Titanium dioxide is a physical sun blocking agent that work primarily by reflecting/absorbing ultraviolet light. It bocks UVB and short UVA (320-340 nm, a.k.a UVA-2). Titanium dioxide is less effective against long UVA (340-400 nm, a.k.a UVA-1). Hence, in regard to the range of protection, titanium dioxide is inferior to zinc oxide, another popular physical sun block.

Titanium dioxide has a long history of safe use as a sun block and cosmetic ingredient. It is not irritating and more compatible with sensitive skin than chemical sunscreens. However, unlike zinc oxide, it is not an anti-irritant or skin protector. Furthermore, it has significant photocatalytic activity and may promote formation of free radicals when exposed to sunlight. However, since it is not absorbed into the skin, this effect may not be an issue in topical use on unbroken skin. More research is required.

The main complaint about titanium dioxide-based sunscreens is that they leave unsightly white residue. In fact, titanium dioxide creates stronger whitish tint than zinc oxide at comparable concentrations. This problem has been partly addressed by the advent of titanium dioxide nanoparticles as a sunscreen agent. Titanium dioxide nonoparticles have different optical properties and tend to produce much less whitish tint than regular powdered titanium dioxide. Yet, early research indicates that titanium dioxide nanoparticles retain the capacity to protect from UVB and short UVA (UVA-2) light.

However, the safety of titanium dioxide nanoparticles remains a controversial subject. When exposed to sunlight (and possibly even other light), titanium dioxide nanoparticles may promote generation of free radicals and increase the risk of mutations - more so that regular titanium dioxide. Furthermore, in one UCLA study, titanium dioxide nanoparticles administered orally to mice in water were shown to cause genetic damage, such as mutations and DNA breaks. However, this effect may not be relevant to topical use as long as titanium dioxide nanoparticles do not penetrate stratum corneum (the outer skin layer consisting mostly of dead cells). Furthermore, manufacturers typically cover titanium dioxide nanoparticles with special coating, which increases the stability and reduces photoreactivity of the particles. Finally, there is a question whether titanium dioxide nanoparticles, could penetrate the skin, reach systemic circulation and accumulate in body organs. At present, there is no clear evidence that they do, but this is a valid concern that needs to be researched. Until then, it may be prudent to avoid the products with titanium dioxide nanoparticles (regular titanium dioxide should be ok). For more details on see our article on the potential risks associated with nanoparticles in skin care.

Titanium dioxide is in many ways similar to zinc dioxide as a sun blocking agent. However, zinc oxide has advantages of having broader range, anti-irritant properties and less opacity at comparable concentrations. In commercial sunscreens, titanium dioxide is usually combines with chemical UVB blockers. Sometimes it is also combined with other physical (zinc oxide) and/or chemical UVA blockers.