"Of the few studies carried out, one has shown that long–term exposure to musk xylene causes cancer in mice." Read more about Synthetic Musk Fragrances (pdf).
"Though there is no available research on a link between musk ketone and cancer, animal studies indicate that a related nitromusk, musk xylene, may have carcinogenic properties (Maekawa 1990; Apostolidis 2002" Human Toxome Project, www.bodyburden.org
"Musk Ketone: A synthetic compound with a typical musk odor is widely used in cosmetics and is permitted as a food additive. Exposure to it, experiments in animals and with human cells indicate it might increase the susceptibility to health hazards caused by cancer causing agents in humans." You May Attract More Than the Opposite Sex With Musk by Ruth Winter, MS, Ingredient Blog