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Context - Zinc oxide (ZnO) and titanium dioxide (TiO2) nanoparticles (NPs) are used in sunscreens.
Are they safe?
This is a faithful summary of the leading report produced in 2016 by The Australian Therapeutic Goods Administration (TGA): "
Because zinc oxide and titanium dioxide are very effective at blocking UV-light, these are largely used in sunscreens but the fact that their common forms (i.e. particles in the micrometre range) have a white colour made them less appealing to customers for application on skin. However, when they are in the form of nanoparticles (NPs), they become transparent and therefore sunscreens are one of the applications where the emerging nanotechnologies have been developed.
This report reviewed the scientific literature on the safety of zinc oxide (ZnO) and titanium dioxide (TiO2) when in the form of nanoparticles, and the concerns surrounding their use in sunscreens. The two main issues considered in this review were on one side the evidence for the ability of these nanoparticles to penetrate the skin and to reach viable cells, and on the other side their potential toxicity.
It is the second update of the report published by the Australian Therapeutic Goods Administration (TGA) first in 2006 and already updated in May 2013.
Powders of inorganic zinc oxide (ZnO) and titanium dioxide (TiO2) have been used as ingredients in the majority of physical sunscreens because of their strong UV light (UV-A and UV-B) absorbing capabilities. When particles in these powders are in the micrometer range, they keep their color and UV-absorbtion properties when exposed to UV for long periods of time, which enhances the stability of sunscreens and their ability to protect the skin from ultraviolet light.
One disadvantage of ZnO and TiO2 particles is that, when their size is in the range of a micrometre, they are visible on the skin as an opaque white layer and this resulted in reluctance of consumers to use sunscreens products containing them. However, when used in nanoparticle (NP) form, these oxides are transparent and are not visible on the skin but they retain or even augment their UV-sunscreening properties.
The present Literature review showed that there is evidence that:
However, the majority of experimental studies have shown that both ZnO and TiO2 nanoparticles either do not penetrate or minimally penetrate through the outer layers of skin and suggest that the absorption of nanoparticles into the body, and the accompanying toxicity, is highly unlikely.
In addition, neither TiO2 nor ZnO NPs when used in sunscreens on humans in the absence of UV or non-UV light were found to possess notable skin irritation (reversible skin damage), corrosion (irreversible necrotic damage extending into the dermis), or sensitisation properties.
In order to reduce their potential to generate reactive oxygen species (ROS), even after UV exposure, which largely mediate their toxicity for cells and genotoxicity, the surface of nanoparticles can be modified by coating them with aluminium hydroxide, polymers or inert oxides of silica.
Anti-oxidant compounds (such as vitamins A, E or C) can also be added to the sunscreen formulation. Dispersing ZnO particles in some kinds of triglycerides was shown also experimentally to reduce ROS formation.
However, it has been shown that the integrity of the coating layer can be disrupted and the NP coating be stripped under certain circumstances, principally by calcium and hypochlorite ions, which, for example are present in swimming pool water.
Based on the current evidence, the report concluded that neither titanium dioxie nor zinc oxide nanoparticles in the way they are used as coated ingredients in sunscreens making are likely to cause harm.
On the contrary, the report underlined that the current state of knowledge strongly indicated that the minor risks potentially associated with nanoparticles in sunscreens are vastly outweighed by the benefits that nanoparticles containing sunscreens afford against skin damage and, importantly, skin cancer.
The report also mentioned that, in 2012, the Scientific Committee on Consumer Safety (SCCS) of the European Commission concluded that in sunscreens, zinc oxide nanoparticles can be considered to not pose any risk of adverse effects in humans after application on healthy, intact or sunburnt skin. A similar position was upheld in their review of TiO2 nanoparticles2.
1 © Commonwealth of Australia 2017. This work is copyright. You may reproduce the whole or part of this work in unaltered form for your own personal use or, if you are part of an organisation, for internal use within your organisation, but only if you or your organisation do not use the reproduction for any commercial purpose and retain this copyright notice and all disclaimer notices as part of that reproduction. Apart from rights to use as permitted by the Copyright Act 1968 or allowed by this copyright notice, all other rights are reserved and you are not allowed to reproduce the whole or any part of this work in any way (electronic or otherwise) without first being given specific written permission from the Commonwealth to do so. Requests and inquiries concerning reproduction and rights are to be sent to the TGA Copyright Officer, Therapeutic Goods Administration, PO Box 100, Woden ACT 2606 or emailed to tga.copyright(@t)ga.gov.au>.
2 It should be noted that titanium dioxide dust, via the inhalation route, has been classified by the International Agency for Research on Cancer as “possibly carcinogenic to humans”. (Group 2B)
https://monographs.iarc.fr/ENG/Publications/techrep42/TR42-4.pdf 
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