Last week, the treaty attempted to control the amount of potent neurotoxins acceptable in many items. Some lightbulbs, batteries, and thermometers will no longer be available, due to mercury levels. But, the treaty exempted soaps and makeups, which sometimes use the silvery chemical as a preservative.
In an interview with Scientific American, Stacy Malkan, cofounder of the advocacy group Campaign for Safe Cosmetics, said there's no reason that "a known neurotoxin should be allowed in any of these products." Many U.S. companies have developed alternative, non-mercury-based preservatives. But, since manufacturers aren't required to list any ingredient that is present as less than 1% of the product — and mercury is often present in very small amounts — it might not be possible to judge whether your particular product is free of it.
No tests have been done to determine the consequence of mercury in mascara, but the toxin is readily absorbed through the skin. At higher levels, it can cause neurological effects and kidney damage. When pregnant women are exposed at even very small levels, it can affect the brain of a developing fetus.
Your other options? Try to find a mascara that has a shorter shelf life. These are less likely to contain harmful preservatives. (Scientific American)