Response: Edible seaweed is a marine algae that can come in many forms, including the type that you mention, kombu, as well as the commonly eaten wakame and nori seaweeds. Although long part of the Asian diet, edible seaweed has increasingly become a part of Western diets due to its well-documented nutritional and often discussed medicinal qualities. In comparison to information on seaweed's benefits, there is little information about the possible negative effects of eating seaweed. A couple of years ago, several governments, including the Canadian and British governments, issued warnings advising people to not eat one type of seaweed – hijiki – because of concern over its high levels of inorganic arsenic, a toxic element that has been linked to cancer. Since little was (and still is) known about the specific risks of arsenic in hijiki, hijiki was not banned. The warnings, which were intended to give the consumers a choice, stated that eating hijiki every now and then was probably not dangerous. Importantly, the warnings applied only to hijiki and not to other more commonly eaten seaweeds, including arame, nori, kombu, and wakame, which were found to be free of arsenic.
Based on these and other concerns, several scientific studies have been performed to measure the amount of arsenic and other heavy metals present in seaweed. Results from these studies show that metal contamination of seaweed depends on three major factors, including where the seaweed was harvested, the type of seaweed, and the specific metal. For example, in a small Canadian study (Van Netten et al., Science of the Total Environment, 2000), seaweed grown in waters near British Columbia, Canada generally had lower amounts of heavy metals, especially of mercury, than seaweeds grown in Japan and Norway, possibly due to lower amounts of these metals in British Columbia waters. All seaweed samples – even those grown in Japan and Norway – however, had metal levels that are generally thought of as safe to eat.
What this means for you and your family is that you can continue to enjoy your seaweed salads, soups, and sushi (although given the governmental warnings I would probably stay away from hijiki). Given seaweed’s many nutritional benefits and relatively low pollutant levels, seaweed is probably not only a safe food but also one that is good for your health.
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Helen Suh MacIntosh is a professor in environmental health at Harvard University and studies how pollution behaves in the environment and how it affects people's health. Please keep in mind that her answers are just her interpretation of available information and should not be taken as the only viewpoint or solution to a problem. Use this column at your own risk. Having said this, please feel free to post any of your environmental health questions to firstname.lastname@example.org.