Table 4 The importance of crystallinity to TiO₂ toxicity

Dunford et al., [58]

TiO₂ extracted from sunscreens (content ranging from 50/50 anatase/rutile, to 100% anatase or rutile)

Pure Anatase (100%)

Pure Rutile (100%)

Oxidation of organic material (phenol)

DNA plasmids in vitro

Comet Assay (MRC-5 fibroblasts)

(ALL conducted in presence of sunlight)

TiO₂ stimulates oxidation of organic materials (due to production of hydroxyl radicals), on illumination

Strand breaks in plasmid DNA. The damage suppressed by free radical quenchers (mannitol &DMSO) - illustrates that it is oxidant (hydroxyl) driven

DNA damage observed in comet assay, and again is oxidant driven

Anatase > rutile.

Derives from greater photocatalytic activity.

Lu et al., [51]

Pure anatase (5 nm)

Pure rutile (50 nm)

Anatase/rutile mixture (21 nm)

Protein tyrosine nitration

(Conducted in presence of UV light)

TiO₂ increased protein tyrosine nitration (indicative of oxidative and nitrative stress)

Anatase >anatase/rutile > rutile

BUT other physicochemical differences such as size were, not controlled for, which may contribute to response

Nakagawa et al, [59]

Anatase form (21 nm)

Anatase form (255 nm)

Rutile form (255 nm)

Rutile form (420 nm)

In vitro genotoxicity assays:

Microbial mutation assay-S. Typhimurium

Mammalian cell mutation assay (L5178Y cells)

Chromosomal aberration assay (CHL/IU cells)

(experiments conducted in presence or absence of UV light)

Weak genotoxicity in absence of UV light

With irradiation, TiO₂ particles were genotoxic in all tests

Anatase > rutile

Phototoxic component to response

21 nm anatase sample most toxic (illustrates that size may also contribute to response)

Pan et al. [72]

Rutile (15 nm)

Anatase (200 nm)

Human dermal fibroblasts

Cell area, morphology & actin

Cell proliferation

Wound healing function

Cell migration

Particle internalisation

Cell morphology detrimentally affected and cell function impaired by TiO₂

Anatase > rutile

Sayes et al., [71]

Pure anatase

Anatase/rutile

Pure rutile

Human dermal fibroblasts and lung carcinoma cells

Cell viability (LDH, MTT)

Inflammation (IL-8)

Ex vivo ROS production

Cytotoxicity, ROS production & cytokine release is crystal phase dependent

Anatase greater than anatase/rutile > rutile

Oxidant driven response & phototoxic component

Size of particles did not contribute to the response

Wang et al., [17]

Rutile (80 nm)

Anatase (155 nm)

Mice (intranasal)

Particle distribution in brain

Neurone morphology & toxicity

Oxidative stress (lipid and protein oxidation)

Neurochemical levels

Accumulation of particles in brain

Both particle types translocate to brain

Anatase elicit greater neurotoxicity

anatase > rutile

Warheit et al., [11]

Rutile

Anatase/rutile

Rats (intratracheal)

Inflammation (BALF cells, LDH, protein)

Histopathology

Pulmonary inflammation (nature, and length of response dependent on particle sample)

Rutile/anatase > rutile

Other factors such as particle size & agglomeration may also contribute to the response