Arsenic in seaweed - the truth behind the numbers.

The Arsenic Story in Seaweed: What’s Real, What’s Measured, and What Actually Matters

When it comes to seaweed, one of the most common safety questions we’re asked is about trace elements and metals.

And it’s a fair question.

Seaweeds are natural biofilters — they interact with their environment and can accumulate minerals from seawater. That includes beneficial nutrients, and also elements that require careful monitoring.

Let’s start with iodine — the “good” trace element

One of the most talked-about elements in seaweed is iodine.

• Iodine is essential for human health, particularly for thyroid and hormonal function
• But like many powerful nutrients, it is required in very small (micro) quantities

Some seaweed species — especially brown seaweeds — can contain very high iodine levels.

Our cultivated Sea Lettuce (Ulva) is different.
We have consistently measured low iodine levels, making it a more balanced and accessible seaweed for regular consumption.

(We explore this in more detail in our blog on “The Irony of Iodine” — where an essential element can become excessive depending on species.)

What about arsenic?

Unlike iodine, arsenic is not considered an essential nutrient.

However, nature is rarely simple.

We are still learning how ultra-trace elements behave in biological systems. For example:

• Boron and strontium are now recognised as important for bone health
• Yet they still don’t have clearly defined intake guidelines

Arsenic sits in this broader category of poorly understood trace elements — but importantly, it has one key distinction:

👉 Not all arsenic is the same

The critical distinction: Organic vs inorganic arsenic

Seaweeds are known to contain arsenic — this has been well established for decades.

But what matters is the form of arsenic, not just the total amount.

There are two main categories:

• Inorganic arsenic (As III and As V)
  → the form associated with toxicity and tightly regulated
• Organic arsenic (arsenosugars)
  → the dominant form in most seaweeds
  → generally considered much lower risk

Our own work, including our early AgriFutures report (2017), identified this clearly:

👉 Most arsenic in seaweed exists as organic arsenosugars, not inorganic arsenic

Arsenic in Seaweeds: Why Species Matters

🌿 Where PhycoHealth seaweed sits

Key Takeaway

• Most arsenic in seaweed (often >80–90%) is present as organic arsenosugars
• Food safety regulations focus on inorganic arsenic, not total arsenic
• Species selection matters more than the headline number
• Values represent typical ranges reported across peer-reviewed studies (EFSA, Roleda et al. 2023, Food & Chemical Toxicology reviews). Actual values vary depending on species, location, and cultivation conditions.

Regulation focuses on the part that matters

Because of this distinction, food safety regulations do not focus on total arsenic alone.

They focus specifically on:
👉 inorganic arsenic

In Australia and New Zealand (FSANZ):

• The limit for inorganic arsenic in seaweed is 1 mg/kg

This is the number that matters for safety and compliance.

What does the latest research say?

Recent European research reinforces what we’ve known — but with better data:

• Green seaweeds (like Ulva / Sea Lettuce)
  → consistently show the lowest total arsenic levels
• The vast majority of arsenic present is organic (arsenosugars)
• Inorganic arsenic is only a small fraction

In one recent dataset, the inorganic arsenic fraction in green seaweeds is typically well below regulatory limits, often a very small percentage of total arsenic. Health risk indices indicate that there is a low toxicity risk from Green species of seaweeds and these levels are on par with the levels that we have measured in our own cultivated species (Yu et al. 2024, Survey of arsenic content in edible seaweeds and their health risk assessment,
Food and Chemical Toxicology, 187).

Why total arsenic can be misleading
Most arsenic measured in seaweed (often >80–90%) is present as arsenosugars — a naturally occurring organic form with much lower toxicity. This is why modern food safety standards focus specifically on inorganic arsenic, not total arsenic.

Putting numbers into perspective

Let’s take a realistic example.

If:

• Total arsenic = ~3 mg/kg (typical for many edible seaweeds)
• Inorganic arsenic = <0.1 mg/kg (common for Ulva-type species)

Then a daily intake of:

• 10 g dried seaweed

Would provide approximately:

• <1 microgram (µg) of inorganic arsenic

👉 That is extremely low.

To put this in context:

• Background dietary exposure to inorganic arsenic from everyday foods (like rice, grains, and vegetables) is typically in the range of:
  • ~0.1–3 µg per kg body weight per day globally

So the contribution from a serving of green seaweed is:
👉 comparable to, or smaller than, everyday dietary exposure

Why species matters (and why hijiki is different)

It’s important to say this clearly:

👉 Not all seaweeds behave the same

• Most edible seaweeds → low inorganic arsenic
• Hijiki (a brown seaweed) → known exception
  → accumulates high levels of inorganic arsenic
  → widely advised against for regular consumption

This is why species selection is critical.

Our approach: control, testing, and transparency

At PhycoHealth, we recognised early on that trace elements are both a nutritional opportunity and a safety responsibility.

Since our early work in 2017, we have focused on:

• Species selection → working with green seaweed (Ulva)
• Controlled cultivation systems → clean, monitored inputs
• Ongoing analytical testing → including trace element profiles
• Understanding fractions and processing effects

And importantly:
👉 We continue to invest in research and publish findings as we learn more

The bottom line

• Yes — trace elements in seaweed are a real and valid concern
• But the story is far more nuanced than total numbers

For arsenic:

• The form matters more than the amount
• Most seaweeds contain low-risk organic arsenic
• Inorganic arsenic is the regulated and relevant fraction
• Green seaweeds like Ulva consistently sit at the lower-risk end of the spectrum

Why did my arsenic test go up after eating seaweed?

Seaweed contains naturally occurring organic arsenic compounds called arsenosugars. These are very different from the inorganic arsenic associated with toxicity. Standard urine tests measure total arsenic and cannot distinguish between these forms, so levels may appear elevated after eating seaweed or seafood. This reflects normal dietary exposure, not toxicity.

If you’ve recently eaten seaweed or seafood, it’s completely normal to see elevated arsenic levels in standard urine tests. These tests measure total arsenic and don’t distinguish between the harmless organic forms found in food and the inorganic forms that are of concern. To assess risk properly, a speciated arsenic test is needed.

What you should do to confirm your situation: pause seaweed/seafood, speak with your clinician, and ask for "speciated" arsenic testing — usually urine speciation — before drawing toxicity conclusions. If your As levels are lower again (organic arsenic clears quickly from blood and urine), and the speciation show safe levels of inorganic As, then there should be no need for concern under normal circumstances. Always consult with your clinician on this front.

And we’re still learning

Nature operates in complexity — and seaweed is no exception.

We are continuing to explore:

• Elemental profiles across species and growth systems
• How processing affects trace element distribution
• The role of ultra-trace elements in human health

And as always:
👉 We’ll share what we find

The future of seaweed as food depends not just on what we grow, but how well we understand it. At PhycoHealth, we believe transparency in trace elements is not a risk — it’s a responsibility, and an opportunity to lead.

Meanwhile - here are some references on the topic - if you want to dive deeper:

Our foundational work

• Winberg, P. et al. (2017)
  Seaweed as a Novel Food: Safety and Nutritional Assessment
  AgriFutures Australia Publication No. 17-043
  https://agrifutures.com.au/wp-content/uploads/publications/17-043.pdf

Regulatory & guidance (Australia)

• FSANZ (Food Standards Australia New Zealand)
  Arsenic in Food
  https://www.foodstandards.gov.au/consumer/chemicals/arsenic
• FSANZ Imported Food Risk Statement – Hijiki Seaweed
  https://www.foodstandards.gov.au/sites/default/files/2023-11/Hijiki%20seaweed%20and%20inorganic%20arsenic.pdf

European / international research

• Roleda et al. (2023)
  Element concentrations in European seaweeds
  Journal of Applied Phycology
  (Table showing speciation proportions incl. arsenosugars ~major fraction)
  https://link.springer.com/article/10.1007/s10811-023-03131-8
• EFSA (European Food Safety Authority)
  Risk assessment of arsenic in food (and updates on organoarsenic)
  https://www.efsa.europa.eu
• Taylor et al. / Food & Chemical Toxicology (2024)
  Trace elements and arsenic speciation in seaweeds
  (Confirms dominance of arsenosugars and species variation)

CEVA / France guidance

• CEVA (Centre d'Étude et de Valorisation des Algues, France)
  Industry guidance widely used in France for edible seaweed:
  • ~3 mg/kg inorganic arsenic (upper guideline used in practice)
    (Note: often applied as a pragmatic threshold acknowledging arsenosugar dominance)

👉 Important nuance for blog:

• CEVA operates more as industry guidance, not EU-wide law
• EU is still moving toward harmonised limits

Human metabolism / exposure

• Francesconi & colleagues (various papers)
  Arsenosugars metabolism in humans
  (Estimates ~1 mg/day arsenosugar intake in Japan)
• WHO / JECFA
  Evaluation of arsenic exposure
  (background dietary exposure ranges)