Neither hollow-fibre membrane filters nor activated-charcoal filters remove fluoride from fluoridated tap water.
OBJECTIVE
Previous reports of the reduction of fluoride concentrations in fluoridated water by domestic water treatment systems have indicated that further supplementation with fluoride is required. However, the absorption of fluoride by filters has not yet been directly identified. If these filters do not absorb fluoride, further fluoride supplementation may increase fluorosis. In this study, we determined whether filtering systems absorb fluoride ions.


METHODS
We directly measured the amounts of fluoride absorbed by activated-carbon filters or hollow-fibre membrane filters using pyrohydrolysis of the filters and flow-injection analysis, the sensitivity of which is more than 100 times greater than that of conventional methods. We made fluoride solutions of pure or tap water and determined changes in fluoride concentration as a result of filtering with a fluoride electrode.


RESULTS
Hollow-fibre membrane filters did not affect fluoride concentrations in the fluoridated water, but activated-carbon filters removed some fluoride, especially from the pure-water solution. Filtering a pure-water solution with a fluoride concentration of 0.8 mg F/L reduced the fluoride concentration until 210 L of the solution had been filtered. However, filtering a tap-water solution of 0.8 mg F/L reduced the fluoride concentration only until 8 L had been filtered. The concentration of absorbed fluoride in the filter at 10 L of filtration was 4.7 mg/kg activated carbon.


CONCLUSIONS
Further fluoride supplementation of fluoridated water should not be necessary, regardless of whether an activated-carbon or hollow-fibre membrane filter is installed on a domestic water treatment system.
Department of oral health, School of Life Dentistry at Tokyo, Nippon Dental University,Tokyo, Japan.
Hideki
H
Konno
Ken
K
Yaegaki
Tomoko
T
Tanaka
Tsutomu
T
Sato
Kazuyoshi
K
Itai
Toshio
T
Imai
Takatoshi
T
Murata
Mayumi
M
Herai
D002327:Cariostatic Agents / Q000032:analysis*; D002606:Charcoal; D005374:Filtration / Q000295:instrumentation*; D005457:Fluoridation; D005459:Fluorides / Q000032:analysis*; D005618:Fresh Water / Q000737:chemistry; D008567:Membranes, Artificial; D018508:Water Purification / Q000295:instrumentation*
D002327:Cariostatic Agents; D008567:Membranes, Artificial; D002606:Charcoal; D005459:Fluorides