Although barium sulfate is almost completely inert, zinc sulfide degrades upon exposure to UV light, leading to darkening of the pigment. The severity of this UV reaction is dependent on a combination of two factors; how much zinc sulfide makes up the pigments formulation, and its total accumulated UV exposure. Depending on these factors the pigment itself can vary in shade over time, ranging from pure white all the way to grey or even black. To suppress this effect, a dopant may be used, such as a small amount of cobalt salts, which would be added to the formulation. This process creates cobalt-doped zinc sulfide. The cobalt salts help to stabilize zinc sulfide so it will not have as severe a reaction to UV exposure.
Some small test-tube research has shown that these nanoparticles are absorbed by intestinal cells and may lead to oxidative stress and cancer growth. However, other research has found limited to no effects (13Trusted Source, 14Trusted Source, 15Trusted Source).
When E171 isn’t combined with other ingredients and administered in water, some studies suggest that under these artificial conditions, E171 may be processed differently in the body resulting in some biological changes in experimental animals that are poorly understood.