It had already been known that triclosan could be converted to dioxin in the laboratory, and it was also known that sunlight causes triclosan to degrade in the environment. But it had not been known that the natural degradation resulted in dioxin, said researchers Kristopher McNeill, an assistant professor of chemistry, and William Arnold, assistant professor of civil engineering. They discovered that the reaction could occur in Mississippi River water exposed to ultraviolet light.
"This form of dioxin is at least 150,000 times less toxic than the most dangerous form," said McNeill. "But repeated exposure to chlorine, perhaps in water treatment facilities, could chlorinate triclosan. After chlorinated triclosan is discharged from the facility, sunlight could convert it into more toxic dioxins. Such a process could be a source of highly toxic dioxin in the environment."
"This study also shows that the disappearance of a pollutant such as triclosan doesn't necessarily mean an enviromental threat has been removed," said Arnold. "It may just have been converted into another threat."
The researchers began their study after reading numerous environmental studies that reported the presence of pharmaceutical compounds in surface waters around the nation. McNeill and Arnold decided that the logical next step was to examine the natural processes that led to the loss of such materials in the environment. Last year, the U.S. Geological Survey published a widely circulated study of chemicals in surface water, in which triclosan was found in 58 percent of natural waters tested. Its median concentration was 0.14 parts per billion; the maximum was 2.3 ppb. McNeill and Arnold chose to study triclosan because they could tell from its structure that it would likely break down in sunlight.
In their study, McNeill and Arnold added triclosan to river water, shined ultraviolet light on the water, and found that between one percent and 12 percent of the triclosan was converted to dioxin.
"The fact that this conversion can happen in surface layers of rivers may not cause harm by itself, but it suggests that more serious reactions--leading to more toxic forms of dioxin--may also happen when triclosan enters the environment," said Arnold. "We want to determine if this is the case." As a first step in sorting out the relations, if any, between triclosan and more toxic dioxin, McNeill and Arnold plan studies to determine whether they tend to occur together in natural waters.
The researchers said that even low levels of very toxic dioxin are worrisome because dioxin readily accumulates in organisms and becomes more concentrated in tissues as it moves up the food chain.
The study was funded by the U.S. Geological Survey, through the National Institutes of Water Resources. Triclosan is manufactured by Ciba-Geigy.
To access the study, go to www.sciencedirect.com; click on "Journals"; search under "J"; select the Journal of Photochemistry and Photobiology A: Chemistry; and click on "Articles in press." It will be the fourth article.
Kristopher McNeill, Chemistry dept., 612-625-0781
William Arnold, Civil engineering dept., 612-625-8582
Deane Morrison, University News Service, 612-624-2346