Sea vegetables have been a topic of ongoing debate and research concerning their potential to be contaminated by heavy metals found in the waters in which they grow.
In the world of marine biology and marine ecology, sea vegetables are widely recognized as plants with an excellent ability to take up minerals from the water and hold onto these minerals in their cells. This ability makes sea vegetables a rich source of many wonderful minerals, including magnesium, calcium, iron, and iodine. However, in waters that have become polluted with heavy metal elements—including arsenic, lead, and cadmium—sea vegetables can also act like a sponge in absorbing these unwanted contaminants. Some marine ecologists actually use sea vegetables as a kind of "biomonitor" to determine levels of heavy metal pollution in bodies of water.
Among all of the heavy metals, arsenic appears to be most problematic when it comes to sea vegetable toxicity risk. Virtually all types of sea vegetables have been determined to contain traces of arsenic. These types include arame, hijiki, kombu, nori, and wakame. Among all types of sea vegetable, however, hijiki stands out as being particularly high-risk when it comes to arsenic exposure. During the period 2000-2005, government-related agencies in England, New Zealand, and Canada issued public health recommendations advising against consumption of hijiki sea vegetable unless verified as containing very low levels of inorganic arsenic. Based on these reports, we recommend avoidance of hijiki as a sea vegetable unless available in the form of certified organic hijiki.
The levels of arsenic found in other types of sea vegetable have been relatively small. For example, after preparation using water soaking, a British study found wakame to contain an average of 3 milligrams arsenic per kilogram of sea vegetable. In practical terms, this amount represents about 43 micrograms per half ounce of wakame. However, even in this case of relatively small exposure, health risks appear possible.
Our reason for discussing information about these possible risks involves a decision by the U.S. Environmental Protection Agency (EPA) in 1993 to set an oral Reference Dose (RfD) level of .0003 milligrams per kilogram of body weight per day for inorganic arsenic. In practical terms, this maximum safe dose level would allow an adult weighing 150 lbs. to consume about 20 micrograms of inorganic arsenic every day and stay beneath the RfD level.
While a person might be unlikely to eat sea vegetables on a daily basis, you can see from this example how an arsenic-related health risk might be possible with routine consumption of an arsenic-containing sea vegetable. It's important to note here that scientists continue to debate the health risks associated with inorganic (versus organic) forms of arsenic, and that the arsenic found in sea vegetables exists primarily in an inorganic form. It is also important to note that methods of preparing sea vegetables can make a difference in the amount of arsenic found in edible portions.
We continue to include sea vegetables among the World's Healthiest Foods because of their incredibly rich mineral content and other unique health benefits and because the toxicity risks described above can be prevented through the purchase of certified organic sea vegetables! Because most certified organic sea vegetables can be purchased in dried form and reconstituted at home, they can often be ordered from outside of your local area and shipped to you at a relatively low cost.