The Consumer Products Inventory lists over 1,600 products which are identified by the manufacturer as containing nanoparticles – particles between one and 100 nanometres (between one and 100 billionths of a metre) across. So let's take a look at what's inside your household items. Last month in this series on nanotech in household items, we looked at clothing. This month: sunscreen.

There are two types of nanoparticles already being added to sunscreen; titanium dioxide (TiO2) and zinc oxide (ZnO).

Why use them?

Bulkier particles of zinc oxide and titanium dioxide have been used in sunscreens for decades to reflect or absorb cancer-causing ultraviolet light. The reason traditional sunscreens look white when you rub them onto your skin is because particles of this size reflect visible light. But when these sunscreen ingredients are manufactured into nanoparticles – usually 25 to 50 nanometres wide – they behave differently.

Despite clumping together when mixed into sunscreen, nanoparticles of titanium dioxide and zinc oxide not only retain their highly effective UV light-absorbing capacity, but also absorb and scatter visible light, rendering them transparent on the skin. And in comparison to other UV filters, they are more stable – requiring less reapplication – and are low irritant and low allergen materials.

"Another advantage is that, at the nanoscale, ZnO and TiO2 feel 'lighter' on the skin," says Megan Osmond-McLeod, researcher at Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO), "rather than heavy and cakey."

So in the fight against skin cancer, nanoparticle formulations are onto a winner. You're more likely to slather yourself in it, better protecting your skin from sun damage.

Are they safe?

The potential for metal oxide nanoparticles in sunscreens to cause harm primarily depends upon the ability of these objects to penetrate the skin. The current weight of evidence suggests that such nanoparticles do not do this. "There's a negligible penetration of sunscreen nanoparticles," says Paul Wright, toxicology expert at RMIT University. "They don't get past the outermost dead layer of human skin cells, of which millions are shed each day."

Simon James, research fellow at the Australian Synchotron, recently showed that immune cells collect and break down zinc oxide nanoparticles. "Our study demonstrates that the human immune system has the right 'equipment' to remove any nanoparticles that somehow make it through the skin – assuming some do at all," he explains. James is aware that their work so far only looks at cells in the lab, and 'raw' nanoparticles rather than a sunscreen formula. "These issues are important when considering the bigger picture," says James. "More work is needed before we understand the details of how this process occurs inside the body, but we at least now know the immune system has the capacity to degrade zinc oxide nanoparticles."

Osmond-McLeod and researchers at CSIRO are doing similar work, looking at biological responses to metal oxides in both the short- and long-term. "It is always important to emphasise the crucial point that we know the negative effects of prolonged exposure to the sun without some form of protection," stresses Osmond-McLeod. "And that must be factored into the argument."

Wright reiterates that sunscreens are safe to use, although looking at whether there is likely to be any difference in skin penetration for individuals with skin diseases or naturally thinner skin would be worthwhile. "Even so," he says, "the minimal amount of zinc that may end up in the body is still a negligible fraction of its normal levels in the blood and tissues of a healthy person."