The National Academy of Sciences has released a report indicating that the current legal levels of fluoride in drinking water are dangerous and should be lowered. Although the U.S. government states that only 0.7 to 1.2 milligrams of fluoride per liter of drinking water is necessary to help prevent tooth decay, up to four times that amount is present in some municipal water supplies. Excessive fluoride ingestion is known to weaken bones. The fluoride debate has raged on for over 60 years, with opponents pointing to data showing that it's effective when applied topically but not ingested. Consumers can learn how much fluoride is in their tap water by asking their local utility, and most of it can be removed through filtration.