According to the Environmental Working Group: "We found that consumers using sunscreens without zinc and titanium would be exposed to an average of 20% more UVA radiation — with increased risks for UVA-induced skin damage, premature aging, wrinkling, and UV-induced immune system damage — than consumers using zinc- and titanium-based products"
The common complaint by users of sunscreen products containing zinc has been regarding the white coating it leaves on the skin. This is where nanoparticles come in - particles that are smaller than 100 nanometers (or "nm" — a nanometer is roughly half the size of a strand of DNA). Zinc is shrunk into tiny particles measuring under 100nm, providing better protection and eliminating the white cast.
Nanoparticles have come under close scrutiny by some watchdog groups concerned over the possible health effects of nanoparticles entering the deep tissues, blood or lungs. Research on the subject to date has had varied outcomes. The level of nano-scale zinc found in the body has ranged from zero to little, or greater amounts, dependant on how it is introduced to the system and its size. In their most recent sunscreen guide, the Environmental Working Group (EWG) writes:
In 15 peer-reviewed studies, nanosize zinc and titanium were shown not to penetrate through unbroken skin at concentrations exceeding 1.5%. A recent review for the EU decision-making body found that, "There is currently little evidence from skin penetration studies that dermal applications of metal oxide nanoparticles used in sunscreens Lead to systemic exposure" (Börm 2006).
The majority of sunscreens available for purchase in the US include nanoscale zinc, typically 30 to 200 nm in size. And because labeling laws are unclear where nanoparticles are concerned, it can be difficult to know if you are purchasing a product that contains nanoparticle ingredients or not. The FDA has not yet set standards for nanoparticle claims or defined the minimum size of a nanoparticle. This has caused some individuals and groups to recommend consumers avoid zinc and titanium-based sunscreens altogether.
But this recommendation is not necessarily the safest option. A study by the EWG shows that those using chemical sunscreens are exposed to both greater UV radiation and more hazardous ingredients. Micronized zinc is shown to provide safer protection than either chemical sunscreens or unprotected sun exposure.