Manufacturers have been adding the germ fighter triclosan to soaps, hand washes, and a range of other products for years. But here’s a dirty little secret: Once it washes down the drain, that triclosan can spawn dioxins.

Dioxins come in 75 different flavors, distinguished by how many chlorine atoms dangle from each and where those atoms have attached (their locations indicated by the numbers in the front part of a dioxin’s name). The most toxic is 2,3,7,8-tetrachlorodibenzo-p-dioxin, or TCDD. Some related kin bearing four to eight chlorines are also toxic, just less so.

Triclosan’s dioxin progeny belong to this infamous family, but aren’t the ones that have typically tainted the environment. And, before you ask: No one knows how toxic triclosan’s dioxins are. Few investigations have been conducted because chemists considered them arcane and too rare to pose a threat.

Patented in 1964, triclosan quickly found use in medical supplies. By 1987, manufacturers were adding it to liquid hand soaps for the consumer market. Within a little more than a dozen years, three-quarters of all such liquid hand soaps would contain the chemical. And as these soaps were used, triclosan washed down residential drains along with chlorinated tap water, forming super-chlorinated triclosan.

In wastewater treatment plants, the bonus chlorine atom or two that tap water had added to the molecule tends to be stripped off, notes William Arnold, an environmental engineer at the University of Minnesota in Minneapolis. But in the finishing stage at those treatment plants, most water gets one last chlorine-disinfection step, which “will re-chlorinate the triclosan,” he says, before the water is released out into rivers.

Arnold’s group and others have demonstrated in the lab that that in the presence of sunlight, the super-chlorinated triclosan can undergo transformations that beget a series of dioxins. They include 2,8-dichlorodibenzo-p-dioxin, 2,3,7- and 1,2,8-trichlorodibenzo-p-dioxin, and 1,2,3,8-tetrachlorodibenzo-p-dioxin.

The genesis of these compounds isn’t just some laboratory curiosity. Triclosan’s odd dioxins also develop in the environment – big time, Arnold’s group reported May 18 online, ahead of print, in Environmental Science & Technology.