Fluorine [F, 9]
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15.6 HUMANS
Fluorine occurs naturally as the negatively charged ion fluoride (F – ). Some amounts
of F are present in the body, at about 2.6 g in adults, of which about 95% is found in
the bones and teeth. Concentrations of F in the blood of Americans, receiving fluoridated drinking water, ranged from 20 to 60 μg/L. Once in the body, F is absorbed
into the blood through the digestive tract.
The mean F plasma level in human, with 5.03 mg F/L in their drinking water, is
106 μg/L. Concentration of F in bones varies with age, sex, and part of bones, and is
believed to reflect an individual’s long-term exposure to F (WHO 2002a). Fluoroapatite
forming in bones, after the reaction of F with hydroxyapatite, hardens tooth enamel
and stabilizes bone minerals. At usual intake levels, F does not accumulate in soft tissues. Fluorine therapy may be beneficial for the treatment of osteoporosis, but serious
side effects have been associated with the high doses of F used to treat osteoporosis.
Fluoride crosses the placenta and is transferred from the mother to the fetus. It is
eliminated from the body primarily in the urine. In infants, about 80%–90% of an F
dose is retained; in adults, it is approximately 60% (WHO 2002a). There is a possibility of an adverse effect in case of high F exposure on children’s neurodevelopment
(Choi et al. 2012). Fluoride has both beneficial and detrimental effects on human
health, with a narrow range between the amounts of its intake. The lowest dose that
could trigger adverse symptoms is considered to be 5  mg/kg bw, with the lowest
potentially fatal dose considered to be at 15 mg/kg bw.
Both Ca and Mg form insoluble complexes with F and are capable of significantly decreasing F absorption. Diet low in chloride (salt) may increase F retention
by reducing its urinary excretion. In humans, the only clear effect of inadequate F
intake is an increased risk of dental caries (tooth decay). Calcium supplements, as
well as Ca- and Al-containing antacids, can decrease the absorption of F.
Elevated F intake may involve gastrointestinal irritation, joint pain in the lower
extremities, and the development of Ca deficiency, as well as stress fractures. One of
the most important contributions of F chemistry to the quality of human life has been
the creation of compounds that induce anesthesia. Modern inhalation anesthetics is
almost entirely dependent on F chemistry. Ingestion of excess F, most commonly
in drinking water, can cause fluorosis, which affects the teeth and bones. Moderate
amounts of F lead to dental effects, but long-term ingestion of large amounts can
lead to severe skeletal problems. The dental effects of fluorosis develop much earlier
than the skeletal effects in people exposed to large amounts of F. Dentifrice products
for adults generally contain F at concentrations ranging from 1000 to 1500 mg/kg;
some products for children use contain lower levels, ranging from 250 to 500 mg/kg.
Dental products, such as toothpaste, mouthwash, and F supplements, have been
identified as significant sources of F.
In skeletal fluorosis, F accumulates in the bone progressively over many years.
The early symptoms of skeletal fluorosis include stiffness and pain in the joints. In severe
cases, the bone structure may change and ligaments may calcify, resulting in pain
and impairment of muscles. Early stages of skeletal fluorosis are characterized by
increased bone mass. If very high F intake persists over many years, joint pain and
stiffness may result from the skeletal changes.