SCIENCE OBSERVER
Persistently Clean?
Christopher Brodie
More than a million pounds of antimicrobial chemicals from soap and
other products flow into the nation's sewers every year. Do these
compounds pose a risk? Product manufacturers say no, pointing to
data that show only traces of the two most common antibacterials,
triclosan and triclocarban, in treated wastewater. What happens to
the remainder is less certain. The stock explanation has been that
the majority is broken down during the treatment process. The
fraction released into surface water was thought to meet the same
fate sooner or later. Thus, much of the claim that these products
are safe rested on the fact that they were rendered harmless in
treatment plants or just beyond.
New data puncture that conclusion: 50 percent of triclosan and 76
percent of triclocarban remain unchanged by aerobic and anaerobic
digestion in a typical wastewater facility, according to a pair of
recent reports. This large intact fraction isn't going out with the
treated water—the old estimates are correct in that respect.
Rather, it is trapped in the sludge at the bottom of the treatment
tanks. Most of that sludge gets spread on the ground to fertilize
pasture, forests and human food crops.
Rolf U. Halden, a scientist and engineer at Johns Hopkins
University, and his coworkers are the chemical detectives behind
this work, which appeared in the June 1, 2006 issue of
Environmental Science & Technology and in
Chemosphere, published online June 9, 2006. By comparing
the amounts that entered a wastewater plant with the amounts that
exited or were broken down, the Hopkins team pieced together a much
more complete picture of the life cycle of these compounds in the environment.
Triclosan and triclocarban are small organic molecules that give
antimicrobial properties to personal-care products such as soap,
deodorant and toothpaste as well as durable goods such as cutting
boards, baby carriers and socks. Overall, Halden's team estimates
that more than 100,000 pounds of triclosan and over 300,000 pounds
of triclocarban are spread on the ground as sludge each year in the
United States, based on data from a dozen sites around the country.
Of the total mass that enters a typical sewage-treatment facility,
two percent of triclosan and three percent of triclocarban remain in
the clean-water output. Thus, only 48 percent of triclosan and 21
percent of triclocarban are transformed or lost in the treatment
process—much less than industry estimates. At 50 and 76
percent, respectively, sludge is the biggest repository.
According to a 2002 report by the National Research Council, 63
percent of the 5.6 million tons of dried sludge made in the United
States each year is applied to the land. (When used as fertilizer,
municipal sludge goes by the more polite name of
biosolids.) This recycling is viewed as being good for the
environment, because the alternatives are incineration, burial or
(before 1992) offshore dumping. But combined with the numbers from
the National Research Council, Halden's analysis indicates that
hundreds of thousands of pounds of triclosan and triclocarban are
spread on the ground every year. Remarkably, this massive
contamination is unregulated and unmonitored. The ecological effects
are similarly unexplored.
In fairness, the reason that no one noticed the organic compounds
(such as triclocarban) in sludge is that the technique used to
measure such things wasn't up to the job. Sludge is a complex matrix
that adheres to and masks molecules that are strongly hydrophobic,
as are triclosan and triclocarban—so much so that this
municipal gunk acted as a "chemical black hole," according
to Halden. "It used to be you could dump [manmade] chemicals in
the sludge and they'd disappear," he explains, referring to how
they became invisible to detection. But in 2004, Halden's group
described a method that allowed chemists to peer inside the sludge,
something he describes as "one of the last frontiers in
analytical chemistry."
What they saw was an accumulation of triclosan in the sludge up to
concentrations of 30,000 micrograms per kilogram—more than
6,000 times more concentrated than the incoming sewage. The numbers
were even higher for triclocarban: 51,000 micrograms per kilogram in
the sludge, which worked out to 8,400 times the concentration of
sewage and 300,000 times that of the sewage treatment plant's outflow.
The biosolids industry is regulated by the U.S. Environmental
Protection Agency, which dictates the conditions under which the
substances can be used. But in terms of sludge composition, the EPA
only set limits for metals and certain pathogenic bacteria. There is
no oversight of organic chemicals and no categorical prohibition of
the use of biosolids on food crops. Current rules do govern the
types of food that can be grown with biosolids fertilizer, the
amount of time between application and harvest, and other practical
details. But the EPA's official stance is that the practice of
growing food in dewatered municipal sludge is acceptable.
And perhaps it is. Manufacturers consider triclosan and triclocarban
to be safe—even healthy, to judge by the tone of their
advertisements. Some bar soaps, for example, are five percent
triclocarban by weight. With the exceptions of triclocarban causing
outbreaks of "blue-baby syndrome" in the 1960s and
‘70s (pediatricians still advise against exposing newborns to
triclocarban) and the trace amounts of dioxin, a known carcinogen,
that tag along with triclosan, the compounds have a clean safety
record in people. The problem, as any toxicologist will tell you, is
that the dose makes the poison.
The estimated annual production of triclocarban exceeds one million
pounds. From this massive starting amount, consider that triclosan
and triclocarban resist degradation (Halden estimates their
half-life in sediments to be 540 days) and that their chemical
structure suggests that they build up in fat, and it's easy to see
the potential for accumulation in the food chain.
Chemical stability by itself is hardly damning, and neither compound
has killed anyone. But direct effects on hand-washers and
tooth-brushers are not the only relevant outcomes. For example,
triclosan disrupts the functions of the endocrine system in cultured
cells. Furthermore, the risk of fueling the evolution of
antibiotic-resistant bacteria—as yet unproven—remains
plausible. Halden's concern is that in sludge, the combination of
concentrated microorganisms, some of them capable of causing
disease, with extremely high concentrations of antimicrobials is a
recipe for drug resistance.
In the end the decision of what to do about sludge will be up to the
risk managers. The only thing environmental scientists like Halden can
do is to show their data as best they can with the resources they have.
Fortunately, the EPA seems to be listening. Their 2001 survey of
biosolids examined only metals and dioxins, but the 2006 survey (as yet
unreleased) will be more comprehensive, checking for more metals, a
short list of organics and, at least in some samples, triclocarban
specifically. Getting rid of the things that get rid of microbes may
turn out, paradoxically, to be the healthy thing to do.
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