However, the safety picture of mineral sunscreens is greatly skewed when nanoparticles enter the picture for two reasons, toxicity and free radical generation. 

  Toxicity: concerns about the overall safety picture of nanoparticles used in cosmetics have been raised first by scientists outside the industry. Dr Charles Vyvyan Howard, head of research with the developmental toxico-pathology research group at Liverpool University, says the tiny particles have a toxicity that seems to be directly related to their size, which can cause inflammation. He also points out that studies have shown that fine particles of the size now being used in sunscreen have gone through the skin and ended up in the lymphatic system. 

  The evidence is accumulating that nanoparticles may be more dangerous than sunscreen manufacturers would have us believe. The argument that nanoparticles do not pass the epidermal barrier has been refuted in at least two instances: one, when the epidermis is damaged (by sunburn or other trauma), and two, particles in the 250 nm size range (considered large in the nano world) have been shown to cross the placental barrier. (Environmental Health Perspectives 118(3): 432-436) 

  Free radicals: while we wait for more studies related to nanoparticle risks to be done, we do know that “Both nano-size zinc oxide and titanium dioxide, including forms extracted from sunscreen, react strongly with UV light (Dunford 1997) and may damage skin cells (Sharma 2009).” (EWG Report) An interesting study (Progress in Organic Coatings62: 313:320. 2008] by Phil Barker and Amos Branch) looked at the interaction of sunscreens with roof surface coatings after it was observed that freshly treated roofs were degrading very rapidly in places where the surface coating had come into contact with sunscreen worn by the roofers. 

“Out of ten sunscreens tested—four containing no nanoparticles, five containing titanium dioxide nanoparticles, and one containing zinc oxide nanoparticles—all but one of the nanoparticle-based sunscreens consistently degraded samples of pre-painted roofing surface exposed to sunlight for 12 weeks. In contrast, the non-nano products had no obvious deleterious effect. In the worst case, the roofing lost over 85% of its gloss (a measure of degradation) in just six weeks.” 

Even if existing studies do not convince us that nanoparticles pose a danger, we do know that they can contribute to accelerated aging, which is the primary reason most of us are using sunscreen to begin with. The answer? The study quoted above did establish that the non-nano products did not degrade roofs—yet another indication that non-chemical AND non-nano inorganic sunscreens remain your best bet for safe, effective and anti-aging sun protection.