As you can see, rice contains more of the toxic inorganic arsenic than seafood, with one exception. Hijiki, an edible seaweed—a hundred times more contaminated than rice, leading some researchers to refer to it as the “so-called edible…seaweed.” Governments have started to agree. “In 2001, the Canadian [government] advised the public not to eat hijiki.” Then, the UK, the rest of Europe, Australia, New Zealand, then China “advised the public not to eat hijiki, and banned imports and sales” of the stuff. Japan, where they actually have a hijiki industry, just advised moderation.

So, I’d recommend to avoid hijiki due to its excess arsenic content, and avoid kelp due to its excess iodine. But all other seaweeds should be fine, as long as you don’t eat them with too much rice.

However, some seaweed should be used with caution due to its overly high iodine content, like kelp. Too much iodine can cause hyperthyroidism, a hyperactive thyroid gland.

The recommended average daily intake is 150 mcg/day for non-pregnant, non-breastfeeding adults, and we may want to stay below 600 mcg/day on a day-to-day basis, whereas a tablespoon of kelp may contain 2,000 mcg. I'd stay away from kelp because it has too much, and stay away from hijiki because it contains too much arsenic.