Sunscreen nanotechnology

Powders of titanium dioxide or zinc oxide form a physical barrier against the sun's rays. They reflect UVA as well as UVB rays and reduce the need to use chemicals that may irritate the skin.

In the past they came as thick white pastes that were messy to use and left a ghostly white sheen on your skin. Then scientists found that shrinking the particles of these two ingredients to less than 100 nanometres (a nanometre equals one millionth of a millimetre) made them transparent while keeping their sunscreen action.

But nanoparticles are tiny enough to slip through cell membranes of the skin.

Concerns have been raised that if nanoparticles of zinc oxide or titanium dioxide are absorbed into skin cells they could possibly interact with sunlight to increase the risk of damage to these cells. The Australian Therapeutic Goods Administration has said this would be a concern only if the nanoparticles penetrated living skin cells. "The weight of current evidence is that they remain on the surface of the skin ..."

However, it's not known whether skin damaged by eczema, acne or cuts is more vulnerable to penetration.

Unfortunately, most sunscreens don't say whether or not any titanium dioxide or zinc oxide they contain is in nano form. So if you want to avoid exposure to nanoingredients, choose sunscreens that don't list them on the label.

If a sunscreen containing either of these ingredients is transparent when applied, it probably contains nanoparticles. The word "micronised" on the label is also a clue.

More information

• Cancer Society of New Zealand: www.cancernz.org.nz
• Information on skin cancer: www.sunsmart.org.nz
• International Council on Nanotechnology: http://icon.rice.edu

Report by Bev Frederikson.