Final answer:
Excessive fluoride ingestion is associated with systemic diseases like skeletal fluorosis, which affects mobility and can also harm the thyroid gland. The US EPA regulates these levels to minimize such health risks.
Step-by-step explanation:
Antifluoridationists associate a variety of systemic diseases and conditions with fluoride ingestion, particularly when exposure is excessive. One of the most severe conditions is skeletal fluorosis, caused by excessive fluoride intake leading to stiffened joints and thickened bones, which can notably impact mobility.
Due to these serious health risks, the US Environmental Protection Agency sets the maximum level of fluoride in drinking water at 4 ppm (4 mg/L), to help prevent the prevalence of fluorosis. Besides skeletal fluorosis, excessive fluoride exposure may also have negative effects on the thyroid gland and is a matter of concern for public health, especially in regions where fluoride levels in groundwater are not adequately regulated.