Literature review on the safety of titanium dioxide and zinc oxide nanoparticles in sunscreens
Scientific review report
This scientific review report is limited to the review of safety concerns surrounding zinc oxide (ZnO) and titanium dioxide (TiO2) nanoparticles (NPs) present in sunscreens. The two main issues considered in this review are the evidence for the ability of these NPs to penetrate the skin to reach viable cells and the potential toxicity exerted by them.
The TGA has been continuously monitoring the emerging scientific literature in this area and working cooperatively with international regulatory agencies to ensure that appropriate regulatory action is undertaken if any unacceptable risk of harm/toxicity is identified.
A review on these issues was first published by the Therapeutic Goods Administration (TGA) in 2006, updated in 2009. The current review is based on several published papers (up to May 2013), as well as on reviews of international authorities in the public domain.
Several in vitro and in vivo studies using both animal and human skin have shown that these NPs do not penetrate the underlying layers of skin, with penetration limited to the stratum corneum. This suggests that systemic absorption is unlikely.
In conclusion, on current evidence, neither TiO2 nor ZnO NPs are likely to cause harm when used as ingredients in sunscreens.
Inorganic UV filters zinc oxide (ZnO) and titanium dioxide (TiO2) have been used as ingredients in sunscreens for over three decades. However, one apparent disadvantage of ZnO and TiO2 is that in their macroparticulate (bulk) form in sunscreens, they are visible on the skin as an opaque layer resulting in reluctance of consumers to use the products. This visual effect has been addressed by decreasing the particle size of these metal oxides to nanoparticle (NP) form (see Section 2). When used in this NP form, these oxides cannot be seen on the skin but retain or even augment their UV-sunscreening properties.
In the USA, patents on TiO2 and ZnO NPs were filed in the 1980s (Wang & Tooley 2011), although in Australia the use of TiO2 and ZnO NPs in sunscreens began later. These NPs are particularly useful in sunscreens because of their intrinsic ability to filter ultraviolet (UV)A as well as UVB wave length spectra, thus providing broader protection than any other sunscreening agent.
The TGA has been continuously monitoring the emerging scientific literature in this area and working cooperatively with international regulatory agencies to ensure that appropriate regulatory action is undertaken if any unacceptable risk of harm/toxicity is identified. A review on these issues was first published by the TGA in 2006, which was updated in 2009.
This scientific review report is limited to the review of safety concerns surrounding ZnO and TiO2 NPs present in sunscreens. The two main issues considered in this review are the evidence for ability of these NPs to penetrate the skin to reach viable cells and the potential toxicity exerted by them.
The review is based on several published papers (up to May 2013), as well as on reviews of international authorities in the public domain as listed in the Bibliography (Section 6). An on-line search on the safety of nanoparticulate TiO2 and ZnO in sunscreens was undertaken on the following databases: Medline, Embase, Biosis, Cabi and a Dialog search on a large number of medical and pharmaceutical databases as well as Google. The search strategy used was:
(Nanoparticle OR nanoparticles OR nanoparticulate OR nanoscale OR nanosize OR
nanomaterials) AND (zinc oxide OR titanium dioxide) AND (sunscreen OR sunblock OR
sun block OR sun screen OR UV blockers OR physical sunscreen) AND (safety OR
toxicology OR toxic OR safe)
2. Nanoparticle characteristics
For the purpose of this report, the 'working' definition of TiO2 and ZnO NPs includes materials within the nanosize range of 1 to 100 nm. The review also includes materials that contain aggregates and agglomerates (Section 2.1). The latter have been included because of their yet unclear potential to disaggregate and disagglomerate when applied on the skin in a sunscreen formulation.
2.1 Nanoparticle size
Nanosized TiO2 and ZnO exist in three separate states: primary particles (5–20 nm), aggregates (30–150 nm) and agglomerates (1–100 microns). Primary particles cluster together to form aggregates and they are the smallest units that are present in a final sunscreen formulation (Butler et al., 2012; SCCP, 2007; Schilling et al., 2010; Wang & Tooley 2011). The larger agglomerates form when the aggregates bind loosely as a result of processes during manufacture (Schilling et al., 2010). These are not efficient UV absorbers so they need to be broken down into the more efficient aggregates which are chemically bound.
Although agglomerates are normally not found in sunscreen formulations, they may form on the skin surface after application of sunscreens, suggesting fewer primary NPs would be available for skin penetration (Tran & Salmon, 2010). A preliminary study by Bennett et al. (2012) suggests that exposure to sunlight can lead to disaggregation of TiO2 NPs which facilitated penetration of isolated pig skin sections.
Any potential hazard linked with nanosized particles is likely to be reduced if these particles aggregate and form structures above the nanoscale, which then do not dissociate into smaller NPs, thus reducing their ability to penetrate the skin.
Both ZnO and TiO2 NPs possess photocatalytic properties and are catalysts in the production of reactive oxygen species (ROS). Manufacturers of NP-containing sunscreens attempt to block the potential production of ROS by coating NPs or adding anti-oxidant compounds to the sunscreen formulation (Tran & Salmon, 2010). Coating material includes aluminum hydroxide (Al[OH]3), polymers and inert oxides of silica while anti-oxidant compounds include vitamins (A, E, C).
Fisichella et al. (2012) demonstrated that such surface-treated NPs were not toxic to intestinal cells tested in vitro. It has been postulated that if the Al[OH]3 surface layer is removed, e.g., by chlorine in swimming pools, the photo-active TiO2 NPs can react with water and form compounds, which may lead to skin damage and cancer (Virkutyte et al., 2012). When the TiO2 NPs were immersed in the chlorinated pool water the Al[OH]3 slowly degraded leaving the TiO2 NPs exposed. The study also showed that the coated NPs generated free radicals when exposed to sunlight, but only in the presence of water containing chlorine. Further research in this area is warranted to assess whether there is a risk to human health.
Nano-sized TiO2 has different crystal forms: rutile, anatase and amorphous. The rutile form, or a mixture of rutile and anatase, is generally used in sunscreens (Dussert & Gooris 1997). UV radiation of the anatase form of TiO2 can lead to ROS (hydroxyl radical) formation resulting in in vitro cellular toxicity (Uchino et al., 2002). Recently, Barker & Branch (2008) reported that several sunscreens contained the anatase crystal form of TiO2. They reported that this form of the NP was the cause of rapid deterioration of paint surfaces on steel roofing via a photocatalytic mechanism resulting in the production of ROS.
3. Dermal exposure
3.1 Skin irritation/sensitisation
Potential effects of photoirritation and photosensitisation of ZnO were discussed in the SCCS (2012) report; there was no evidence of any positive findings in two photoirritation studies and two photosensitisation studies after topical application to intact skin of human volunteers.
In a review of photoprotection, Lautenschlager et al. (2007) reported that neither TiO2 nor ZnO NPs possess notable skin irritation or sensitisation properties when used in sunscreens on humans.
3.2 Skin penetration
Skin penetration of TiO2 and/or ZnO NPs through the outer layer to the viable cells within the deeper skin layers was investigated in a large set of in vitro and in vivo studies assessed for this report (Tan et al., 1996; Dussert & Gooris, 1997; Lademann et al., 1999; Pflücker et al., 1999; Bennat & Muller-Goymann, 2000; Pflücker et al., 2001; Lademann et al., 2001; Schulz et al., 2002; SCCNFP report, 2003; Alvarez-Roman et al., 2004; Popov et al., 2005; Gamer et al., 2006; Nash, 2006; NANODERM project, 2007; Cross et al., 2007; Mavon et al., 2007; Zvyagin et al., 2008; Lademann et al., 2008; Nohynek et al., 2008; Newman et al., 2009; Kuo et al., 2009; Wu et al., 2009; Filipe et al., 2010; Senzui et al., 2010; Nohynek et al., 2010; Gulson et al., 2010; Schilling et al., 2010; Sadrieh et al., 2010; Monteiro-Riviere et al., 2011; Christensen et al., 2011). The findings of the great majority of these studies indicated an apparent inability of TiO2 and ZnO NPs to reach viable cells in the dermis.
3.2.1 in vitro studies
There has been a plethora of in vitro studies investigating the properties of nanoparticulate TiO2 and ZnO, using various cultured cell lines and different methodological approaches. This multiplicity and variability in methodology has generated uncertainty regarding the relevance of individual study findings. For example, Lanone et al. (2009) and Park et al. (2009) identified a clear difference in the sensitivity between cytotoxicity assays with cell type being investigated. Further, numerous issues were identified by Park et al. (2009) concerning the conduct of in vitro studies leading to a conclusion that "much work has yet to be done before in vitro toxicity assays can play any role of importance within the risk/hazard assessment of nanomaterials".
The complex nature of interactions of nanomaterials (nanoparticles/quantum dots) with cell cultures was highlighted in an in vitro study (Ryman-Rasmussen et al., 2007), where difficulties with interpretation of results from this type of study were identified. A review (Stone et al., 2009) on the development of in vitro systems for nanotechnology highlighted the complexities associated with in vitro studies investigating safety of nanoparticles and the need to establish validated experimental approaches to determine any biologically relevant nanoparticle-induced hazard.
For example, Nohynek et al. (2008) provided a comparative analysis of dermal penetration between different animal species, rating them in the order: rabbit >rat >pig >monkey >man. They noted that pig and rat skin is up to 4 and 9–11 times, respectively, more permeable than human skin. This inter-species variability in skin penetration demands caution when extrapolating positive findings from animal studies to potential for hazard in humans. The results of Wu et al. (2009) can at present be only accepted as preliminary, and speculative in terms of the hazard in humans, particularly since their findings in porcine skin in vitro and in vivo clearly indicate that TiO2 NPs penetrated into the stratum corneum, stratum granulosum, prickle cell layer and basal cell layer, but not into the dermis. Importantly, only 4 nm TiO2 NPs reached the basal cell layer; hence the ability of TiO2 NPs to penetrate porcine skin was shown to be size-dependent. This is an important finding considering that a great majority of TiO2 NPs exist in sunscreen formulation as relatively large aggregates or even larger agglomerates after the skin application (Schilling et al., 2010).
A few studies with findings which suggested that TiO2 NPs could penetrate beyond the stratum corneum suffered from important methodological limitations that put the validity and extrapolation of these findings to humans in doubt (Tan et al., 1996, effect not statistically significant; Wu et al., 2009, effect observed in hairless mouse only; Sadrieh et al., 2010, tissue contamination likely).
Evidence that other forms of nanomaterials can penetrate into the skin has been presented. Vogt et al. (2006) examined dermal penetration of NP fluospheres and showed that 40 nm particles can penetrate hair follicles and reach the Langerhans cells surrounding the follicles in the dermis. However, this penetration was only achieved after tape-stripping debris from the follicle opening as well as removing a significant proportion of the protective stratum corneum and subjecting the human skin samples in vitro to a prolonged (16 h) exposure to the test material in a special humidified chamber. Such conditions are artificial and unlikely to mimic typical use of sunscreens. Zhang & Monteiro-Riviere (2008) showed that Fluosphere NPs (quantum dots of 14 and 18 nm) did not penetrate normal, flexed or even tape-stripped skin in rats, while minimal penetration was observed in abraded skin. This was confirmed by Campbell et al. (2012) who demonstrated that Fluosphere polymeric NPs (20–200 nm) penetrated only into the superficial layers of the stratum corneum of partially damaged pig skin strips, again suggesting that the NPs are unlikely to reach viable cells below the stratum corneum.
The effect of absorption enhancers (oleic acid and/or ethanol) on the skin penetration of ZnO NPs was studied (Kuo et al., 2009) in vitro using skin samples from hairless mice. Results indicated that ZnO NPs penetrated into the stratum corneum, but not into viable cells in the presence of these penetration enhancers. This finding suggested that ZnO NPs are unlikely to penetrate into viable skin cells in humans, since this could not be achieved in an animal model with higher dermal permeability compared with human skin, even in the presence of penetration enhancers.
The possibility that hair follicles may play a part in percutaneous absorption of nanomaterials has also been examined. Otberg et al., 2008 found that hair follicles represented very limited sites for potential absorption (up to 0.1% of the skin surface), assuming they were permeable to NPs, although ZnO or TiO2 NPs were not investigated. In another study, Lademann et al. (1999) failed to detect TiO2 in the epidermal or dermal tissue outside of the follicles. Similarly, Pflücker et al. (1999) found TiO2 NPs only in trace amounts in the upper part of the hair follicle without any evidence of uptake into follicular epithelium. Although Bennat & Muller-Goymann (2000) concluded that their findings in vitro suggested that TiO2 NPs seems to penetrate human skin probably via the sebum lipids of the hair follicle, they could not detect any TiO2 NPs within the sebum matrix of hair follicles.
The currently available evidence suggesting that the likelihood of penetration of TiO2 NPs beyond the surface layers into viable cells of the dermis is extremely low is supported by a recent review (Christensen et al., 2011).
3.2.2 in vivo studies
An Australian study investigated the potential for systemic exposure to nano-ZnO in a sunscreen formulation, under real-life conditions of sunscreen use by humans at a beach over a period of five days (Gulson et al., 2010). The results indicated that blood and urine levels of 68Zn (based on changes in 68Zn/64Zn ratio) from 68ZnO particles in sunscreens increased in all subjects over the period of exposure, with significant higher levels of 68Zn in females exposed to a sunscreen containing NPs of 68ZnO, compared to females exposed to larger 68ZnO particles and males exposed to particles of both sizes. However, there are several concerns relating to the methodology and conduct of the study that may impact on the validity of these findings. Most critically, the determination of 68Zn ion concentration in the blood as evidence of systemic or dermal exposure to ZnO NPs does not necessarily prove the dermal penetration of, or systemic exposure to, ZnO NPs. The method used to detect 68Zn from the sunscreens could not distinguish between 68ZnO particle or 68ZnO2+ ions. However, these observations may still be relevant to human risk assessment because they show that the level of exposure to Zn following short term and real-life use of ZnO NP-formulated sunscreen was negligible compared with the Zn levels normally found in the body and in a typical daily diet. Thus, such exposure is unlikely to be of any concern to human health.
Subsequently, Filipe et al. (2010) investigated dermal penetration of TiO2 and ZnO NPs in human volunteers after in vivo application, using punch biopsy analysis. Localisation of TiO2 and ZnO NPs in damaged skin was evaluated using skin tape stripping before sunscreen application. Interestingly, the removal of the stratum corneum resulted in negligible adhesion of the sunscreen, with no Ti detectable beyond the stratum corneum, and Zn levels were found to be similar to non-treated skin (i.e., normal physiological levels). The authors concluded:
"Layers deeper than the stratum corneum were devoid of TiO2 and of exogenous ZnO, even after a 48 hour exposure of the sunscreen under occlusion."
In a review article (Newman et al., 2009), the issue of altered penetration of nanosized ingredients through damaged (i.e., diseased, sunburnt) skin was discussed. Overall, the authors suggested that penetration through compromised skin was likely to be similar to normal skin (findings of other studies were cited), but more work was needed to improve our understanding of this important safety issue. For psoriatic skin, application resulted in the sunscreen only being found on the top layers of the stratum corneum. Additional in vitro data (Senzui et al., 2010) for TiO2 NPs applied on skin (from micropig) damaged using tape stripping showed no penetration of Ti into viable skin cells.
Most recently, Sadrieh et al. (2010) conducted a dermal penetration study in minipigs using three different forms of TiO2 NPs, including one TiO2 NP currently used in a commercial sunscreen formulation. Using detailed transmission electron microscopy analysis, they detected minimal penetration of TiO2 NPs into sub-epidermal layers of the skin, but found no evidence of TiO2 NP penetration via expected routes such as follicular lumens. In concert, these results suggest that the risk of TiO2 NP penetration into the dermis via hair follicles is very low.
Several in vitro and in vivo studies using both animal and human skin have shown that these NPs do not penetrate the underlying layers of skin, with penetration limited to the stratum corneum. This suggests that systemic absorption is unlikely.
With respect to ZnO, this conclusion is also supported by the opinion expressed in a comprehensive review of the toxicological profile of ZnO NPs recently completed by the European Commission’s Scientific Committee on Consumer Safety (SCCS, 2012). After analysing the data provided and the published literature, the group of independent scientists found no evidence that ZnO NPs are absorbed through the skin.
None of the studies provided any evidence that nano-sized ZnO particles are able to cross the skin barrier in intact or compromised skin. Dermal penetration was largely determined by methods which did not discriminate between particulate and solubilized forms. They were of the opinion "that, until proven otherwise, it is assumed that any transdermal penetration following application the nano-ZnO containing cosmetic product is that of Zn ions released from the ZnO nanoparticles". The SCCS concluded that the use of ZnO NPs, at a concentration of up to 25% as a UV filter in sunscreens "can be considered not to pose a risk of adverse effects in humans after dermal application".
4. Cytotoxicity and genotoxicity
Nano-TiO2 (uncoated anatase and coated rutile, average crystal size 20 nm for both) was not cytotoxic or genotoxic in vitro (using rat liver epithelial cells in micronucleus assay) in the presence or absence of UV light (366 nm) (Linnainmaa et al., 1997). The authors concluded that TiO2 NPs do not cause direct chromosomal damage and suggested that the tumour responses to TiO2 NP dust observed in experimental animals were most likely a consequence of inflammatory effects rather than genotoxicity.
Recent investigations into the cytotoxic properties of ZnO NP cores coated with a TiO2 NP shell revealed that the inherent cytotoxic properties of the former were markedly reduced by curtailing the release of Zn2+ ions and decreasing the contact area of the ZnO NP by the TiO2 NP shell (Hsiao & Huang, 2011b). Thus, if cytotoxicity is largely related to release of Zn2+ ions from the particle surface, it is likely that these potentially harmful effects of ZnO NPs can be effectively controlled by particle coating (Section 2.2).
Cellular toxicity following exposure to TiO2 or ZnO NPs has been linked to free radical generation (Cai et al., 1991; Dunford et al., 1997; Wamer et al., 1997; Afaq et al., 1998; Serpone et al., 2001; Uchino et al., 2002; Long et al., 2006; Kang et al., 2008; Braydich-Stolle et al., 2009; Liu et al., 2010; Yin et al., 2010 and Xue et al., 2010). Evidence from these in vitro studies using cultured cells suggests that both TiO2 and ZnO NPs possess an inherent ability to induce generation (through excessive oxidation) of free radicals (e.g., hydroxyl radicals) which, if not neutralised by specific intracellular defences, can cause cellular damage, ultimately resulting in cell death. However, Warheit et al. (2007) who investigated in vivo and in vitro toxicity of ultrafine TiO2 (average size 140 nm) concluded that this form of oxide exhibited low hazard potential in mammalian and aquatic species/cell lines following acute exposure. In another in vitro study of TiO2 biological activities (Kang et al. 2008), it was noted that while ROS levels in RAW 264.7 cells were generally higher following treatment with TiO2 (fine and ultrafine – 21 nm and 100 nm, respectively), the detected increase in ROS was not concentration-dependent. The level of intracellular ROS was slightly lower at high concentrations compared with low concentrations of TiO2.
Genotoxicity/mutagenicity was addressed by several studies and reviews. Three unpublished studies investigating photomutagenicity effects (possibly linked to free radical formation) of micronized (≤200 nm) ZnO had been reviewed by the Scientific Committee Cosmetic Non-Food Products (SCCNFP 2003) to the European Commission, which concluded that:
"[M]icronised material (ZnO) has been found to be clastogenic, possibly aneugenic and inducing DNA damage in cultured mammalian cells in vitro, under the influence of UV light".
In the same review, the SCCNFP (2003) noted that micronised ZnO was non-photomutagenic in bacterial cultures (Ames assay). However, the general validity of Ames assay findings in nanoparticulate genotoxicity testing has been questioned by Landsiedel et al. (2009) and Warheit & Donner (2010) because NPs may not penetrate the bacterial cell wall. Nevertheless, the OECD guidance manual on nanoparticle genotoxicity testing includes the Ames assay in the list of in vitro methods that should be included in application dossiers submitted in the EU (Warheit & Donner, 2010).
A review by Nohynek et al. (2008) suggests that possible photogenotoxicity associated with in vitro exposure to ZnO NPs may be influenced by UV radiation-induced increases in background sensitivity of experimental cell cultures. This could partly explain why levels of photogenotoxicity determined by Dufour et al. (2006) using the micronucleus assay (following exposure to ZnO) were relatively small (2 to 4-fold increase) compared with a potent effect of a known photo-clastogenic agent. Consequently, Nohynek et al. (2008) questioned the validity of evidence from in vitro studies presented by Dufour et al. (2006) implicating ZnO NPs as a photogenotoxic agent, and the same review documented data suggesting that the TiO2 NPs were not genotoxic. Furthermore, TiO2 NPs were found to be of low hazard based on a series of toxicity studies conducted by Warheit et al. (2007), and Nohynek et al. (2008) concluded that the hazard associated with the use of TiO2 and ZnO NPs appears to be low. This view was recently confirmed by the review conducted by industry representatives (Schilling et al., 2010), which stated that "the in vitro genotoxic and photogenotoxic profiles of these nano-structured metal oxides are of no consequence to human health".
The findings of another study (Gopalan et al., 2009), which investigated photogenotoxic effects of TiO2 and ZnO NPs in human lymphocytes and in sperm cells, were inconsistent. Whereas lymphocytes showed significant levels of photogenotoxicity in a concentration-dependent manner for both oxides, the sperm showed greater photogenotoxicity at the lowest concentrations of both TiO2 and ZnO; no explanation for this unexpected finding was provided by the authors. The relevance of these findings to dermal exposure remains unclear. As discussed by McCall et al. (2013), a better interpretation of the results could be made if it had been accompanied by physico-chemical characterisation of the NPs used in this biological study.
Further investigations of Hsiao & Huang (2011b) examining the effect of the TiO2 NP shell coating on the cytotoxicity of nano-ZnO particles demonstrated that the nano-ZnO/TiO2 core/shell structure appeared to moderate the toxicity of nano-ZnO by curtailing the release of Zn2+ ions and decreasing the contact area of the nano-ZnO cores. Thus, these findings suggest that the nano-ZnO cytotoxicity is mediated, at least in part, by the release of Zn2+ ions from the ZnO particle surface.
It is noteworthy that HaCaT cells (human keratinocytes – living cells of epidermis) have been tested, with no evidence of any DNA damage after exposure to ZnO (presumably micronised form; ≤200 nm) in the presence or absence of UV irradiation (SCCNFP 2003).
Although some forms of TiO2 and ZnO NPs have been shown to possess cytotoxic and proinflammatory properties, this does not necessarily represent a health hazard associated with sunscreen usage unless there is also conclusive scientific evidence demonstrating their ability to penetrate human skin in sufficient doses to cause an effect. Even if such evidence were available for certain forms of nanoparticulate TiO2 and ZnO, their potential to cause damage is unlikely to be uniform, considering the large variability in their physicochemical characteristics. Moreover, any identified hazard may be largely reduced by nano-engineering solutions, such as coating, or utilisation of sunscreen formulations which would reduce or eliminate rather than enhance the risk of dermal penetration.
The SCCS (2012) comprehensive review of ZnO NPs assessed both in vitro and in vivo studies on photo-mutagenicity/genotoxicity, concluding that there is no conclusive evidence to ascertain whether or not ZnO NPs pose a mutagenic/genotoxic, photo-toxic or photo-mutagenic/genotoxic risk to humans. Since skin penetration is limited to the upper layers, it is very unlikely that harmful effects would occur at the systemic level in humans.
If, as seen in cell cultures in vitro, TiO2 and ZnO NPs can trigger the generation of ROS in the presence of sunlight in vivo, the potential toxicity associated with such an outcome would be negated if it were shown not to take place in viable cells under the stratum corneum. Regardless of potential hazard, covering of TiO2 and ZnO NPs with coating agents (e.g., inert oxides of silica) has been used as a prophylactic approach to address the concern of potential ROS generation and associated hazard, with promising results. Tsuji et al. (2007) discussed the usefulness of this process in reducing or eliminating ROS generation by TiO2 NPs (anatase) following UV irradiation. In another study (Carlotti et al., 2009), investigators examined ROS production induced by TiO2 NPs coated with various materials. The study demonstrated that coatings based on silicon dioxide were most effective, and that not all coating materials effectively reduce the oxidative activity of TiO2 NPs.
The current weight of evidence suggests that TiO2 and ZnO NPs1 do not reach viable skin cells or the general circulation, but rather they remain on the skin surface and in the outer layer of the stratum corneum which is composed of non-viable, keratinized cells. Importantly, potential for harm has not been demonstrated in vivo following exposure to these nanomaterials from normal sunscreen usage in short-term studies.
There is conclusive in vitro evidence that in the presence of UV light, specific forms of ZnO and TiO2 NPs can induce free radical formation, which may damage cells (e.g., ZnO-induced photogenotoxicity). However, it should be noted that during the manufacturing process the surface of the NPs are coated to reduce the formation of free radicals, even after UV exposure.
Several expert reviewers caution about extrapolating the in vitro data to the in vivo setting to identify possible risk(s) associated with sunscreen use, highlighting the complexities and/or uncertainties inherent in such in vitro studies. This position is based on:
- The lack of validated and internationally-endorsed experimental approaches to determine biologically relevant NP hazard prevents a meaningful and conclusive interpretation of the in vitro data as a signal for potential human risk from exposure to ZnO and TiO2 NPs through the use of sunscreens.
- These cytotoxic and pro-inflammatory effects appear to be dose-dependent and, therefore, very low tissue exposure are unlikely to present a hazard, and may indeed be adequately controlled by natural cellular defences.
- Protective mechanisms, such as endogenous antioxidant activity mediated by a range of intracellular enzymes and factors, are likely to be protective against the negative effects of oxidative stress generated by any exposure to nanoparticles.
- For ZnO and TiO2 NPs in sunscreens to cause harm requires them to reach vulnerable cells in the skin or enter the systemic circulation, and the current evidence indicates that dermally applied ZnO and TiO2 NPs do not reach viable cells or the systemic circulation, even via diseased or damaged skin.
- The systemic exposure to Zn following use of an uncoated ZnO NP-containing sunscreen is multiple orders of magnitude below the levels of Zn naturally present in diet and deposited within the body2.
- The current state of knowledge suggests that skin damage (e.g., skin cancer) is caused by free-radical generation following repeated exposure to UV radiation or other similar assaults, and sunscreens containing ZnO and TiO2 NPs (and other molecular UV-abosrbers) offer protection against such assaults.
Although the current evidence in relation to potential genotoxicity of ZnO NP is not conclusive, the use of ZnO NPs in cosmetic products should not pose a risk to the consumer in the absence of a significant systemic exposure.
In conclusion, on current evidence, neither TiO2 nor ZnO NPs are likely to cause harm when used as ingredients in sunscreens.
- NB: zinc ions are not nanoparticles.
- This study should be repeated with more robust scientific methodology.
Includes additional reference material consulted but not cited herein.
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