Confused about SPF ratings in physical / mineral sunscreens
Hi everyone - this is my first post on this forum - feel lucky to have found you.
One of the things I've noticed lately is that sunscreens' SPF ratings "seem" all over the place by looking just at the active ingredient concentrations. For example:
Titanium Dioxide 4% and Zinc Oxide 6%
rated at 50+ (Avene Face & Body)
Titanium Dioxide 5% and Zinc Oxide 10%
rated 30+ (Blue Lizard Sensitive)
Titanium Dioxide 2% and Zinc Oxide 17%
rated 30 (MDSolarSciences Mineral Creme)
What am I not understanding or taking into consideration? Is it the inactive ingredients that could be making the difference in the SPF rating?
Can we trust SPF ratings? Or should we ignore ratings and always look for high concentrations of Zinc Oxide (how high is "enough") and Titanium Dioxide (how high is "enough"?)
I am referring only to physical sunscreens because those are the only ones I try to use (I like that they scatter the rays rather than letting them through...wish I could find one that does not leave a white cast though...or leave you greasy...)