For normal skin care, any copper concentration is too much copper. Why? Because we need copper in minute concentrations, and we are already getting enough from the environment. No reason to add it more, unless perhaps, for healing skin. Don’t use copper when you are using vitamin C (it will oxidize it). Let’s make things easy: just don’t use copper. I am sure there is copper somewhere in your house, that will supply enough to fulfill the amounts of this metal require at microelement amounts.
From all the bad skincare products that came to the market in the last few decades, those containing copper are an example of what is bad with the industry. Because there is no requirement for testing of cosmetics, ingredients that don’t make sense can reach the marketplace. This means you have to use your knowledge.
If you look at the experiments using copper in vitro, it is possible to see that excess copper is toxic for human cells. But this should be self-evident because copper can be toxic. For example, drinking water, by law, can’t exceed 1.3 mg/l copper.
Other examples of stuff that should never have made it to skincare products are idebenone, Nerium oleander extract, silver, benzoyl peroxide, hydroquinone, and more.
It is your skin, know before you buy!