Hi all,
I have followed Specktra discussions as a guest and then as a non-commenting member for a few years now, but I find the sudden urge to join this discussion now.
For the safety of your health, please favour using products containing titanium dioxide particles that are large enough to cause light dispersion (flashback). This is because when it doesn't cause a flashblack, it means that the particles are nano-sized. I am not sure if general consumers are already aware of it, but the cosmetic industry which so eagerly jumped on the nano train a few years ago is now slowly shying away from nano anything because nanoparticles are extremely toxic. Their small size allows them to penetrate skin and blood vessel barriers and accumulate in certain vital organs, which academic studies show leads to cancer in animal subjects. There are other on-going studies at universities across the world on the long-term effects of nanotoxicity. Legislation governing the use of nanoparticles is slowly taking shape, but too slowly, unfortunately, because scientific bodies and industries have been unable to properly define and agree on what nanoparticles are, and which ones should be regulated. I've spent the last few years studying nanoparticles and their toxicity for my Masters course in Materials Engineering and I was personally shocked to learn that companies were putting Titanium Dioxide nanoparticles in our sunblock so that they spread more "invisibly", but due to the related health dangers have stopped using the word "nano" on their products. Neither do they specify the size of their particles so the only way to know, as Erine mentioned, is by trial and error. Therefore, for your own safety, it is really better to have a sunscreen/ sunblock that blends less easily (larger particles) than one that completely disappears.
I hope that my post has been of help.