Is Our Local Seaweed Contaminated?

by Luisa Loi

Jennifer Hahn sampling seaweed.
courtesy photo: Jennifer Hahn

Armed with a ceramic-bladed knife and years of experience as a wilderness guide and forager, Jennifer Hahn paddled the cold waters of the Salish Sea aboard her kayak, sometimes snacking on some freshly cut seaweed.

Hahn was on a mission to collect data for her research project, which would have answered questions she would often hear during her wild-food harvesting and cooking workshops, particularly from indigenous participants:

Is our seaweed contaminated? And, if it is, how much is safe to eat?

Existing studies on seaweed in the Salish Sea couldn’t provide the needed data on contaminant levels in these algae. 

“Without this knowledge,” her research paper says, “health-based consumption advisories cannot be determined for consumers that include tribes and First Nations, Asian and Pacific Islander community members, and recreational harvesters.”

Hahn and her team found some contaminants in concentrations high enough to pose health risks, but no greater than in other commonly consumed foods. The findings, the paper argues, shouldn’t discourage people from eating locally sourced seaweed. These sea vegetables are high in fiber and nutrients, low in fat and associated with many health benefits, Hahn said. 

In recent years, seaweed has become more popular in this corner of the Pacific Northwest. Hahn said the Department of Natural Resources is considering six seaweed aquaculture applications in the Salish Sea as of Dec. 10, 2022. 

Seaweed Sequesters Carbon
Seaweed is also gaining more interest as it has the ability to sequester carbon, thus improving the health of nearby species, like mussels. For this reason, as Hahn learned from the Washington Seaweed Knowledge Symposium, some of the proposed farms will dedicate part of their seaweed to habitat restoration and part to harvesting. A recording of the event will be available in January 2023 (6). 

“Farmed seaweed is going to be very available in the future,” said Hahn, who fell in love with this algae at first bite when she was a child. “It’s going to be a wonderful thing, having fresh seaweed not far from our table.”

Interest in seaweed is particularly growing among indigenous communities that are working to achieve food sovereignty (2).

Historically, seaweed was a traditional food for Coast Salish people. However, due to colonization, the loss of harvesting sites, the forced assimilation of Western diets and the loss of intergenerational knowledge sharing, many indigenous communities struggle with disproportionately high diabetes and heart disease rates among these groups (2) (3). 

The SeaDoc Society is a nonprofit that funds scientific research that informs environmental policies. The organization funded Hahn’s project, contributing $90,000. Hahn said most of the funds went to lab work, as testing a single sample costs over $1,000.

“This was a very clear case of people needing information to make decisions about what they felt safe to eat,” said Joe Gaydos, the organization’s science director. “If you look at it from a human rights issue, it’s something that they have traditionally eaten since time immemorial. Many of these contaminants were not around before we created them.”

June to September 2015
From June to September 2015, Hahn collected swaying blades of seaweed from the left, middle and right side of beaches or kelp beds, often helped by members of the tribes and nations whose land she was given access to.

“One of the really strong aspects of this study is the number of places Jennifer was able to sample,” said Kathryn Van Alstyne. “You don’t really see somebody sampling over 40 different sites very often.” Van Alstyne is a seaweed ecologist and marine scientist at Western Washington University who assisted Hahn with her research.

The study focused on three seaweed species: bull kelp, rockweed and spiral bladderwrack.

At each of the 43 American and Canadian sites she visited, Hahn would place her sample in a mesh cotton bag, rinsing it 12 times with seawater. After draining it, she would transfer the sample to the inside of a Ziploc bag and place it in a cooler with ice packs before driving to her next destination (1).

Within hours or two days, Hahn would transfer the samples again, this time inside of a -80˚C ultra-low freezer, which she said could have fit a six-foot-tall person inside.

After chopping the seaweed into a slurry with a ceramic knife — a metal blade would change the concentration of metal in the sample — and pouring it into an amber bottle, the sample would be rushed to either the National Oceanic and Atmospheric Administration lab in Seattle, the lab at the University of California, Davis, or the AXYS Analytical Labs in Sidney, British Columbia.

The lab also analyzed seaweed from Post Point in Bellingham, the only sample Hahn took in Whatcom County. She had arranged to go with people who worked for the Northwest Indian College to collect samples in Lummi territory, but the winds were too strong. “You only have so many low tides and if the weather is dangerous, you have to keep moving through on your schedule,” she said.

“I thought I was going to be done with this study in about a year and a half or two years,” Hahn said. Instead, it took much longer: the paper was submitted for approval in December 2021, and published in September 2022.

It takes a long time to do community-based research and to interpret results in a way that people can understand and apply to human health, she said.

Risks vs. Benefits
The toxicological analyses detected some contaminants at potentially harmful levels. 

Three contaminants — cadmium, total mercury and lead — exceeded international limits at some sites (1). According to the World Health Organization, cadmium is a carcinogen; mercury may affect the nervous, digestive and immune systems; lead may affect the neurologic, gastrointestinal, cardiovascular, renal and hematologic systems.

For rockweed, polycyclic biphenyls (PCBs) levels exceeded the United States-based screening levels at 26 out of 38 sites (1). Some studies, according to the Washington State Department of Health, suggest PCBs may be carcinogenic, while the most common adverse reactions known are acne and skin rashes (4).

“I was surprised there were PCBs in the seaweed, and that they were high in some of these very highly industrialized areas,” Hahn said. “People told me ‘Oh, seaweed. It’s at the bottom of the food chain, we don’t think it’s gonna be that polluted.’” 

Benzo[a]pyrene (BaP) — a carcinogen that is also associated with developmental, reproductive and immunological adverse effects (5) — exceeded national screening levels at 1 out of 38 sites for rockweed and 1 out of 3 sites for the spiral bladderwrack, while other contaminants were found at low concentrations. 

When concentrations of all the 162 contaminants are considered in the three species, most were below the smallest measurable levels or were below national and international levels (1). 

Based on the level of contamination found at each site, the team recalculated consumption rates for specific seaweeds at these locations (1). For example, based on the amount of PCBs found in a sample taken from Post Point in Bellingham in 2015, the paper recommends consuming no more than 4.4 grams of rockweed per day. The same goes for spiral bladderwrack from Fidalgo Bay in Anacortes (1). 

Hahn’s team also calculated the amount of contaminants that would be consumed in a five-gram portion of dried bull kelp and rockweed and compared them to a portion of commonly consumed food for each focal contaminant. The paper defines focal contaminants as PCB, cadmium, lead, BaP, mercury, and arsenic (1). 

Because only three samples of spiral bladderwrack were collected, the scientists did not include them in the comparison (1).

When Hahn’s research team compared a portion of rockweed to a portion of commonly consumed food, they found that the focal contaminants were at very similar levels in both foods. 

“If you were to eat a portion of a really well-done steak, it would have more benzo[a]pyrene than a portion of rockweed from the most polluted site in the Salish Sea … A very well-done hamburger [has concentrations] higher than all the other sites with rockweed. And, if you eat a plate with a portion of kale, broccoli, pretzels, spinach, fish, all those have got more than [the samples from] all the other sites where we harvested.”

Health Benefits
Some cautions must be considered in interpreting these findings.

“It’s important not to scare people away from traditional foods, because then they go to the store and buy high-sugar, unhealthy fat foods, low-fiber foods,” Hahn said. 

Seaweed can be contaminated while also offering health benefits, such as compounds that can reduce risk factors for type 2 diabetes, Alzheimer’s disease, cardiovascular disease, obesity, cancer, high blood pressure and depression.

“You don’t see that necessarily with chicken or steak,” Hahn said. “If [someone] consumes a high seafood diet, they have to think about what’s the total amount of contaminants they’re exposing themselves to. And that can be overwhelming for people, but that’s the kind of stuff we need to be sharing. There’s so much consumer education to be done.”

There are no regulations on maximum levels of contaminants allowed in seaweed in either Canada or the United States (1). To reduce exposure to contaminants, Hahn recommended not harvesting at polluted sites, or eating less seaweed from those locations.

The results, Hahn and Gaydos said, should not discourage consumers from including seaweed in their diets. 

“I was pleasantly surprised,” SeaDoc’s Gaydos said. “Not terribly surprised — the fact that you get results that don’t blow you away feels good to me. We hear a lot of bad stories. People right now are struggling to find healthy food sources, and to feel comfortable harvesting wild food.”

Gaydos believes the findings aren’t concerning enough to create new policies to protect consumers. At the same time, Hahn hopes it will represent another step toward reducing pollution and inspiring people to write to officials to take action to clean up the Salish Sea.

Some questions remain unanswered. 

1. How are other edible seaweeds affected?

2. Since Hahn collected the samples during the summer of 2015, what would the results be if they were collected today? How would they change seasonally?

3. How do these contaminants affect the food chain?

4. What guidelines would be necessary for seaweed farms?

“I hope that more research will be done, because this is a good foundational beginning for more research to happen,” Hahn said. 

Tips on How to Safely Eat Seaweed

1. Get a license from the Washington Department of Fish and Wildlife or dealers, and follow local rules on harvesting.

2. Avoid harvesting from areas where there are homes nearby that use septic systems, as they may leak. Avoid harvesting grounds near industrialized sites.

3. Rinse the seaweed before consuming it to remove some surface contaminants. Rinsing with freshwater causes the seaweed to degrade, so Hahn recommends rinsing the seaweed with cool water right before cooking or eating it. 

4. To rinse, Hahn recommends soaking dry seaweed and rinsing fresh or frozen seaweed under running water. 


1. Hahn JL, Van Alstyne KL, Gaydos JK, Wallis LK, West JE, Hollenhorst SJ, et al. (2022) “Chemical contaminant levels in edible seaweeds of the Salish Sea and implications for their consumption.” PLoS ONE 17(9): e0269269.







Luisa Loi is a freelance reporter based in Bellingham with an interest in covering local environmental issues. You can learn more about Luisa through her LinkedIn (Luisa Loi), or by reaching out to

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