Reaction mass of Potassium aluminium tetrafluoride and Dipotassium aluminium pentafluoride monohydrate

Endpoint summary

Description of key information

Short description of key information on absorption rate: 
A dermal absorption percentage for workers of 0.6% is assumed. For the general population a dermal absorption percentage of 1.7% is assumed.

Additional information

No data were available on the toxicokinetics of aluminium potassium fluoride / KAIF4 in general. Therefore, it is referred to available data of a structural analogue of aluminium potassium fluoride, cryolite (trisodium hexafluoroaluminate / Na3AlF6) as reported in the European Risk Assessment Report (RAR) of cryolite. The two substances are structural analogues of each other, being complex salts of two homological fluoroaluminic acids and differing primarily in the alkali metal cation, namely potassium vs. sodium. Their human toxicological profiles are expected to be mostly governed by the presence of fluoride anions formed upon dissociation of the fluoroaluminate moieties. As sodium and potassium cations are essential constituents and two of the most abundant ions in all humans, as well as in all animal species, it is considered acceptable to derive the lacking data on toxicokineticsof aluminium potassium fluoride by read-across from cryolite.

The available data on the toxicokinetics of fluoride-containing substances are very limited and are rather dated.

Fluorides can be absorbed though the lungs and gastro-intestinal tract. Absorption and excretion of HF is rapid during and after inhalation. Based on limited data water-soluble fluorides, such as HF and NaF, and fluorides in solution may be nearly completely absorbed by the gastro-intestinal tract. The presence of strongly fluoride-binding ions, such as Al³⁺, and Ca²⁺may reduce the absorption of fluoride, possibly by decreasing the solubility.

Roughly half the amount of absorbed fluoride will be stored, predominantly in the bones, the other half being rapidly excreted in the urine. Faecal excretion normally accounts for less than 10% of daily intake. Another route of excretion is by perspiration, mainly through sweat, but this route is considered only of some importance during excessive sweating, e.g. sporting activities.

High fluoride contents in the bones will occur after abnormally high rates of intake. This process seems reversible: after terminating this increased intake, the excessive fluoride stored will be released slowly over a long period and excreted, mainly in the urine.

After occupational exposure to fluorides, including cryolite, correlations were found between fluoride concentrations in both postshift urine and serum and the dust exposures. The urinary aluminium excretion correlated with the urinary fluoride levels. It was suggested that the minimal amounts of fluoride released from cryolite dust could be complexed with aluminium in the form of AlF₆^3-.

The suggestion of complexion of fluoride and aluminium in the form of AlF₆^3-could not be verified. It is unclear what part of fluoroaluminates is absorbed, or whether or how the molecule is dissociated. In excretion measurements, it was usually not addressed what fluoride compound was determined. It is, therefore, not clear whether the data on e.g. NaF are descriptive for multiconstituent aluminium potassium fluoride.

Regarding the inhalation and oral route of exposure, an absorption percentage of 100% is assumed.

Discussion on absorption rate:

Solids substances will only penetrate the skin in (aqueous) solution. Aluminium potassium fluoride is a salt with a water solubility of circa 5 g/L. Once solved, the salt is hydrolysed to its ions F-, Al3+ and K+. The ions are hydrophylic and due to lack of lipophilicity, they will not have any affinity to skin(lipids). Therefore, skin absorption can only occur through the water that penetrates the skin and the maximum skin absorption is defined by the maximum water solubility of the salts and the maximum amount of water that can penetrate the skin.

The maximum amount of water that can penetrate the skin is determined to be 17 µL per 1 cm2 per 24 hours (Ten Berge, W. A simple dermal absorption model: derivation and application.Chemosphere 2009; 75(11):1440-5), which equals 6 µL per cm2 per 8 hours.

Skin penetration, dermal absorption percentage for workers

Since 6 µL of water can maximally penetrate 1 cm2 of skin per 8 hours, 6 x 5 = 30 µg of hydrolysed salt may penetrate 1 cm2 of skin per 8 hours. In an in vitro skin absorption experiment (according to OECD guideline 428), the application should mimic human exposure, normally 1-5 mg/cm2 (1000 -5000 µg/cm2). Thus, in case the skin penetration of aluminium potassium fluoride would be experimentally be determined according to OECD guideline 428 using 5 mg/cm2 as exposure condition, a skin penetration of 0.6% (30/5000) would be observed maximally. Therefore a skin penetration of aluminium potassium fluoride of 0.6% is considered a worse-case situation for workers.

Skin penetration, dermal absorption percentage for general population

Since 17 µL of water can maximally penetrate 1 cm2 of skin per 24 hours, 6 x 5 = 85 µg of hydrolysed salt may penetrate 1 cm2 of skin per 8 hours. In an in vitro skin absorption experiment (according to OECD guideline 428), the application should mimic human exposure, normally 1-5 mg/cm2 (1000-5000 µg/cm2). Thus, in case the skin penetration of aluminium potassium fluoride would be experimentally be determined according to OECD guideline 428 using 5 mg/cm2 as exposure condition, a skin penetration of 1.7% (85/5000) would be observed maximally. Therefore a skin penetration of aluminium potassium fluoride of 1.7% is considered a worse-case situation for the general population.