Which seaweeds are high in heavy metals?

November 19, 2025 •

3 min read

According to a study on edible seaweeds, concentrations of certain heavy metals were found to be thousands of times higher in some species than in the surrounding seawater. This guide explores which seaweeds are high in heavy metals, focusing on specific species known to accumulate toxins like inorganic arsenic and cadmium.

Quick Summary

This article outlines which types of seaweed are most likely to contain elevated levels of heavy metals such as inorganic arsenic and cadmium. It identifies specific species, discusses influencing factors like environmental conditions, and provides practical advice for consumers to mitigate health risks and enjoy seaweed safely.

Key Points

Hijiki has High Inorganic Arsenic: The brown seaweed hijiki (Sargassum fusiforme) is known for exceptionally high levels of inorganic arsenic, a potent toxin and carcinogen, leading to advisories against its consumption.
Brown Seaweeds Accumulate Cadmium: Species within the brown seaweed group (Phaeophyceae), including certain kelps (Laminaria) and Sargassum, are particularly effective at absorbing and concentrating cadmium and other heavy metals.
Source Location is Key: Contamination levels depend heavily on where the seaweed is grown or harvested. Locations near industrial activity, mining, or urban runoff tend to produce seaweed with higher heavy metal concentrations.
Environmental Factors Alter Uptake: Conditions like salinity, pH, and water temperature influence how and how much heavy metal seaweed absorbs. Lower salinity can sometimes increase metal bioavailability and uptake.
Safe Seaweed Alternatives: Nori and dulse generally contain lower levels of heavy metals and are considered safer options for regular consumption, provided they are sourced from unpolluted areas.
Consume in Moderation: Even with lower-risk species, consuming seaweed in moderation is recommended as part of a varied diet. This helps balance the intake of both beneficial minerals and potential contaminants.
Check for Contamination Sources: When purchasing seaweed, especially higher-risk types, look for products sourced from pristine, certified organic waters to minimize the risk of heavy metal exposure.

Seaweeds are celebrated for their rich nutritional profile, providing a host of vitamins, minerals, and bioactive compounds. However, their powerful ability to absorb minerals from the marine environment means they can also accumulate harmful heavy metals. Not all seaweeds carry the same risk; the level of contamination is highly dependent on the species, growing location, and environmental conditions. This makes understanding which seaweeds are high in heavy metals crucial for safe consumption.

High-Risk Seaweed Species to Watch For

Certain species of seaweed are notoriously prone to absorbing and retaining specific heavy metals, posing a higher potential health risk to consumers. Awareness of these species is the first step toward reducing exposure to toxins.

Hijiki: The Inorganic Arsenic Hazard

Hijiki (scientifically known as Sargassum fusiforme or Hizikia fusiformis) is a brown seaweed with one of the highest risks for inorganic arsenic accumulation. Inorganic arsenic is a known carcinogen, and health authorities in several countries, including Canada and the UK, have advised consumers to avoid or limit its consumption. While the total arsenic content in seaweed is typically high due to harmless organic forms (arsenosugars), hijiki contains an unusually high proportion of the more toxic inorganic variety.

Brown Seaweeds: Potential for Cadmium and Lead

Many brown seaweed species (Phaeophyceae) are efficient at accumulating heavy metals due to the polysaccharides in their cell walls. Research has identified several brown seaweeds with elevated levels of cadmium (Cd) and lead (Pb). For instance, studies have found brown algae like Padina tetrastromatica and Sargassum species to contain high levels of both cadmium and other heavy metals. Similarly, the brown seaweed kelp (Laminaria species) can accumulate significant levels of cadmium, sometimes exceeding international food safety limits, and high levels of iodine.

Factors Influencing Heavy Metal Bioaccumulation

The contamination risk is not solely determined by the seaweed species. Several environmental and physiological factors contribute to a seaweed's capacity for heavy metal uptake.

Geographic Origin: Seaweeds harvested from areas with high industrial, urban, or agricultural pollution are more likely to contain elevated heavy metal levels. Coastal regions near mining operations or factory effluents are particularly concerning.
Water Chemistry (Salinity, pH, Temperature): The bioavailability and uptake of heavy metals are significantly affected by water conditions. For example, lower salinity can increase metal bioavailability and uptake in some species, while pH influences metal speciation and binding to the seaweed's cell wall.
Growth Method: For farmed seaweeds, the cultivation method plays a role. Species grown in off-bottom systems, where they are in contact with or near contaminated sediment, tend to accumulate higher metal levels than those cultivated on floating long-lines.
Cell Wall Composition: The structure and chemical composition of a seaweed's cell wall are critical to its biosorption capabilities. Brown seaweeds, rich in alginates and sulphated polysaccharides, often have a higher affinity for heavy metals like cadmium and lead than green or red varieties.

Comparison of Common Seaweeds and Potential Contaminants


Seaweed Type	Scientific Name(s)	Associated Heavy Metals	Contamination Potential	Consumption Advice
Hijiki	Sargassum fusiforme	Inorganic Arsenic	High	Consume sparingly, if at all, due to high inorganic arsenic content.
Kelp (Kombu)	Laminaria spp.	Cadmium, Arsenic, Lead	Moderate to High	Moderation is key due to potential heavy metal and high iodine accumulation.
Wakame	Undaria pinnatifida	Cadmium, Arsenic	Low to Moderate	Generally lower risk than hijiki, but source-dependent. Moderation is recommended.
Nori	Pyropia spp.	Arsenic (mostly organic)	Low	Generally safe for regular consumption, though organic arsenic levels vary. Inorganic arsenic is typically low.
Dulse	Palmaria palmata	Arsenic (mostly organic)	Low	Typically contains lower levels of heavy metals compared to many brown species.

Note: The contamination potential and metal levels are generalizations and can vary significantly based on environmental factors.

Conclusion

While seaweeds are valuable for their nutritional content, consumers should be mindful of the risks associated with heavy metal accumulation. Brown seaweeds like hijiki, kelp (kombu), and certain Sargassum species have a higher propensity for concentrating toxic heavy metals such as inorganic arsenic and cadmium. The geographical origin, water conditions, and growth methods are all significant factors influencing contamination levels. By being selective about the type of seaweed consumed and understanding its source, it is possible to mitigate the health risks. A balanced approach that includes consuming seaweed in moderation and opting for lower-risk species like nori can allow individuals to enjoy the benefits while minimizing exposure to toxins.

For more detailed information on safe consumption, it is advisable to consult health advisories from reputable organizations. For example, see guidance from the Canadian Food Inspection Agency regarding hijiki.

Frequently Asked Questions

The most concerning heavy metal in seaweed is inorganic arsenic, primarily because certain species, most notably hijiki, can accumulate it in high concentrations. Inorganic arsenic is more toxic than the organic forms commonly found in marine life.

Most seaweeds contain trace amounts of heavy metals, which they absorb naturally from the marine environment. However, the type and concentration of metals vary widely by species and location. Not all seaweeds are considered high-risk.

Soaking can reduce the concentration of certain contaminants, particularly inorganic arsenic, in high-risk species like hijiki. However, it may not remove all heavy metals, and it's not a failsafe method. It's best to rely on low-risk species from clean sources.

Yes, brown seaweeds generally have a higher capacity for accumulating heavy metals, especially cadmium, due to the presence of polysaccharides like alginates in their cell walls. Hijiki and kelp are brown seaweeds with known heavy metal issues.

Factors such as water salinity, pH, temperature, and proximity to industrial pollution sources significantly influence heavy metal accumulation in seaweed. For example, lower salinity can increase the uptake of certain metals.

Kelp can accumulate heavy metals and very high levels of iodine, so consuming kelp products in moderation is advisable. For individuals with thyroid conditions, monitoring iodine intake is particularly important. The risk depends on the sourcing and processing of the product.

To find safer seaweed products, choose those that specify a clear origin from clean, uncontaminated waters. Organic certification often provides an extra layer of assurance. Focus on lower-risk species like nori and dulse.

Long-term consumption of high heavy metal levels can lead to health problems. Inorganic arsenic is a carcinogen, while cadmium can cause kidney damage and bone fragility. However, regular moderate consumption from clean sources is generally low risk.