This article was researched and written by a student at Mount Holyoke College participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.
What are synthetic musks?
Musk is a word used to describe pleasant smelling fragrances found in many personal and home care products. There are two major types of synthetic musks used in today’s products, nitro-musks and polycyclic-musks. The musks discussed in this article are used commonly in consumer products and have been found to persist in the environment (they have been detected in various fish and mollusk species, as well as in humans). The potential toxicity of these compounds in either humans or wildlife is unknown.
Why were synthetic musks developed?
Synthetic musks were developed for scent enhancement of such products as perfumes, detergents, and soaps. They came into favor during the 1950s because of their relatively easy and inexpensive production—as compared to natural musks from oxen and other animals. Today synthetic musks are used in thousands of products which include:
- Bar soap, liquid soap, detergent, perfume, cologne, shampoo, hand and body creams, make up, deodorants, facial cleansers, aerosol sprays, and plug-in deodorizers among many others.
Although not all products use the musks discussed, the majority of these products use some type of synthetic musk.
“Nitro-musk” is the common name for nitrogen-containing musks. There are 2 common nitro-musks:
- Musk xylene (MX) (1-ter-butyl-3,5-dimethyl-2,4,6-trinitorbenzene) which is widely used in bar soaps and detergents; and
- Musk ketone (MK) (4-ter-butyl-2,6-dimethyl-3,5-dinitoracetophenone) which is found most frequently in cosmetics.
Other Musks Include:
- Ambrette (6-tert-butyl-3-mthyl-2,4-dinitroanisole);
- Tibetene ((1-tert-butyl-3-metheyl-2,4-dinitoranisole); and
- Musk moskene (1,1,3,3,5-pentamethyl-4,6-dinitorindane).
The use of musk Ambrette was discontinued after studies demonstrated its tendency to irritate the skin through photosensitization. The musks tibetene and moskene are still used in small quantities for detergents
“Polycyclic-musk” is the common name for a group of synthetic musks whose chemical structure has many rings. The most commonly used musks in production and found in the environment that fall under this category are:
- Galaxolide (HHCB) (1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta-gamma-2-benzopyran), used most commonly in a wide category of products including make up, soap and shampoo; and
- Tonalide (AHTN) (6-Acetyl-1,1,2,4,4,7-hexamethyltetralin), used less commonly but found in many different products.
Potential impact on the environment
How do synthetic musks get into the environment?
Synthetic musks are released into the environment through household grey water and wastewater. Since synthetic musks are used in many home cleaning products as well as in detergents, they are washed down the drain readily. Also, synthetic musks and their metabolites are found in human feces and urine.
Wastewater treatment is designed to clean water and treat it before its discharge to natural water bodies. There is no process, however, for extracting synthetic musks—and many other personal care product and pharmaceutical residues—from the effluent. As a result, the concentration and type of musk found at each plant varies according to the density of the human population in its service area and the density of industries relating to the production or use of synthetic musks. Although evidence suggests that fungal and bacterial breakdown of some musks can occur, musk metabolites are present, as yet, in the water effluent of many wastewater treatment plants.
Synthetic musks' effect on mollusks
Synthetic musks have been measured in various fish and mollusk species. Although there are few studies on the health impacts of musks on these organisms, recent studies demonstrate that they can inhibit efflux transporters of the mollusk Mytilus californianus. Efflux transporters allow xenobiotics to be eliminated from within the cell and they are an important part of an organism’s xenobiotic defense mechanisms.
Synthetic musks and their metabolites are found in human feces and urine. This is most likely due to dermal exposure and absorption through the skin. Because of the predominance of synthetic musks in household as well as personal care products, almost everyone has some measurable concentration in their blood stream.
Further, studies have shown that airborne concentrations of musks are higher in urban areas compared with rural areas. A study done in the Great Lakes and in rural and urban Iowa, measuring concentrations of several different musks in rural, suburban and urban areas, reported a positive relationship between population density and airborne musk concentrations.
Although the overall impact of synthetic musks on the environment and on humans is currently unknown, musks are an active area of international research. Some preliminary studies demonstrate adverse effects on wildlife and suggest the same may be possible in humans.
United States and Canada versus Europe
Western Europe and the United states (US) differ when it comes to regulation of synthetic musks. Following studies indicating bioaccumulation of nitro-musks, Western Europe responded by phasing out nitro-musks through increased restriction of use—replacing them with polycyclic-musks. Studies show that levels of polycyclic-musks are two times higher in European aquatic biota than in US and Canadian aquatic biota. Nitro-musks are found in higher concentrations in United States and Canadian aquatic biota.
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- Kupper, T. Berset, J.D. et al. (2004). Concentrations and specific loads of polycyclic musks in sewage sludge originating from a a monitoring network in Switzerland. Chemosphere 54: 1111-1120.
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