Heavy metals are metals of relatively high density or of high relative atomic weight
Heavy Metals are mostly used in dyes/pigments. They have been linked to skin irritation, abdominal cramps/nausea, organ poisoning + cancer. Neurotoxicity caused by heavy metals have been linked to problems with learning/language development and behavioral issues. Reduced fertility in both men and women has been reported. Hormonal and menstrual irregularities fall under this category of dysfunction as well. Delayed onset of puberty in girls and development of testes in boys is tied to cumulative/long-term exposure to heavy metals. Environmental persistence and poisoning of the food chain is also an ongoing problem.
Both Lead and Mercury are banned in Canada. The US FDA has banned the use of Lead Acetate in cosmetic products intended for hair color additives. The U.S. state of California introduced AB 495, the “Toxic-Free Cosmetics Act” in 2019. It aims to ban asbestos, lead, formaldehyde, and 17 other toxic chemicals in cosmetics. The bill is still in process and would be the first of its kind in the United States if passed. 
 The Government of Canada. Cosmetic “Hotlist” prohibited ingredients for cosmetics. (2019). https://www.canada.ca/en/health-canada/services/consumer-product-safety/cosmetics/cosmetic-ingredient-hotlist-prohibited-restricted-ingredients/hotlist.html#tbl1
 CDC. Department of Health and Human Services. Toxicological profile for Lead. Updated (2019). https://www.atsdr.cdc.gov/toxprofiles/tp13.pdf
 EPA. Minamata Convention on Mercury. (2013). https://www.epa.gov/international-cooperation/minamata-convention-mercury.
 CDC. Department of Health and Human Services. Toxicological profile for Mercury. (1999). https://www.atsdr.cdc.gov/toxprofiles/tp46.pdf
. California bill AB 495, the “Toxic-Free Cosmetics Act” (2019). https://leginfo.legislature.ca.gov/faces/billTextClient.xhtml?bill_id=201920200AB495
 NRDC. Mercury Guide. (2016). https://www.nrdc.org/stories/mercury-guide