Soleo Organics is a new sunscreen that, amongst other things, proclaims that it is titanium dioxide free. Now why, thought I, should it do that? Is titanium dioxide, a physical sunscreen, something to be avoided? Having only recently put myself off most chemical sunscreens beginning with the letter O, was I now going to have to reconsider titanium dioxide as well? Many hours on Google later......

The basic distinction that is made between chemical and physical sunscreens is that chemicals absorb the UV rays and radiation, while physical sunscreens reflect and/or scatter UV rays and radiation. Actually, it turns out that this isn't entirely true, but I'll come back to that. In the meantime, you'll probably have noticed that titanium dioxide turns up almost every sunscreen with a physical blocker, because it reflects UVA and UVB rays and it doesn't discolor under ultraviolet light.  As a not entirely pointless aside, it is worth noting that titanium dioxide is a pigment and is sometimes used in toothpaste and to make skimmed milk look less like dishwater.

Controversy erupted when the International Agency for Research on Cancer (IARC) pronounced titanium dioxide to be a group 2B carcinogen, eg ''possibly carcinogenic to humans''. Now, any manufacturer that uses TD will be quick to point out the IARC conclusions were based on very specific evidence of high concentrations titanium dioxide dust in the respiratory tracts of rats. So not being a dust-inhaling rat why worry, just slather on the sun cream.

Well, it turns out that there might be one or two causes for concern.

For a start, TD doesn't just reflect rays, it also absorbs them. And this means that, like chemical sunscreens, TD is a photosensitizer, absorbed by the skin and resulting in an increased production of free radicals. In a book called Sunscreen Photobiology—Molecular, Cellular and Physiological Aspects, the author Francis Gasparro says TD illuminated by short wave UV kills human cells. "The distinction between 'chemical' sunscreens and 'physical' sunscreens, attractive though it may be to those who market them, is not based on any significant difference. Both varieties have the potential to produce reactive species that can attack biological materials (human skin cells) when they are exposed to normal sunlight... What is established is that particles of titanium dioxide as large as 220 nm can enter human cells in culture."

A study in 2001 concluded that sunlight-illuminated sunscreen TiO2 (titanium dioxide) particles catalyze DNA damage both in vitro and in human cells. The researchers said that the "results raise concerns on the overall effects of sunscreens and raise the question on the suitability of photoactive TiO2 as a sunscreen component without further studies. The photocatalytically active nature of these metal oxides necessitates some changes since even the TiO2 specimens currently used in suncreams cause significant DNA strand breaks."

The Australian government, which has a particular interest in sun protection since its country is under a big hole in the ozone, was (in 2006) more sanguine: "There is evidence from isolated cell experiments that zinc oxide and titanium dioxide can induce free radical formation in the presence of light and that this may damage these cells (photo-mutagenicity with zinc oxide). However, this would only be of concern in people using sunscreens if the zinc oxide and titanium dioxide penetrated into viable skin cells. The weight of current evidence is that they remain on the surface of the skin and in the outer dead layer (stratum corneum) of the skin."

UPDATE: In July 2016, the EU confirmed that accepted UV filters includes nano titanium dioxide with caveats that concentrations should not exceed 25% and be in products that could cause exposure to lungs.

From what I have read, there seems to be little doubt that TD induces free radicals in the presence of light - in fact some industries make use of this to, for example, eradicate bacteria. What is less clear, is whether the skin readily absorbs TD and the radicals with it. But in the meantime, I am relieved that my Abella sunscreen is zinc oxide only.