Hazards of nanoscale titanium dioxide

The cosmetics industry defends its use of nano-scale titanium dioxide in suntan lotions with own investigations demonstrating the innocuousness of titanium dioxide. The safety of nano-scale titanium dioxide was also confirmed by the Scientific Committee on Cosmetic Products and Non-food Products Intended for Consumers (SCCNFP), an EU body, in October 2000. The committee decided that along with its conventional counterpart, nano-scale titanium dioxide could also be included in the list of approved UV filters: "The SCCNFP is of the opinion that titanium dioxide is safe for use in cosmetic products at a maximum concentration of 25% in order to protect the skin from certain harmful effects of UV radiation. This opinion concerns crystalline titanium dioxide, whether or not subjected to various treatments (coating, doping, etc.), irrespective of particle size, provided only that such treatments do not compromise the safety of the product" (Europäische Kommission 2004). The SCCNFP decision was based however solely on industrial studies not accessible to the public.

In the application of suntan lotions containing nano-scale titanium dioxide, the question arises as to whether it can reach living cells, enter, and either damage them or migrate even further into the body. Moreover, it is not known whether titanium dioxide particles can transport other undesirable substances into the body. Colvin assumes that the risk from titanium dioxide risk is much lower than that of sun bathing itself (Colvin 2003a).

Scientific investigations on coated nano-scale titanium dioxide report that when applied to the skin, the majority of the titanium dioxide remains localized in the upper layers of the skin (stratum corneum). The concentration of titanium dioxide in the upper layers of the stratum corneum decreases drastically over the time, as the skin regenerates (exfoliation). In the deeper layers of the skin, however, the concentration of titanium dioxide particles decreases much more slowly over time (Rickmeyer 2002).

Pflückner reports that coated nano-scale titanium dioxide can be found in hair follicles, which are found in the deeper layers of the skin. However, there are no indications for their movement from the follicles into living cells (Pflücker et al. 2001). This confirms the results of earlier investigations (Lademann et al. 1999). Rickmeyer reports that only very low concentrations of coated titanium dioxide were found in the tissue surrounding the follicles. Contact with living cells "exists only in an evanescently minimum scope" (Rickmeyer 2002). Bennat/Müller-Groymann report that nano-scale titanium dioxide in fatty solutions can penetrate the skin better than in aqueous solutions and do not rule out the possibility of penetration of particles into deeper layers of the skin (Bennat/ Müller-Groymann 2000).

However, Tinkle et al. report that beryllium particles of one-half to one micrometer in size rubbed into the skin have reached the epidermis and in rarer cases the deeper dermis thus bringing them into contact with living cells (Tinkle et al. 2003). Admittedly this result does not say anything about the impact of titanium dioxide on the skin, but it makes clear that in addition to particle size substance class must also be considered in judging possible effects.

The safety of nano-scale titanium dioxide is also being called into question elsewhere. A report by the Royal Society of Engineering e.g. suggests that with the reduction of particle size the number of free radicals on the surface of titanium dioxide increases, possibly leading to skin damage

(Royal Academy of Engineering 2003). This view is backed by an investigation by Dunford et al. and Uchino et al., who report that when titanium dioxide is exposed to sunlight, in vitro as well as with human cells, the cell's DNA is damaged by photocatalysis (Dunford et al. 1997, Uchino et al. 2002, Colvin 2003a). Rickmeyer assumes that these photocatalytic effects may be reduced drastically by coating, but cannot be entirely avoided (Rickmeyer 2002). Moreover it is known that nano-scale titanium dioxide causes cell death in the fetuses of Syrian hamsters (Rahman et al. 2002).

Butz (2006) reports on the results of the Nanoderm research project being promoted by the EU, which is specifically investigating the absorption of titanium dioxide by the skin. His conclusion is that, normally, penetration is restricted to the stratum corneum disjunctum, occasionally Ti is found in s.c. compactum, Ti is rarely detected in the stratum granulosum, Ti is detected in the stratum spinosum in most cases, and Ti spots in the dermis are identified as contaminations.

Moreover Butz states: "To our surprise, the particle shape had no influence, it appears that TiO2 particles are mechanically rubbed into the horny layer / hair follicles / furrows without diffusive transport and thus far there is very limited exposure to vital tissue, but there are open questions: particles in the 1-2 nm range might behave like small macromolecules and penetrate; transglandular pathway clearance from follicles (+ glands?)"

All in all, these research results suggest that nano-structured titanium dioxide as used in the cosmetics industry does not penetrate into the deeper layers when used on healthy skin and that possible negative effects on the cells do not represent a problem of the first priority. Regarding potential effects within the individual environmental compartments, however, little is known.

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