It is very important to choose right sunscreen. The best are mineral (not chemical) sunscreens. They have active ingredients like titanium dioxide and/or zinc oxide. Getting to the truth about the use of nanoparticles in cosmetics, like many issues in the cosmetics industry- with its confusing screen of distorted information- isn't easy. But getting it right is important, particularly because the sunscreen ingredients titanium dioxide and zinc oxide, so essential to the health of your skin, are involved.
Nanotechnology is about changing any material from its original size and making it much, much smaller. This technology is used in a wide variety of industries, from medicine to agriculture to cosmetics. In the case of cosmetic products and over-the-counter drugs such as sunscreens, making particles nano-sized has two chief advantages: it can make the product more aesthetically pleasing (this is often the case with mineral sunscreens- making the particles of the active mineral smaller allows them to be applied without leaving a noticeable white cast), and it can enhance penetration of certain ingredients, such as vitamins and other antioxidants.
You may have seen concerns expressed in the media, online, and by certain lobbying groups about the use of nanoparticles in cosmetic products, both in general and in particular, as when it comes to using nanoparticles of titanium dioxide and zinc oxide as the active ingredients in sunscreens. What's been reported about these benign sunscreen ingredients often sounds scary, with some reports going so far as to state that nanoparticles of these sunscreen actives reach the bloodstream and are potentially dangerous. Some articles about sunscreen nanoparticles have even stated these can interact with sunlight and cause cellular damage to skin. As alarming as this sounds, these assertions are not supported by any published information and they are without support from the medical world or the FDA.
Reviews of scientific data by major regulatory agencies have concluded that nanoparticles of titanium dioxide and zinc oxide remain on the surface of the skin and in the outer dead layer (stratum corneum) of skin. They do are not absorbed into the bloodstream and do not affect living skin cells. Studies coming to these conclusions have tested these nanoparticles on healthy, intact human skin and on various types of human and animal skin samples.
Based on these conclusions and those of other studies, the question of nanoparticle risk from the mineral sunscreen actives is not a human health issue. There is no proof that these sunscreen actives absorb into skin. And in fact, regardless of any potential risk, you would never want that to happen since sunscreen actives need to remain in the surface layers of skin in order to protect it from UV damage.
Further, in terms of the potential risk that titanium dioxide could generate free radicals in the presence of sunlight, it has been shown that adding antioxidants to the mix (whether they're in your sunscreen or already naturally present in skin) eliminates this risk, and other research has established that both zinc oxide and titanium oxide are stable substances that don't elicit free-radical damage at all.
Avoid using your dirty hands to wash your face. While it's pretty much common sense, we sometimes forget and may launch straight into washing our face without thinking about the state of our hands. As a thumb of rule, wash your hands with soap prior to cleansing your face. 
