Seaweed has long been recognised as a rich source of iodine. In recent years its medicinal properties have been promoted, leading to increased consumption worldwide. The benefits derivable from consuming seaweed, however, may be outweighed by negative properties, depending on where it was grown. Seaweed's ability to absorb chemicals is not limited to beneficial ones. It is also rather good at absorbing arsenic, mercury, uranium, copper, molybdenum, aluminium, chromium and some radioactive isotopes.

A small study of eight samples picked at random from, primarily, Japan and Norway, found worrying levels of mercury. The main value of the study is to highlight the need to regulate seaweed sales in much the same way as sales of vegetables and fruit.