Measured fluoride concentrations in air in the United States and Canada typically range from 0.01 to 1.65 µg/m³, with most of it (75%) present as hydrogen fluoride (CEPA 1996). The highest concentrations (>1 µg/m³) correspond to urban locations or areas in the vicinity of industrial operations. Historically, concentrations ranging from 2.5 to 14,000 µg/m³ have been reported near industrial operations in various countries (reviewed by EPA 1988). Ernst et al. (1986) reported an average concentration of airborne fluoride of about 600 µg/m³ during the 1981 growing season in a rural inhabited area (Cornwall Island) on the U.S.-Canadian border directly downwind from an aluminum smelter. Hydrogen fluoride is listed as a hazardous air pollutant in the Clean Air Act Amendments of 1990 (reviewed by ATSDR 2003), and as such, its emissions are subject to control based on “maximum achievable control technology” emission standards. Such standards are already in effect for fluoride emissions from primary and secondary aluminum production, phosphoric acid manufacture and phosphate fertilizer production, and hydrogen fluoride production (ATSDR 2003).

For most individuals in the United States, exposure to airborne fluoride is expected to be low compared with ingested fluoride (EPA 1988); exceptions include people in heavily industrialized areas or having occupational exposure. Assuming inhalation rates of 10 m³/day for children and 20 m³/day for adults, fluoride exposures from inhalation in rural areas (<0.2 µg/m³ fluoride) would be less than 2 µg/day (0.0001-0.0002 mg/kg/day) for a child and 4 µg/day (0.00006 mg/kg/day) for an adult. In urban areas (<2 µg/m³), fluoride exposures would be less than 20 µg/day (0.0001-0.002 mg/kg/day) for a child and 40 µg/day (0.0006 mg/kg/day) for an adult. Lewis and Limeback (1996) used an estimate of 0.01 µg/kg/day (0.00001 mg/kg/day) for inhaled fluoride for Canadians; this would equal 0.1 µg/day for a 10-kg child or 0.7 µg/day for a 70-kg adult.

Occupational exposure at the Occupational Safety and Health Administration (OSHA) exposure limit of 2.5 mg/m³ would result in a fluoride intake of 16.8 mg/day for an 8-hour working day (0.24 mg/kg/day for a