Fluoride is primarily a byproduct of the phosphate fertilizer industry. When manufacturers process phosphate rock into fertilizer, the reaction releases toxic fluoride gases that are captured and converted into a liquid compound called fluorosilicic acid. This same substance is the most common form of fluoride added to public drinking water in the United States.
Phosphate Fertilizer Production
Phosphate rock naturally contains fluoride. When the rock is treated with acid to extract phosphorus for fertilizer, the process releases hydrogen fluoride and silicon tetrafluoride, both highly toxic gases. For decades, these gases vented directly into the atmosphere, damaging crops and livestock in surrounding areas. By the 1960s, complaints from farmers and ranchers pushed manufacturers to install pollution control equipment called scrubbers.
These scrubbers capture the fluoride gases and convert them into fluorosilicic acid, a hazardous but containable liquid. This liquid is then either neutralized with lime and stored in waste ponds, sold for water fluoridation, or processed into other fluoride compounds like sodium fluorosilicate. The phosphate fertilizer industry remains the single largest source of industrial fluoride byproducts worldwide, and researchers have described fluorosilicic acid as “an abundant by-product and industrial waste” of this sector.
Aluminum Smelting
Aluminum production is the other major industrial source. Smelting aluminum requires an electrolyte bath containing a mineral called cryolite, which is rich in fluoride. During the process, fluoride vapors escape from the molten bath in several forms: gaseous hydrogen fluoride, sodium tetrafluoroaluminate particles, and other fluoride compounds. Aluminum fluoride can make up 9 to 12% of the electrolyte by weight, and it gets depleted through chemical reactions, moisture exposure, and simple evaporation.
Modern smelters use a technique called dry scrubbing to recapture these emissions. Alumina powder is passed through the exhaust gases, where it adsorbs the hydrogen fluoride and traps fluoride particles. This “secondary alumina” is then fed back into the smelting cells, recycling over 98% of the fluorides. Still, some fluoride dust remains in the air inside smelter buildings, and older facilities that couldn’t meet tightening emissions limits have been shut down in recent years.
Other Industrial and Natural Sources
Fluoride gases are also released from coal-fired power plants, glass manufacturing, brick and tile works, and plastics factories. Any process that heats fluoride-containing materials to high temperatures, including coal, certain minerals, and clays, can produce hydrogen fluoride emissions.
Outside of industry, the biggest natural source of airborne fluoride is volcanic eruptions. Fluoride also occurs naturally in the earth’s crust, embedded in rocks, coal, clay, and soil. The mineral fluorite (also called fluorspar) is a naturally occurring form of calcium fluoride and has been mined for centuries. This natural calcium fluoride is chemically distinct from the fluorosilicic acid produced by the fertilizer industry, a difference that matters when discussing water fluoridation.
Natural Fluoride vs. Industrial Fluoride
Calcium fluoride is the form found in nature. It’s a simple compound of calcium and fluorine, present in groundwater and mineral deposits around the world. Many water supplies contain some fluoride naturally, leached from surrounding rock.
The fluoride used in most municipal water systems, however, is fluorosilicic acid or its sodium salt, sodium fluorosilicate, both derived from phosphate fertilizer production. A third option, sodium fluoride, is a processed compound that tends to be purer but more expensive. These three chemicals behave differently before they’re diluted but all release fluoride ions once dissolved in water.
The industrial origin of fluorosilicic acid raises questions about trace contaminants. The EPA requires that fluoride additives meet safety standards under NSF/ANSI Standard 60, which limits contaminants to one-tenth of the EPA’s maximum contaminant level after the chemical is diluted in finished drinking water. Testing has found that fluorosilicic acid samples can contain trace amounts of arsenic and lead. Sodium fluoride additives have tested free of arsenic and lead but have contained small amounts of barium. At the concentrations used in water treatment, these trace contaminants are diluted well below regulatory limits, though some researchers have argued the standards deserve closer scrutiny.
What Happens to Fluoride Waste
Not all fluorosilicic acid gets sold for water fluoridation. The portion that isn’t purchased is typically neutralized with lime and stored in waste ponds or sent to landfills. This disposal creates its own problems. Fluoride can leach from stored waste into surrounding soil and groundwater over time, and much of this solid waste receives no further treatment before being landfilled. In some facilities, fluorosilicic acid is instead processed into higher-value chemicals like anhydrous hydrogen fluoride, which is used in petroleum refining, semiconductor manufacturing, and producing refrigerants.
The conversion of a pollution problem into a commercial product is one of the more unusual stories in industrial chemistry. What was once a toxic emission that destroyed farmland became, through scrubber technology and creative reuse, both a public health tool and a feedstock for the chemical industry.