A new source of dioxins, the antimicrobial soap additive Triclosan

There are 75 variations of dioxins.  They differ by the number and location of the chlorine atoms in the relevant molecule (the location of the chlorine atoms is indicated by the numbers in the front part of a dioxin’s name).  The most toxic dioxin is 2,3,7,8-tetrachlorodibenzo-p-dioxin (“TCDD”).  The toxic forms of dioxin generally have four to eight chlorines, though each is less toxic than TCDD.

Triclosan, an antimicrobial, was patented in 1964 and used in many medical supplies.  By 1987 soap manufacturers started to use it as an additive.  When such soaps are used, triclosan is washed into the sewer lines with chlorinated tap water; in turn, this combination forms super-chlorinated triclosan.  In wastewater treatment plants, the additional chlorine atom or two that tap water adds to the molecule tends to be removed.  However, in the finishing stage at waste treatment plants, most wastewater receives a final chlorine-disinfection step, which re-chlorinates the Triclosan just before the wastewater is released into the environment (e.g., rivers).

Researchers have now demonstrated demonstrated in the labatory that, in the presence of sunlight, the super-chlorinated triclosan can undergo transformations that create a series of dioxins.  These new products include 2,8-dichlorodibenzo-p-dioxin, 2,3,7- and 1,2,8-trichlorodibenzo-p-dioxin, and 1,2,3,8-tetrachlorodibenzo-p-dioxin.  It is unknown how toxic these dioxins are.  Few assessments of them have been conducted because they have been considered “arcane” and too rare to pose a threat.

Researchers then collected roughly 80-centimeter-deep cores of sediment from a lake downstream of several wastewater treatment plants serving Minneapolis and St. Paul.  TCDD and its related kin were found at the bottom of the core samples reflecting deposition from the 1940’s, but there were none of the unusual dioxins associated with the phototransformation of triclosan.  As the researchers assayed these cores from the bottom up, concentrations of the conventional dioxins gradually diminished and quantities of triclosan’s dioxins increased.  In the sediment’s topmost strata, triclosan-related byproducts were found to account for roughly one-third of all dioxins present.  The researchers thus conclude that, at least in the aquatic world, these compounds represent a previously unrecognized and increasingly important source of di-, tri-, and tetrachlorinated dibenzo-p-dioxins.

This study can be found at http://pubs.acs.org/doi/abs/10.1021/es1001105 and http://pubs.acs.org/cen/news/88/i21/8821news8.html.

In another study of Triclosan, it was found that it has weakly estrogenic effects in the environment and acts thus as an endocrine disruptor.  This study can be found at http://www3.interscience.wiley.com/journal/123280978/abstract?CRETRY=1&SRETRY=0.