I take kelp, but not everyday. I take them about 4 times a week. The reason, I don't regularly, is due to the iron content. No other reason. My bottle of Nature's Way, says no more than one per day.
Kelp contains mercury, as well as other not so desirable trace minerals that are listed by Prop 65. Kelp is not an extract. It is a whole food. As a whole food, it is quite safe. Otherwise the Japanese population would diminish. The minerals are balanced in such a way that makes the more undesirable minerals bound and safe.
Even bromate, used by nearly all bread makers instead of iodine, is a known cancer toxin according to prop 65. The only bread, I know, that specifically labels the bread with "NO BROMATES", is Nature's Own sold by many grocers. Bromine is a halogen that displaces iodine in the thyroid, also a halogen.
https://www.oehha.ca.gov/prop65/prop6...ngle100810.pdf