speciation was not conducted in this study; the authors recommend future iAs-specific analysis.
LOQ for mercury: <0.02 mg/kg DW (samples below LOQ counted as <LOQ in means).
PCBs and benzo[a]pyrene were also measured; all samples were below applicable screening values for PCBs and BaP.
Methods (brief)
Seaweed samples collected from 43 Salish Sea sites (2017–2019) by hand-picking at low tide. Dried and homogenized. Metal analysis by ICP-MS following microwave digestion (EPA Method 3052 / EPA Method 6020). Mercury by cold vapor AFS (EPA Method 1631). PAHs and PCBs by GC-MS. No arsenic speciation performed; only total arsenic reported. Authors compare results to French, Australian/New Zealand, Chinese, Mauritius, and EU regulatory limits.
Sample sizes: F. distichus N=38; F. spiralis N=3; N. luetkeana N=17.
Implications
Certification: Seaweed and seaweed-derived ingredients (kelp, dulse, bladderwrack) used in dietary supplements, functional foods, and organic mineral supplements are a significant Cd exposure pathway. This dataset documents that wild-harvested Salish Sea seaweeds exceed international Cd limits universally, which is relevant for HMT&C certification of products containing seaweed-sourced minerals or kelp ingredients.
Courses: Key teaching case for the point that total arsenic in seaweed is not the appropriate risk metric — speciation to iAs is required. Also illustrates geographic variation in Pb contamination (industrially impacted sites show sharp spikes vs background levels).
App: Seaweed ingredient pages should flag Cd as the primary risk, tAs as needing speciation qualification, and Pb as site-dependent (spike risk near industrial zones).
Microbiome: Not addressed in this paper.