454 JOURNAL OF COSMETIC SCIENCE cosmetic, paint, textile, plastic, and food industries (47). In the pigment industry, two crystallographic forms of titanium dioxide are used: rutile (the most common and more stable) and anatase (42). Titanium dioxide is produced synthetically because resources of the naturally occurring rutile form are limited (40). Generally, there are two types of titanium dioxide, pigment-grade and ultrafine-grade, according to their particle size and use. Pigment-grade titanium dioxide usually has a particle size greater than 100 nm to optimize the scattering of visible light. The ultrafine-grade titanium dioxide is used as a UV filter and has a primary particle size below 100 nm, which enhance its UV radiation absorption. The most important physical property of titanium dioxide is that it has high refractive index (2.70 for rutile and 2.55 for anatase), which are higher than that of diamond (2.42) (42). In a manner similar to that used for the regulations on synthetic colorants, the purity criteria for titanium dioxide were compared among countries (Table VI). The standards of each country are well aligned with one another, except for a few tests. In addition, an international standard method is necessary for each test. In addition to its use as a white colorant, titanium dioxide is used as a component of pearlescent pigments. A pearlescent pigment consists of multiple layers for example, mica is coated with metal oxides such as titanium dioxide or iron oxides (Figure 7). Multiple layers with a low refraction index (e.g., mica, synthetic mica, silica, alumina, and borosilicate) and high refractive index (e.g., titanium dioxide, iron oxides) produce interference or reflect light at the interface or surface of the layers causing a pearl-like luster. In the EU and the other countries, pearlescent pigments do not appear on the positive lists. Pearlescent pigments are rather considered as mixtures of mica, titanium Table VI Comparison of Purity Specifications of Titanium Dioxide Purity Specification EU United States Korea Name CI 77891 Titanium dioxide Titanium dioxide Loss on drying ≤ 0.5% — ≤ 0.5% Loss on ignition ≤ 1.0% ≤ 0.5% ≤ 0.5% Aluminium oxide and/or silicon dioxide ≤ 2.0% — ≤ 2.0% Acid soluble substances ≤ 0.5% (alumina, silica -free basis) or ≤ 1.5% (product containing alumina/silica) ≤ 0.5% ≤ 0.5% Water soluble matter ≤ 0.5% ≤ 0.3% ≤ 0.25% Cadmium ≤ 1 mg/kg — ≤ 1.0ppm Antimony ≤ 2 mg/kg ≤ 2ppm ≤ 2.0ppm Arsenic ≤ 1 mg/kg ≤ 1ppm ≤ 1.3ppm Lead ≤ 10 mg/kg ≤ 10ppm ≤ 10ppm Mercury ≤ 1 mg/kg ≤ 1ppm ≤ 1.0ppm Zinc — — ≤ 50ppm Assay ≥ 99% ≥ 99.0% ≥ 99.0% EU: European Union. “—”: does not have a criteria. China has no specified specification.

455 COSMETIC COLORATION: A REVIEW dioxide, and/or iron oxides and can be used if they meet each specification requirement (48,49). Iron Oxides. Iron oxides make up the second largest sales market of pigments after titanium dioxide. Generally, iron oxide is low-cost, inert to chemicals, insoluble in organic pigments, and of excellent durability, although it has low tinctorial strength and thermal stability (46). Even though iron oxides are common minerals found in nature (40), only synthetic iron oxide can be used in cosmetics because of its color consistency and purity. Synthetic iron oxides are produced with red (α-Fe(III) 2 O 3 ), yellow (α-Fe(III)O(OH)), and black colors (Fe(II)O·Fe(III) 2 O 3 ) (50). The color can vary depending on the oxidation state of iron. Fe3+ represents yellow and red, and Fe2+ appears green and blue. In addition, the particle size of the iron oxides influences the hue of iron oxide pigments (50). For iron oxides, the specifications in each country have limitations on maximum heavy metal (e.g., arsenic, lead) content (Table VII). Although “iron oxides” is used as the Inter- national Nomenclature Cosmetic Ingredients (INCI) name in the United States, the EU, and the other countries adopted distinct INCI names such as CI 77491 (red iron oxide), CI 77492 (yellow iron oxide), and CI 77499 (black iron oxide). metal oxides colorants (optional) substrate with low RI Figure 7. Structure of pearlescent pigments. Table VII Comparison of Purity Specifications of Iron Oxides Purity Specification EU United States Korea Name CI 77491 Iron oxides Red iron oxide Loss on drying — — ≤ 1.0% Water soluble matter ≤ 1.0% — ≤ 0.3% Arsenic ≤ 3 mg/kg ≤ 3ppm ≤ 10ppm Cadmium ≤ 1 mg/kg — — Chromium ≤ 100 mg/kg — — Copper ≤ 50 mg/kg — — Lead ≤ 10 mg/kg ≤ 10ppm ≤ 10ppm Mercury ≤ 1 mg/kg ≤ 3ppm — Nickel ≤ 200 mg/kg — — Zinc ≤ 100 mg/kg — ≤ 50ppm Assay ≥ 68% (as iron) — ≥ 90.0%(as Fe 2 O 3 ) EU: European Union. “—”: does not have a criteria. China has no specified specification.