Sunscreen Today, Dinner Tomorrow?

(NHI Nanoblog) Super-small zinc oxide is in sunscreen, boat paint and other products that end up in rivers, oceans and the public water supply. What do these minuscule particles mean for marine life — and what might that tell us about effects up the food chain?

Shannon Hanna, a graduate student at the University of California, Santa Barbara, is looking at a tiny but important link in that chain for clues. He’s examining Mediterranean mussels, and found that exposure to nanoscale zinc oxide negatively affects both growth and survival. He also found that zinc accumulates in the mussels, raising questions about the larger impact on animals (including humans) that eat the bivalves.

Hanna (pictured) shared his early results at the Third Annual Meeting of the Society for the Study of Nanoscience and Emerging Technologies, held earlier this month in Tempe, Ariz. (Since the work is still in progress, he asked that the specific data not be included in this article.)

Ultra-tiny zinc oxide is one of many materials that are being used in new ways as engineers learn to exploit the useful properties of shrinking substances to the nanoscale (a nanometer is a billionth of a meter). Nano-sized zinc oxide is used as a transparent physical sunscreen, added to paints as both a white pigment and an antimicrobial (making boat paint resist algae, for example) and whipped into lipsticks.

There is a heated debate over whether topical applications of nano-zinc oxide and titanium dioxide pose a threat to humans. There’s no question that they do wash off human skin, however, which is where environmental researchers like Hanna come in. He’s working with faculty scientists at the University of California Center for Environmental Implications of Nanotechnology, and UCSB’s Center for Nanotechnology in Society. 

The experiment was simple, Hanna said: Put mussels in a tank, add water, then add zinc oxide nanoparticles. Then, repeat, at increasingly higher doses, for 12 weeks. The mussels did worse over time, at the higher doses, Hanna said.

A key question, he said, is whether the zinc found in the mussels’ tissues are nanoparticles, or something larger. That gets to the heart of one of the big questions in nanotechnology safety research: Does size matter, or is the baseline chemical composition more important?

“It’s really tough to tell you what the environmental implications of zinc nanoparticles are,” Hanna said.

Nano-sized or not, Hanna’s work is the latest indicator that new applications carry new risks. Through accumulation in various organisms that feed on one another, the impact to the larger environment is magnified (this is why mercury levels in big fish like tuna and swordfish are high). For Hanna, who jokingly acknowledges that he hasn’t eaten a mussel since he started working with them, the next step is scrutinizing just how big that magnified effect is.