In order to achieve the same solids content, the larger filler and the binder should be reduced if necessary.
The skin of an adult person is, in most places, covered with a relatively thick (∼10 μm) barrier of keratinised dead cells. One of the main questions is still whether TiO2 NPs are able to penetrate into the deeper layers of the skin. The majority of studies suggest that TiO2 NPs, neither uncoated nor coated (SiO2, Al2O3 and SiO2/Al2O3) of different crystalline structures, penetrate normal animal or human skin. However, in most of these studies the exposures were short term (up to 48 h); only few long-term or repeated exposure studies have been published. Wu et al.83 have shown that dermal application of nano-TiO2 of different crystal structures and sizes (4–90 nm) to pig ears for 30 days did not result in penetration of NPs beyond deep epidermis. On the other hand, in the same study the authors reported dermal penetration of TiO2 NPs with subsequent appearance of lesions in multiple organs in hairless mice, that were dermal exposed to nano-TiO2 for 60 days. However, the relevance of this study for human exposure is not conclusive because hairless mice skin has abnormal hair follicles, and mice stratum corneum has higher lipid content than human stratum corneum, which may contribute to different penetration. Recently Sadrieh et al. performed a 4 week dermal exposure to three different TiO2 particles (uncoated submicron-sized, uncoated nano-sized and coated nano-sized) in 5 % sunscreen formulation with minipigs. They found elevated titanium levels in epidermis, dermis and in inguinal lymph nodes, but not in precapsular and submandibular lymph nodes and in liver. With the energy dispersive X-ray spectrometry and transmission electron microscopy (TEM) analysis the authors confirmed presence of few TiO2 particles in dermis and calculated that uncoated nano-sized TiO2 particles observed in dermis represented only 0.00008 % of the total applied amount of TiO2 particles. Based on the same assumptions used by the authors in their calculations it can be calculated that the total number of particles applied was 1.8 × 1013 /cm2 and of these 1.4 x107/cm2 penetrated. The surface area of skin in humans is around 1.8 m2 and for sun protection the cream is applied over whole body, which would mean that 4 week usage of such cream with 5 % TiO2 would result in penetration of totally 2.6 × 1010 particles. Although Sadrieh et al.concluded that there was no significant penetration of TiO2 NPs through intact normal epidermis, the results are not completely confirmative.
Though the regulated use of titanium dioxide in food products is legal in the U.S. and Canada, it's banned in some other countries, notably throughout Europe. In May 2021, the European Food Safety Authority announced that titanium dioxide can no longer be considered safe as a food additive.
In industrial settings, people can be exposed to titanium dioxide through inhalation. Inhalation exposure to titanium dioxide is exceedingly rare for most people.
Over the last several years, nanoparticles have come under scrutiny for adverse health effects. Nanoparticles are ultrafine particles between 1 to 100 nanometers in diameter. (To put this in perspective, the average human hair is around 80,000 nanometers thick.) Because of their size, which can be engineered and manipulated at the atomic or molecular level, nanoparticles exhibit unique physical, chemical, and biological properties. Titanium dioxide is one of the most commonly produced nanoparticles in the world.
After oral ingestion, the absorption of titanium dioxide particles is low, however they can accumulate in the body, Maged Younes, chair of the European Food Safety Authority's expert Panel on Food Additives and Flavourings, said in a May 2021 statement.
The disadvantage of Titanium Dioxide is that it's not cosmetically elegant, meaning it's a white, unspreadable mess. Sunscreens containing Titanium Dioxide are often hard to spread on the skin and they leave a disturbing whitish tint. The cosmetic industry is, of course, really trying to solve this problem and the best solution so far is using nanoparticles. The itsy-bitsy Nano-sized particles improve both spreadability and reduce the whitish tint a lot, but unfortunately, it also introduces new health concerns.