While writing the earlier entry Do Mineral Makeup Pose Health Risks, I read that there is evidence that uncoated titanium dioxide (TiO2) and zinc oxide (ZnO) can degrade avobenzone so it is best not to layer non-micronized mineral makeup with an avobenzone containing chemical sunscreen.
This is apparently confirmed by this Stability Study of Avobenzone with Inorganic Sunscreens by Uyen Nguyen and David Schlossman.
Titanium dioxide and zinc oxide are preferable UVA / UVB sunscreens because of their efficiency and low irritation. In order to increase the protection in the UVA range, in many countries, avobenzone has been used in combination with titanium dioxide and / or zinc oxide. However, in the USA, combinations of avobenzone and physical sunscreens are not permitted. Avobenzone has been reported to be unstable when contained in formulations with uncoated physical sunscreens. Surface coating of the pigment has sometimes been shown to increase its stability.
Based on this, it would mean that for those of us who are using mineral makeup, you need to be aware of the ingredients not only in your mineral makeup, but your sunblock as well. If you’re using mineral makeup without micronized ingredients (which is considered to be safer for our health), then ensure that you’re using a physical sunblock with titanium dioxide or zinc oxide without avobenzone.