Environmental Engineering Reference
In-Depth Information
to larger particles was conducted using TiO 2 (Ferin et al. , 1992 ; Oberdorster et al. ,
1994). As described previously, these studies indicated that nanoparticle TiO 2
induced a greater pulmonary infl ammatory response than fi ne TiO 2 . However, it is
worth noting that when compared to other nanoparticles, such as nickel, cobalt or
carbon black, the TiO 2 nanoparticles are less potent at inducing lung infl ammation,
and this seems to be related to their lower capacity to generate free radicals (Dick
et al. , 2003 ).
TiO 2 can be obtained in two crystal forms, rutile and anatase. Warheit et al. (2007)
recently reported that a nanoparticle TiO 2 preparation consisting of both
anatase and rutile (80/20) was signifi cantly more potent in the rat lung than those
containing rutile alone, suggesting that the crystal form is important in determining
toxicity.
Due to the fact that TiO 2 is used as a sunscreen and in cosmetics, dermal penetra-
tion and toxicology are of great interest. There are many studies which have inves-
tigated the ability of TiO 2 to penetrate skin. The techniques used include tape
stripping, placing and removing adhesive tape onto and from the skin repeatedly
and subsequently analysing the components striped from the skin surface (Mavon
et al. , 2007 ). In vitro techniques use skin explants from humans (Pfl ucker et al. , 2001 )
and pigs (Gamer et al. , 2006) cultured in a tissue bath. All of these studies conclude
that there is no signifi cant skin penetration according to these models. However,
questions have been raised in some reports as to the appropriateness of intact
healthy skin models, since sunscreen is often applied to burnt, diseased or damaged
skin (Borm et al. , 2006). Much work is required to address such questions.
The potential toxicity of zinc oxide nanoparticles has also been studied due to
their inclusion in suntan lotions and cosmetics. Much of this work has focused on
skin, but will not be reviewed here as it is beyond the scope of this chapter. Other
studies have investigated effects on the lung due to the potential for occupational
exposure to dust and consumer exposure to aerosols. For example, Sayes et al.
(2007) exposed rats by instillation to a range of particles including nano and micro-
sized zinc oxide particles (1-5 mg/kg). Both sizes induced an infl ammatory response
which was resolved by one month post-exposure. This was refl ected in vitro by
cytotoxicity to L2 epithelial cells as assessed by the LDH assay. Interestingly, the
MTT cytotoxicity assay and markers of infl ammation (e.g. cytokine expression) did
not concur with the in vivo results, suggesting that the in vitro endpoints and time
points used were not good indicators of the in vivo response. This study did not
generate a clear difference between the micro and nano-sized zinc oxide particles.
The solubility of zinc oxide may be an issue relating to its toxicity. Our own work
has demonstrated that soluble zinc salts interact synergistically with carbon black
nanoparticles to activate pro-infl ammatory gene expression by macrophages in
vitro (Wilson et al. , 2007 )
The metal oxide cerium dioxide (CeO 2 ) has been developed as a fuel additive;
for example Oxonica (Kidlington, UK) markets ENVIROX™ diesel fuel borne
catalyst to reduce fuel consumption and exhaust emissions (http://www.oxonica.
com/, accessed 26 March 2008). A small number of studies have investigated the
potential toxicity of CeO 2 nanoparticles. A number of studies demonstrate a rela-
tively low toxicity associated with exposure to this material. For example, Fall et al.
Search WWH ::




Custom Search