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Physical Barriers

Some sunscreens contain not only the chemical barrier cocktails mentioned above but also physical barriers that also are harmful. The most common physical barrier sunscreen ingredients are zinc oxide and titanium dioxide. Zinc oxide has been used for decades as a sun protection cream. There are two areas for concern with zinc oxide - the possible effects of zinc ion that can become available from zinc oxide, and it's possible effect as a nanoparticle.

There is evidence that zinc ion can become available from zinc oxide through several mechanisms and that zinc ion can reasonably be anticipated to be toxic to aquatic organisms. The principle concern regarding zinc oxide is its toxicity to aquatic species and its ability to bioaccumulate. Zinc ion may become available in the environment from zinc oxide via dissolution in aqueous solutions particularly between the pH range of 5 and 7.

Zinc oxide is incorporated into sunscreen products in a very fine nanoparticle form for cosmetic effect. In this form it still provides a physical barrier but lacks the bright white thick look that has characterised zinc oxide creams of the past. As a nanoparticle it is of serious environmental concern, as nanoparticles have been found to accumulate in the organs of laboratory animals and are taken up by cells. The smaller the particle, the higher its likely toxicity and nanoparticles have various routes into the body and across membranes such as the blood-brain barrier. There is a real concern that nanomaterials can enter the food chain with disastrous consequences.

The other commonly used physical barrier is titanium dioxide. Titanium dioxide is a compound whose toxicity remains unclear. Researchers now say the chemical can be absorbed by human skin. Titanium dioxide is a fine, white powder, used in sunscreens because of its ability to reflect and scatter ultraviolet light. The compound's full effects on human health are still under investigation. The U.S. government's National Institute for Occupational Safety and Health (NIOSH) labels the chemical 'a potential occupational carcinogen.'

Both titanium dioxide and zinc oxide nanoparticles have been found to cause free radicals in the skin cells, damaging DNA.