By Edward Tabor, MD, WDA Board Member

The main source of copper for humans is food, and Wilson disease patients usually limit the amount of high-copper food they eat. However, copper is also found in many non-food items that touch our skin. Can copper be absorbed through the skin? Do Wilson disease patients need to be concerned about this?

Sources of copper other than food

Copper can be found in jewelry, eyeglass frames, coins, and clothing. Often the copper is in the form of an alloy, which is a mixture of two or more metals. For instance, 14 karat gold is an alloy with 58.3% gold along with copper and other metals. In one study, 13 samples of gold jewelry were tested and all were found to contain copper, with or without additional metals. Some eyeglass frames are made of copper alloys. All of our currently minted U.S. coins contain copper. The penny is made of zinc covered with a thin layer of copper; the nickel is entirely made of an alloy of copper and nickel; the dime and quarter are made of layers of a silver-colored nickel-copper alloy surrounding a core of pure copper; dollar coins are a mixture of copper and several other metals.

Many items of clothing, such as socks, t-shirts, and nightwear, are sometimes embedded with copper because copper can kill bacteria that cause odors. (However, the sock company Gold Toe says that they stopped adding copper to their socks in 2019.) Some medical dressings are made with copper in them to kill bacteria. Some cosmetics and “personal care products” contain copper. (However, copper is not found in sunscreens; the word “copper” in one well-known brand refers to the depth of the suntan, not the ingredients.) A few violin strings contain copper, although most are now made of other metals. Copper bracelets have been used for many years as folk remedies for arthritis.

How much copper can be absorbed through skin?

It is not known precisely how much copper from these sources can move through intact healthy skin to enter the bloodstream. The skin is a fairly good barrier to chemicals if it is not injured by cuts, burns, or a skin disease. The ability of copper to penetrate healthy skin may be greater if it combines chemically with sweat, compared to absorption under dry conditions.

Only a few studies have measured serum copper levels in humans after skin exposure. In one study, 19 normal adults received up to 3.1 mg of elemental copper in ointment applied to skin every day for four weeks; the mean serum copper level increased from 17 µmol/L to 24 µmol/L. Although the mean level was elevated, it remained within the normal range of 10-25 µmol/L, except in some women taking oral contraceptives in whom absorption was slightly higher. These volunteers were exposed to a relatively large amount of copper on their skin. The amount was greater than the average daily intake of copper from food in the US, which is 1.4 mg per day for men and 1.1 mg per day for women. However, they clearly did not absorb a large amount of copper, since the serum levels remained within the normal range (except for women taking oral contraceptives). Also, this experiment does not necessarily mimic skin absorption of copper from dry items in “real world” conditions.

It is important to distinguish between skin exposure to copper and skin absorption of copper. One report from the European Food Safety Authority (EFSA) stated that copper exposure through “the dermal route (from face cream, copper jewelry, handling of coins and hair products) [is] up to 0.38 mg/day [median exposure] and [in the worst case] up to 2.13 mg/day.” However, the amount of skin absorption was not known, because good studies of the skin absorption rate have never been reported in humans. Some studies of human or animal skin in laboratory dishes (“diffusion chambers”) have suggested that 0.3% of copper in solution or suspension and 0.03% of dry copper would be absorbed through skin. Thus, theoretically, the amount absorbed from intact healthy skin could be 0.6 µg/day through dry skin and 6 µg/day through wet skin, in comparison to up to 700 µg/day from food.

An important related issue is whether copper can be absorbed from copper intrauterine contraceptive devices (IUDs). Absorption of most chemicals through the mucous membranes, such as those surfaces exposed to IUDs, is much greater than absorption through skin. According to the EFSA, serum copper levels in women using IUDs have been found to be higher than in non-users in some studies.

Even if copper can be absorbed through the skin, the amount would be very small compared to the amount absorbed from food; about 50% of dietary copper is absorbed into the body and bloodstream from the average diet (but only 2-7% of the copper in food would be absorbed by a person whose diet contained very high amounts of copper). Thus, while skin exposure to copper in our current world may be significant, the amount of absorption through intact, healthy skin appears to be very low. Wilson disease patients with skin injuries or skin diseases should be cautious about these non-food sources of copper, at least until good scientific studies can be conducted. However, patients with stable, treated Wilson disease and healthy, intact skin probably do not need to be concerned about absorbing copper through the skin.