The titanium dioxide that we use is classed as nano-particulate. Manufacturers use micronised or nano-titanium dioxide in sun lotions because it offers effective protection against UVA rays, the type of radiation that penetrates the skin and which is a leading cause of skin cancers including basal cell carcinoma and melanoma.
The only other way of offering the same level of UVA protection would be to use synthetic chemical sun filters that have been shown to have harmful effects including the ability to mimic hormones in the body.
The concerns over titanium dioxide nanoparticles are two-fold - firstly that due to their greater surface area they have an increased ability to react with other molecules, particularly oxygen, and secondly that they may enter the body through the skin and thereby reach internal organs where they may cause damage due to their increased reactivity.
To counter these two potential problems, the manufacturers of titanium dioxide UV filters in cosmetics treat the minerals during production by coating each particle with a very fine layer of silicates - effectively they coat it in glass. This treatment has the effect of shielding the titanium dioxide from contact with other materials, including oxygen.
This lack of contact prevents any potential reactions from taking place, and renders the mineral absolutely inert. It is therefore incapable of causing oxidative damage, the first of the two concerns.
Secondly, when nano-particles are incorporated into a cream or lotion they clump together, forming aggregations. These are held together by forces of molecular attraction that prevent the primary particles from becoming separated when applied to the skin.
Therefore, although the primary particle size of the minerals is in theory small enough to be absorbed through the skin, in practice the aggregations that are formed in creams and lotions are too large to cross through the skin-blood barrier. Instead, they remain on the surface of the skin where they reflect and scatter UV light.