In unpolluted ocean waters the natural fluoride level is in the range of 1.3 to 1.4 mg/L. The levels are usually lower in estuarine areas due to dilution by fresh water, unless fluoride pollution is occurring upstream. Measuring fluoride in water is important as fluoride is toxic to humans and aquatic life. Fluoride is accumulated in the hard tissues of fish and shellfish, and then enters the food chain when the organisms are eaten.
Marine organisms can accumulate fluoride even at ambient ocean levels, and this accumulation increases noticeably when there are higher fluoride levels are present. As fluoride moves up the food chain, biomagnification occurs at about 1 order of magnitude per level.