RE-EXAMINATION OF ACUTE TOXICITY OF FLUORIDE
http://www.fluoride-journal.com/97-30-2/302-89.htmKenji Akiniwa, Tokyo, Japan
SUMMARY: The acute toxic dose of fluoride has been believed to be 2 to 5 mg or 8 mg/kg of body weight. However, acute fluoride poisonings have occurred at doses of 0.1 to 0.8 mgF/kg of body weight in the USA.
In Japan, a school-based anticariogenic program is being carried out with fluoride mouth rinses containing 500 to 2000 ppm sodium fluoride on approximately 158,000 persons, consisting mainly of elementary and junior high school children. Thus the safety problem of this treatment attracts much attention. Fluoride retention is said to be around 15 to 30% in fluoride mouth rinsing. In this paper, on the basis of toxic doses estimated in outbreaks of fluoride poisoning, the potential for acute poisoning by fluoride ingested during mouth rinsing is assessed.
Acute fluoride poisoning is shown to be caused by exposure to lower doses of fluoride than commonly suggested. The toxic dose of fluoride should therefore be re-examined.
Key words: Acute toxicity; Dental fluorosis; Fluoride mouth rinsing; Sodium fluoride.
INTRODUCTION
Excessive intake of fluoride has been re-examined recently in the USA1 and Canada.2 The World Health Organisation has warned of an unexpected increase in the incidence of dental fluorosis, a form of a chronic fluoride toxication, and recommended procedures to prevent excess fluoride intake.3
Promoters of fluoride mouth rinsing set the acute toxic dose of fluoride at 2 mg/kg body weight,4 the Ministry of Health and Welfare of Japan at 2 to 5 mg/kg,5 and some other investigators at 8 mg/kg. However, 0.1 to 0.8 mg/kg of fluoride has been estimated to have caused fluoride poisoning in the USA, resulting from troubles with fluoridated water systems and from ingestion of fluoride- containing products by mistake.7-15
In Japan, however, a school-based program of fluoride mouth rinsing (500-2000 ppm sodium fluoride) was selected as the second best alternative to fluoridation of the water system, involving almost 158,000 elementary and junior high school children,16 with a further recent increase involving children under 6 years of age in kindergarten and nursery schools.17
The safety problem of this treatment has become increasingly important. Fluoride retention after fluoride mouth rinsing is considered to be 15% to 30% of the rinsing water18-21 (that is, ingested fluoride = fluoride in the rinsing water -- fluoride in the rinsing water spat out = fluoride swallowed + fluoride absorbed through the mucus membrane of the mouth). According to the toxic doses estimated in the cases of fluoride poisoning in the USA, this amount of fluoride retention is able to cause acute fluoride poisoning. However there appears to be no clear basis for estimating toxic doses of fluoride currently used.
ESTIMATES OF ACUTE TOXIC DOSE
Table 1 shows signs and symptoms of acute fluoride poisoning. Table 2 shows the minimum toxic dose (MTD),22 probably toxic dose (PTD), safely tolerated dose (STD) and certainly lethal dose (CLD) of fluoride. The toxic dose of 2 mgF/kg body weight is supposedly based on Baldwin's experiment conducted 100 years ago in which he himself ingested sodium fluoride and reported:4
"Merely tasting small quantities produced a slight feeling of nausea with slight salivation. 0.03 gram swallowed with some bread produced no effect. Neither did 0.09 gram taken one hour later, except a little salivation. 0.25 gram, however, taken two days afterward on an empty stomach, produced nausea in two minutes. This gradually increased in severity for twenty minutes when the period of greatest intensity was reached. There was a largely increased flow of saliva and some retching but no vomiting occurred at that time although the desire was very great. The nausea gradually subsided so that luncheon could be eaten (without relish), but vomiting took place immediately on its completion which was two hours after taking the poison. Slight nausea continued throughout the following day but disappeared on the second day" [sic].
The amounts of fluoride he ingested were calculated and are shown in Table 3. He did not mention the toxic dose of 2 mg/kg in his report, as seen in the above. But the threshold toxic dose in this case is estimated to be 0.25 mgF/kg or less.
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