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Policy Analysis
120 Years of Nanosilver History: Implications for Policy Makers
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† EMPA − Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland
‡ HeiQ Materials AG, CH-5330 Bad Zurzach, Switzerland
Environ. Sci. Technol., 2011, 45 (4), pp 1177–1183
DOI: 10.1021/es103316q
Publication Date (Web): January 10, 2011
Copyright © 2011 American Chemical Society
*E-mail: nowack@empa.ch.
Abstract
Nanosilver is one nanomaterial that is currently under a lot of scrutiny. Much of the discussion is based on the assumption that nanosilver is something new that has not been seen until recently and that the advances in nanotechnology opened completely new application areas for silver. However, we show in this analysis that nanosilver in the form of colloidal silver has been used for more than 100 years and has been registered as a biocidal material in the United States since 1954. Fifty-three percent of the EPA-registered biocidal silver products likely contain nanosilver. Most of these nanosilver applications are silver-impregnated water filters, algicides, and antimicrobial additives that do not claim to contain nanoparticles. Many human health standards for silver are based on an analysis of argyria occurrence (discoloration of the skin, a cosmetic condition) from the 1930s and include studies that considered nanosilver materials. The environmental standards on the other hand are based on ionic silver and may need to be re-evaluated based on recent findings that most silver in the environment, regardless of the original silver form, is present in the form of small clusters or nanoparticles. The implications of this analysis for policy of nanosilver is that it would be a mistake for regulators to ignore the accumulated knowledge of our scientific and regulatory heritage in a bid to declare nanosilver materials as new chemicals, with unknown properties and automatically harmful simply on the basis of a change in nomenclature to the term “nano”.
Introduction
The
potential adverse effects of nanoparticles on humans and the environment currently receive a lot of attention both in academia and
with regulators.
(1, 2)
A lot of the discussion is centered on the asserted assumption that
nanoparticles are something fundamentally “new” and thus cannot be
compared to conventional chemicals or bulk materials. Nanosilver is one
of the nanomaterials that is under the most scrutiny today
(3-5)
and its release and effects are studied widely.
(6-9)
Although changes in nomenclature over the decades have created
confusion among scientists and policy makers, it is undeniable that
products containing nanoscale silver particles have been commercially
available for over 100 years and were used in applications as diverse as
pigments, photographics, wound treatment, conductive/antistatic
composites, catalysts, and as a biocide. With this long and diverse
history of use it is clear that an extraordinary amount of research into
the chemistry of nanoscale silver has been conducted over the past 120
years— it should be noted that most research, until very recently, did
not use “nano” nomenclature.
In this analysis we
critically examine with respect to nanosilver three important
assertions often made when discussing risk assessment of silver
nanoparticles:
Antimicrobial Biocides
Antimicrobial biocides are commonly used to prevent the growth of bacteria on surfaces and within materials and are typically added in small
quantities to many applications to make it more difficult for bacteria to grow on the treated object. Biocidal functionality can be achieved by
employing either organic or inorganic active agents. Compounds such as quaternary ammonium and chlorinated phenols are examples of two widely
used organic chemical biocidal agents. Inorganic active agents are generally based on metals such as silver and copper. Silver has found a
growing presence in many applications due to a desire to shift away from organic chemical agents toward additives, which can be used in much
lower concentrations in a wider variety of products including applications such as plastics where high-temperature processing is not
feasible for organic compounds. Examples of applications are bacteriostatic water filters for household use(10) or swimming pool algicides.(11)
To meet the diversity of application types, many different forms of silver compounds have been developed to service this market.
Whereas biocidal action derives from interaction of silver ions with bacteria,
silver additives are differentiated primarily by the way the silver ions are stored in the product. Common silver products range from additives
that store and release discrete silver ions held within a ceramic (e.g., zeolite) or glass matrix, through to products that store silver ions as
silver salts (e.g., silver chloride) or elemental silver (e.g., nanoscale silver metal). Upon contact with moisture, silver ions are
released from the additive and the object treated with the additive. The biocidal potency of a silver additive is therefore directly related to
the potential for releasing silver ions.
(12, 13)
However, in the case of free nanosilver particles the interactions can potentially be more complex, and catalytic reactions on the particle
surface that can change as a function of size and shape of the nanoparticles can complicate the system.
(14)
It should be noted, however, that most commercial applications of nanosilver involve embedding the particles within a matrix such as a
plastic or a coating.
There are two clear “bookends” for illustrating the extremes of the potential for silver ion
release from silver substances: namely silver sulfide (highly insoluble, hence a low potential for silver ion release) and silver
nitrate (completely soluble, maximum potential for silver ion release). The release potential of different silver materials can be distributed
between the silver sulfide and silver nitrate extremes (Figure 1).
Materials that store discrete silver ions in a matrix show a high potential for releasing silver ions, only marginally less than silver
nitrate. Silver salts such as silver chloride show a lower release potential than the ion-based materials and so are positioned further
from silver nitrate. At the other extreme, bulk silver metal (e.g., silver ingot) releases silver ions to a small extent and so has a
potential closer to the silver sulfide extreme. As the size of silver metal is decreased from bulk through to micrometer-sized particles
through to nanosized particles, the potential for releasing silver ions increases because of increasing surface availability per mass of silver
and because both the solubility and dissolution kinetics of silver may vary as a function of size as silver metal size decreases. Therefore the
potential for releasing silver ions increases and so the behavior moves away from the silver sulfide bookend toward silver nitrate. It is
important to note that while the tendency for higher silver ion release improves with smaller silver particle size, the silver salts and
silver-ion materials still show higher potential and antimicrobial activity than the nanosized silver metal materials.
(12)
Figure 1. Silver release and amount of silver required in products for different biocidal silver formulations.
History of Nanosilver Production and Use
One of the standard definitions of nanotechnology encompasses “research and technology development
at the atomic, molecular, or macromolecular levels using a length scale of approximately 1−100 nm in any dimension;
the creation and use of structures, devices, and systems that have novel
properties and functions because of their small size, and the ability to control or manipulate matter on an atomic scale.”
(15)
The unintentional formation of nanoparticles thus would not fall under this definition of an engineered nanoparticle. It is estimated that
today about 320 tons/year of nanosilver are produced and used worldwide
(16)
(data on its historical production are not available). Now what about the first report of nanosilver? Over 120 years ago, in 1889, M. C. Lea
reported the synthesis of a citrate-stabilized silver colloid.
(17)
The average diameter for the particles obtained by this method is between 7 and 9 nm.
(18)
Their size in the nanoscale and the stabilization by citrate are identical to recent reports about nanosilver formation using silver
nitrate and citrate, e.g., refs
19 and 20.
Also the stabilization of nanosilver using proteins has been described as early as 1902.
(21)
Under the name “Collargol” such a kind of nanosilver has been manufactured commercially since 1897 and has been used for medical
applications.
(22)
Collargol has a mean particle size of 10 nm
(23)
and as early as 1907 its diameter was determined to be in the nanorange.
(24)
Other nanosilver preparations were also invented in the next decades, for example the gelatin stabilized silver nanoparticles patented by
Moudry in 1953 with 2−20 nm diameter
(25)
and silver nanoparticle impregnated carbon with a diameter of silver particles below 25 nm.
(26)
It is important to note that the inventors of nanosilver formulations understood decades ago that the viability of the technology required
nanoscale silver, e.g., by the following statement from a patent: “for proper efficiency, the silver must be dispersed as particles of
colloidal size less than 250 Å [less than 25 nm] in crystallite size”.
(26)
Whereas it is true for many other engineered nanomaterials that they
are novel, e.g., for fullerenes and carbon nanotubes, this is clearly
not the case for nanosilver. This long history of rational fabrication
and use of colloidal nanosilver has resulted in a lot of research and
knowledge about these nanoparticles over the last 100 years, even if
this research is not reported under “nano” terminology.
The nanosilver formulations mentioned in the preceding section have not
only been used by scientists and described in the patent literature, but
have consistently found their way into the market. In the early part of
the 20th century, the commercial sale of medicinal nanoscale silver
colloids, known under different trade names such as Collargol, Argyrol,
and Protargol, began and over a 50-year period their use became
widespread. These nanosilver products were sold as over-the-counter
medications and also used by medical doctors to treat various diseases
such as syphilis and other bacterial infections.
(27)
Registration of Nanosilver Products in the United States
Apart from these medical applications, many biocidal nanosilver products were
developed and registered in the United States. Information on EPA-registered silver products for the last 60 years is contained in the
NPIRS database (National Pesticide Information Retrieval System, http://ppis.ceris.purdue.edu/npublic.htm).
We searched and evaluated this database for all registered biocides
referring to silver as the active substance, and registered labels
relating to each product were reviewed for the form of the silver and
the company name. Additional searches in public patent literature and
general Internet searches specific to the company and product were also
used to evaluate the nature of each registered product. Based on these
reviews the list of registered silver products was divided into five
categories, from surely containing “nano” to not likely containing
“nano” (Table 1).
Table 1. Evaluation of National Pesticide Information Retrieval System (NPIRS; http://ppis.ceris.purdue.edu/npublic.htm) Database for EPA-Registered Silver Products
| category | conclusion | selection basis | number of registrations |
|---|---|---|---|
| confirmed nano | nano | evaluation based on public citations stating nano nature of product or direct measurements on product material (e.g., Figures 3 and 4) | 7 (7%) |
| likely nano | nano | evaluation based on patent literature and/or manufacturing techniques | 42 (46%) |
| ionic | not nano | products contain materials that store and release individual silver ions | 31 (34%) |
| not likely nano | not nano | products contain macro scale materials, e.g., silver-coated fibers | 8 (9%) |
| unknown | not nano | insufficient information available | 4 (4%) |
Figure 2. Registration of biocidal silver and nanosilver products in the U.S. with the categorization according to Table 1.
Figure 3. TEM analysis of silver-impregnated carbon filter, Zodiac Nature2 G (EPA registration no. 67712-1). Top left: larger particle with silver particles, discernible by their brightness; top right: magnification of one silver nanoparticle from the picture to the left; bottom left: small silver nanoparticles on gray matrix; bottom right: EDX spectra of the two areas in the TEM picture on the left (1: silver particles, bottom spectrum, 2: background: top spectrum). For details about methods see Supporting Information.
Figure 4. TEM analysis of the algicide Algaedyn (EPA registration no. 68161-1) (top) and particle size distribution by image analysis (bottom). For details about methods see Supporting Information.
Of
the biocidal silver products, 53% likely or surely contain nanosilver
but only 7% are advertised as containing nanoparticles. For the majority
of the registered products only a combined evaluation of patents and
knowledge on manufacturing techniques and materials science can inform
about the nature of the silver present in the product. Figure 2
shows the frequency of EPA-registered silver products over the years.
The first biocidal silver product registered in the U.S. under the
Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) in 1954 was
Algaedyn, a nanosilver product based on the patent by Moudry(25)
that is still used today as an algicide in residential swimming pools.
After the establishment of EPA in 1970 all silver registrations in the
next 23 years until 1993 were for nanosilver (colloidal silver) or for
silver nanocomposites. The first non-nanosilver product was registered
in 1994. It is also apparent that the number of “non-nano” silver
products has risen substantially in the last 10 years, necessitating
also new scientific studies on potential ecotoxicological effects of
these “conventional” products.
There are at
least three categories of EPA-registered products that employ elemental
silver particles with particle sizes less than 100 nm: (a) silver
biocidal additives; (b) silver-impregnated water filters; (c) silver
algicides and disinfectants. Several examples of each category are
identified below.
Silver Biocidal Additives
EPA has registered numerous biocidal additives based on elemental silver particles. Table 2
contains some examples of currently registered biocidal additive
products that contain metallic (elemental) silver with very small
particle size (<100 nm), including Additive SSB (EPA reg. 83587-3,
company NanoHorizons), MicroSilver BG-R (EPA reg. 84146-1, company
Bio-Gate), and HyGate 4000 (EPA reg. 70404-10, company BASF, formerly
Ciba Corp.). These silver biocides are typically used in plastic and
textile applications where the silver is effectively contained within
polymer substrates.
Table 2. Details about Some Selected EPA-Registered Nanosilver Products
| product type | product name | company | EPA registration number | date registered | reference |
|---|---|---|---|---|---|
| water filter | 989 Bacteriostatic Water Filter Media | Barnebey & Sutcliffe Corp (Now owned by Calgon) | 58295-1 | 1-Dec-1988 | a |
| water filter | NATURE2 G45-VC40 | Zodiac Pool Care, Inc. | 67712-1 | 21-Nov-2002 | b, Figure 3 |
| algicide | Algaedyn | Pool Products Packaging Corp | 68161-1 | 31-Dec-1954 | Figure 4 |
| algicide | Nu-Clo Silvercide | Alden Leeds Inc. | 7124-101 | 15-Jun-1993 | d |
| algicide | ASAP-AGX | American Biotech Laboratories | 73499-1 | 27-Feb-2002 | c |
| additive | Additive SSB | Nanohorizons Inc. | 83587-3 | 28-Sep-2007 | d |
| additive | MicroSilver BG-R | Bio-Gate AG | 84146-1 | 18-Mar-2008 | e |
| additive | HyGate 4000 | BASF Corp (Formerly Ciba) | 70404-10 | 5-Sep-2008 | e |
a
US Patent 3,374,608, “Silver Impregnated Carbon” (1968), Pittsburgh Activated Carbon Company, now owned by Calgon (“activated carbon impregnated with a metallic silver having a crystallite size of not over 250 A [25 nm].”...“[T]he silver must be dispersed as particles of colloidal size (less than 250 A”) (emphasis added).
b
US Patent 6,165,358, “Water Purifier for a Spa” (2000). Zodiac Pool Care, Inc. “purification materials are described, for example, in U.S. Pat. No. 5,352,369”. US Patent 5,352,369, “Method of Treating Water” (1994). Fountainhead Technologies Inc.: “the elemental silver preferably includes at least 2% of silver crystals having crystal sizes between approximately 3 nm and 10 nm” (emphasis added).
c
“These engineered silver particles currently vary in size between about 10−50 nm in diameter”. (April 26, 2005) William D. Moeller, President, American Biotech Laboratories Testimony on Malaria before the U.S. House of Representatives, International Relations Committee, Subcommittee on Africa, Global Human Rights, and International Operations. [http://www.foreignaffairs.house.gov/archives/109/20915.pdf] (p.38)].
d
SNWG “Evaluation of Hazard and Exposure Associated with Nanosilver and Other Nanometal Oxide Pesticide Products”, Presentation to Scientific Advisory Panel (November 4, 2009). Docket ID: EPA-HQ-OPP-2009-0683-0165. [http://www.regulations.gov/search/Regs/contentStreamer?objectId=0900006480a52512&disposition=attachment&contentType=pdf].
e
Ciba Specialty Chemicals (Now BASF) “Ciba Specialty Chemicals forms marketing cooperation with Bio-Gate for silver antimicrobial technology” (December 14, 2005, Basel, Switzerland) [http://cibasc.com/index/med-index.htm?reference=41794&checkSum=C44184952B5155A13ECC5E419C8F7310] Note figure caption “Scanning electron microscopy showing the high porosity HyGate 4000 powder: primary particle size 50−200 nm...” (emphasis added).
Silver-Impregnated Water Filters
EPA
has registered multiple silver-impregnated water filters since the
1970s. Bacteriostatic water filters are generally based on activated
carbon or ceramics that are impregnated with metallic (elemental) silver
exhibiting a very small particle size (<100 nm). It should be noted
that impregnating carbon and ceramic materials with metals is widely
recognized as a standard technique for the synthesis of nanoscale metal
particles. In particular, wet impregnation methods have been employed in
production of nanostructured industrial catalysts for decades. Numerous
peer-reviewed scientific literature publications clearly establish that
impregnation methods lead to nanoscale metal particles (e.g.,
nanosilver) supported on the impregnated substrate (e.g., carbon).(28, 29)
Silver-impregnated
water filters currently registered under FIFRA employ
impregnation-based manufacturing methods to achieve the registered
metallic silver form,(26, 30, 31) in many cases with the clear intention to produce nanoscale silver. An example is given in one patent application from 1994: “the
elemental silver preferably includes at least 2% of silver crystals
having crystal sizes between approximately 3 nm and 10 nm”.(31) Silver-impregnated water filters contain nano-silver particles supported on the filter matrix structure.
To
verify the presence of nanoscale Ag in water filters, commercially
available filters were disassembled and the carbon pellets were crushed
and investigated with TEM equipped with a HAADF (High Angle Annular Dark
Field) detector that is sensitive toward heavy elements (Figure 3). Details of the analysis can be found in the Supporting Information.
Bright nanoparticles are visible distributed on matrix particles having
sizes ranging from a few nm to about 100 nm. EDX (energy dispersive
X-ray spectroscopy) analysis showed that the bright particles contain Ag
whereas spectra of the light gray matrix background have no detectable
Ag signal. These analyses clearly prove that during manufacturing of the
silver-impregnated water filters Ag nanoparticles are formed and that
all silver is present in nanoparticulate form in these filters. However,
it remains to be investigated what the fate of this nanosilver is
during use of the water filters, whether there is only dissolution and
release of ionic silver or whether there is also release of particulate
silver.
Silver-impregnated water filters
have been safely used for domestic water applications such as drinking
water and swimming pool filters for decades. No reports about any health
or environmental effects have been reported, although the absence of
such reports does not mean that no effects occurred. However, the many
decades long use of these EPA registered nanosilver containing products
in numerous households presents a unique opportunity for epidemiologists
to investigate effects of extended use of nanosilver on human health.
Silver Algicides and Disinfectants
Silver
algicides and disinfectants have been FIFRA registered as biocides
since 1954. Colloidal nanosilver algicides are based on elemental silver
particles maintained in a stabilized solution, containing silver in
very small particle size (e.g., <100 nm). Table 2
contains some examples of currently registered biocidal products
including Silver Algaedyn (EPA reg. 68161-1, Pool Products Packaging
Corp), Nu-Clo Silvercide (EPA reg. 7124-101, Alden Leeds Inc.) and
ASAP-AGX (EPA reg. 73499-1, American Biotech Laboratories). Figure 4
shows a TEM micrograph of Algaedyn and the size distribution of the
particles determined by image analysis (>3500 particles, details of
the analysis can be found in the Supporting Information), proving that Algaedyn contains silver nanoparticles with diameters between 2 and 20 nm.
It
should be noted that algicide applications have been used safely in
high-exposure, direct water contact, and down-the-drain applications
such as swimming pool disinfection for decades without any known
damaging impact on humans or the environment. The more than 50-year use
of these nanosilver products presents a unique opportunity for
environmental scientists to study the effects of the discharge of
nanosilver into sewer systems, wastewater treatment plants, and natural
waters, especially in residential areas with a lot of swimming pools.
Also epidemiologists could study populations of home-owners using
silver-based algicides and compare with those using other biocides to
get data on a population that has been exposed to nanomaterials for
decades.
Silver Nanotoxicology
Colloidal nanosilver has been administered as a medication for almost one hundred years.(22)
Regardless of the medicinal claims associated with these products a lot
of research on human and animal body distribution and toxicology was
carried out with these materials. One early example is a study from 1924
in which the behavior of Collargol nanosilver in the human body is
described.(32)
Numerous cases of the nontoxic, cosmetic condition argyria were
documented during this period. Argyria is a condition characterized by a
bluish-gray discoloration of the skin.(33) The toxicity of silver is considered to be relatively low (3)
and toxic effects on humans other than argyria are only observed at
very high concentrations (e.g., acute oral LD50 for rats is higher than
1600 mg kg−1 d−1).(3, 33) Gaul and Staud in 1935(34)
listed 43 cases of argyria of which 27 (63%) were caused by Collargol
or Argyrol, thus originating from the medical use of colloidal
nanosilver. The other cases were caused by silver chloride or silver
iodide. A significant portion of the historical toxicological research
on the effects of silver on humans can thus be considered early examples
of “nanotoxicology”—predating what is currently considered to be
nanotoxicology(35) by more than 80 years.
In 1939 Hill and Pillsbury(36)
collected the available literature on silver and colloidal nanosilver
toxicology and derived exposure limits based on the threshold value
above which development of argyria can be expected. This threshold value
was found to be the intake of 0.9 g of silver over the whole lifetime.
The modern drinking water standard of 100 μg/L for silver is based on
this value(37)
and thus includes data on nanoscale-silver. Only a very few studies are
available that describe toxicity of bulk silver as opposed to
nanosilver or dissolved silver. However, for some standards there is a
distinction between metallic and ionic silver; for instance, the
American Conference of Governmental Industrial Hygienists has
established separate threshold limit values for metallic silver (0.1
mg/m3) and soluble compounds of silver (0.01 mg/m3).(33)
We can thus state that with relation to human toxicology and the legal
standards related to occupational and consumer health, the toxicity of
nanosilver has been taken into account and thus the existing standards
are sufficient to protect consumers also from novel nanosilver, at least
in forms equivalent to those available in the 1930s.
If
we turn our attention to environmental risk assessment of silver we see
a different picture. The data about effects of silver on environmental
organisms were almost exclusively obtained using dissolved silver.(38)
Most research on nonmammalian species was based on the use of dissolved
silver and only recently were ecotoxicological studies with nano-Ag
published, e.g., refs 8, 40, and 41.
However, it has been questioned if data obtained using ionic silver
should be used to derive threshold values in the environment.(42) Silver in natural waters is typically associated with the particulate and colloidal fraction(43) and is thus to some extent naturally present as nanoparticles and metal-sulfide clusters.(44)
Furthermore it should be noted that the latest research indicates that
under real-life environmental exposure conditions nanosilver is rapidly
converted to silver sulfide, resulting in material forms that give no
measurable impact on wastewater treatment plants and are much less toxic
than ionic silver.(44-46)
The conversion to silver sulfide highlights the importance of
accounting for speciation and passivation of silver materials by
ubiquitous environmental species under real-life conditions in
real-world risk assessment of nanosilver materials.(5)
Because
many of the aquatic species are several orders of magnitude more
sensitive to silver than mammals and humans (with lethal concentrations
for some sensitive aquatic organisms of only 1−5 μg/L(3)),
the question whether nanosilver has a different toxicity than dissolved
silver is of eminent importance. However, this should be discussed in
the light of the recent findings that the silver present in the
environment is to a large extent present in particulate form and as
nanoclusters and not as dissolved silver.
It
should also be kept in mind that the expected concentrations of
nanosilver in the surface waters of the U.S. are between 0.09 and 0.43
ng/L(47) whereas total silver concentrations in water were modeled to be between 40 and 320 ng/L for European surface waters.(42) Nanosilver thus contributes only a small extent to the total silver flow in the environment.
Implications for Policy of Nanosilver
Regardless
of what nomenclature is used, any concept of risk must ultimately
derive from chemical and physical characteristics of a specific material.
Applying the general prefix “nano” does not in itself automatically
render a material harmful. Although today’s nanosilver has many
alternative nomenclatures and historical aliases including “colloidal
silver”, the underlying material is the same—extremely small particles
of silver. Contrary to many common assumptions, nanosilver materials
have a deep historical record of demonstrated safe use together with a
long period of formal and successful regulatory oversight. The use of
very high doses of colloidal nanosilver at the beginning of the 20th
century has sparked a vast amount of research on the toxicology of
nanosilver, resulting in the first exposure limits for silver and
subsequent regulations on its use. Clearly nanosilver is a material that
does not fit the paradigm of a “new” chemical with new and unknown
risks. To consider otherwise is to confuse nomenclature (nano) instead
of considering the material itself.
Regulators
rightly state that policy needs to be made on the basis of sound
science. A first principle of science is that assumptions need to be
tested. For example nanosilver is assumed to be a new material because
of the term “nano”. However, on close inspection nanosilver materials
have a long history of relatively safe and regulated use. Historical
perspective also shows us that nanosilver has been intentionally
manufactured and adopted commercially across a wide spectrum of
every-day applications for decades. For example, EPA-registered silver
nanoparticles have been safely used in down-the-drain and high-volume
water-contact applications (e.g., swimming pool algicides and drinking
water filter systems) bringing benefit to millions of consumers over a
period of 50 years. On balance, a substantial amount is known about
silver and silver nanoparticles and that historical experience of use
and exposure actually points to these materials being relatively safe.
While there are naturally topics where there is ample opportunity to
improve scientific understanding about nanosilver (e.g., with respect to
its environmental behavior and effects), it would be a mistake for
regulators to ignore the accumulated knowledge of our scientific and
regulatory heritage in a mistaken bid to declare nanosilver materials as
new chemicals, with unknown properties and automatically harmful simply
on the basis of a change in nomenclature to the term “nano”. However,
this does also not mean that nanosilver should be treated as harmless
without testing its effect, but rather that it is sufficient to apply
the already strict and coherent risk assessment framework for other
silver-containing materials and products that often have a shorter
history of regulated use.
Acknowledgment
We thank Dr. James Delattre and Dr. Rosalind Volpe of the Silver Nanotechnology Working Group (SNWG) for valuable contributions to the background of this article. We thank Dr. Ralf Kaegi from Eawag for performing the TEM analyses.
Supporting Information
TEM analysis of water filters and TEM analysis of Algaedyn. This material is available free of charge via the Internet at http://pubs.acs.org.
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- Published In Issue February 15, 2011
- Article ASAPJanuary 10, 2011
- Received: September 30, 2010
Accepted: December 16, 2010
Revised: December 13, 2010
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