The high concentrations of ascorbic acid in the aqueous humour, together with its ability to absorb UV light, have led to its referral as a physiological "sunscreen" [ 22], preventing the penetration of UV light and protecting tissues from photo-induced oxidative damage [ 23].
A damaged stratum corneum would likely allow for exposure of viable living cells and subsequent penetration by nanoparticles in sunscreen.
Penetration of TiO2 microparticles contained in sunscreen into the stratum corneum and follicular orifice of hair has been reported (Lademann et al. 1999).
The incorporation of two sunscreen couples into SLN leads to a further advantage - penetration of UV absorbers into the skin is thereby reduced, resulting in a positive effect on the toxicological potential of the UV absorbers.
Skin penetration of metal nanoparticles also causes mistrust of sunscreen products usage [ 62].
"With early indications that some nano sunscreens may present hazards, so many unanswered questions about skin penetration, and a lack of effective regulation, there is no need for schools to take a risk," says Georgia Miller, from Friends of the Earth Australia.
Although application of sunscreens is the "gold standard" for protecting the skin from UV radiation, studies have shown that currently used sunscreens can cause adverse skin and systemic reactions, owing to their penetration into the viable cutaneous strata and to transdermal absorption.
The penetration of the nanoparticles was reported in a study where the sunscreen was applied to relatively hairy skin [192].
Synthesis of vitamin D3 can be reduced by factors that reduce the penetration of the UV-B rays into the skin such as sunscreen, clothing and darkly pigmented skin (Reginster 2005).
In most safety testing experiments pretreated chemicals isolated from sunscreens are used, but incorporated sunscreen chemicals in cosmetic creams/lotions presented as oil-in-water (O/W) or water-in-oil (W/O) emulsion enhance the penetration of these pretreated chemicals into the skin [ 60, 64].
Because of the deeper penetration of UVA light, causing photoaging and DNA damage, there has been a shift towards broad spectrum sunscreens.
