Mercury is a metallic element that occurs naturally in the environment. Mercury can take several forms but is most often recognized as a shiny, silver-white, dense liquid. Metallic (liquid) mercury has many non-cosmetic uses, including in thermometers, electrical switches, dental amalgams and some industrial manufacturing. In addition to metallic mercury, the element can exist in combination with other elements to form compounds. Mercury compounds are the most common form that exists naturally in the environment. The use of such compounds in cosmetics products has been strictly regulated by the U.S. Food & Drug Administration (FDA) since 1974. 21CFR700.13
Mercury is generally found at low levels throughout the environment in rocks, sediments, water and soils. Scientists have also found some mineral occurrences and thermal springs that are naturally high in mercury. Most human exposures to mercury occur from low levels found in the environment. We are routinely exposed via unavoidable natural occurrences of mercury compounds in the food we eat and, to a lesser degree, the air we breathe and water we drink.
The toxicity of mercury compounds is extensively documented in scientific literature. Some mercury compounds can be absorbed through the skin on topical application and accumulate in the body. Depending on the form of mercury, exposure to sufficiently high concentrations can result in allergic reactions, skin irritation or neurotoxicity.
Due to the known hazards of mercury, its questionable efficacy as a skin-bleaching agent, and the availability of effective and less toxic non-mercury preservatives, FDA determined there is no justification for the use of mercury in skin-bleaching (also known as skin-lightening) preparations or as a preservative in cosmetics. According to the 1974 FDA regulation (21 CFR 700.13), any cosmetic product that contains more than unavoidable traces of mercury (defined as no more than 1 part per million [1ppm = 0.0001 percent]) is in violation of the Federal Food, Drug and Cosmetic (FD&C) Act and is subject to legal action. The only exception was eye-area cosmetics for which no other effective and safe non-mercury preservatives were available. Historically, the continued use of mercurial preservatives in such eye-area cosmetics was considered warranted because mercury compounds are exceptionally effective in preventing Pseudomonas bacterial contamination of cosmetics. Pseudomonas infection of the eye can cause serious injury, including blindness.
Since these regulations were implemented in 1974, however, suitable alternative non-mercurial preservatives have become available and the use of mercury in eye-area cosmetics has all but ceased.
Several decades ago, mercury-containing products were popular for lightening or bleaching the skin. The same regulation that banned most uses of mercury in cosmetics and personal care products banned this use of mercury. Currently, FDA regulates all skin-bleaching ingredients as Over-The-Counter (OTC) drugs and requires the products containing them be marketed be shown to be safe and effective according to the applicable OTC drug monograph. In fact, FDA has taken regulatory action against mercury-containing skin lightening products that sometimes illegally enter the United States from other countries. For example, here is a 2019 FDA press release about a mercury-containing skin cream.