Published:
January 10, 2011
r
2011 American Chemical Society
1177
dx.doi.org/10.1021/es103316q
|
Environ. Sci. Technol.
2011, 45, 1177
鈥
1183
POLICY ANALYSIS
pubs.acs.org/est
120 Years of Nanosilver History: Implications for Policy Makers
Bernd Nowack,*
,
鈥
Harald F. Krug,
鈥
and Murray Height
鈥
鈥
EMPA
-
Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland
鈥
HeiQ Materials AG, CH-5330 Bad Zurzach, Switzerland
b
S
Supporting Information
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
铿
lters, 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
铿
ndings 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 scienti
铿
c 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
鈥
nano
鈥
.
鈥
INTRODUCTION
The potential adverse e
铿
ects 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
鈥
new
鈥
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 e
铿
ects
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
鈥
nano
鈥
nomenclature.
In this analysis we critically examine with respect to nanosilver
three important assertions often made when discussing risk
assessment of silver nanoparticles:
(1) Nanosilver is new and exhibits unique physical and
chemical properties compared to
鈥
conventional
鈥
silver
(e.g., macroscale
鈥
bulk
鈥
silver).
(2) Nanosilver has been used for only a few years and the
environment and humans have never been exposed to
nanosilver before.
(3) Existing risk assessments of silver have been based on a
data set derived from conventional silver materials, so
they do not apply to nanosilver.
鈥
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 di
铿
cult for bacteria to grow on the treated object. Bio-
cidal 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
铿
lters for
household use
10
or swimming pool algicides.
11
To meet the
diversity of application types, many di
铿
erent 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 di
铿
erentiated 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).
Received:
September 30, 2010
Accepted:
December 16, 2010
Revised:
December 13, 2010
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2011, 45,
1177鈥1183
Environmental Science & Technology
POLICY ANALYSIS
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 nano-
particles 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
鈥
bookends
鈥
for illustrating the extremes of
the potential for silver ion release from silver substances: namely
silver sul
铿
de (highly insoluble, hence a low potential for silver ion
release) and silver nitrate (completely soluble, maximum poten-
tial for silver ion release). The release potential of di
铿
erent silver
materials can be distributed between the silver sul
铿
de 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 sul
铿
de 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 sul
铿
de 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
鈥
HISTORY OF NANOSILVER PRODUCTION AND USE
One of the standard de
铿
nitions of nanotechnology encom-
passes
鈥
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.
鈥
15
The unintentional
formation of nanoparticles thus would not fall under this de
铿
ni-
tion 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
铿
rst 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
鈥
Collargol
鈥
such a kind of nanosilver has been manufac-
tured 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:
鈥
for proper
e
铿
ciency, the silver must be dispersed as particles of colloidal
size less than 250 脜 [less than 25 nm] in crystallite size
鈥
.
26
Whereas it is true for many other engineered nanomaterials that
they are novel, e.g., for fullerenes and carbon nanotubes, this is
Figure 1.
Silver release and amount of silver required in products for di
铿
erent biocidal silver formulations.
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Environmental Science & Technology
POLICY ANALYSIS
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
鈥
nano
鈥
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 di
铿
erent
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
鈥
REGISTRATION OF NANOSILVER PRODUCTS IN THE
UNITED STATES
Apart from these medical applications, many biocidal nano-
silver 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 speci
铿
c 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
铿
ve categories, from
surely containing
鈥
nano
鈥
to not likely containing
鈥
nano
鈥
(Table 1).
Of the biocidal silver products, 53% likely or surely contain
nanosilver but only 7% are advertised as containing nanoparti-
cles. For the majority of the registered products only a combined
evaluation of patents and knowledge on manufacturing tech-
niques 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
铿
rst biocidal
silver product registered in the U.S. under the Federal Insecti-
cide, 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 registra-
tions in the next 23 years until 1993 were for nanosilver (colloidal
silver) or for silver nanocomposites. The
铿
rst non-nanosilver
product was registered in 1994. It is also apparent that the
number of
鈥
non-nano
鈥
silver products has risen substantially in
the last 10 years, necessitating also new scienti
铿
c studies on
potential ecotoxicological e
铿
ects of these
鈥
conventional
鈥
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
铿
lters; (c) silver algicides and disinfectants. Several exam-
ples of each category are identi
铿
ed 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.
Silver-Impregnated Water Filters.
EPA has registered mul-
tiple silver-impregnated water filters since the 1970s. Bacterio-
static 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 meth-
ods have been employed in production of nanostructured
industrial catalysts for decades. Numerous peer-reviewed scien-
tific 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
铿
lters currently registered under
FIFRA employ impregnation-based manufacturing methods to
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
con
铿
rmed 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
铿
bers
8 (9%)
unknown
not nano
insu
铿
cient information available
4 (4%)
Figure 2.
Registration of biocidal silver and nanosilver products in the
U.S. with the categorization according to Table 1.
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2011, 45,
1177鈥1183
Environmental Science & Technology
POLICY ANALYSIS
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:
鈥
the elemental silver
preferably includes at least 2% of silver crystals having crystal sizes
between approximately 3 nm and 10 nm
鈥
.
31
Silver-impregnated
water
铿
lters contain nano-silver particles supported on the
铿
lter
matrix structure.
To verify the presence of nanoscale Ag in water
铿
lters, commer-
cially available
铿
lters 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
铿
lters Ag nanoparticles are formed and that all silver is present
in nanoparticulate form in these
铿
lters. However, it remains to be
investigated what the fate of this nanosilver is during use of the water
铿
lters, whether there is only dissolution and release of ionic silver or
whether there is also release of particulate silver.
Silver-impregnated water
铿
lters have been safely used for
domestic water applications such as drinking water and swim-
ming pool
铿
lters for decades. No reports about any health or
environmental e
铿
ects have been reported, although the absence
of such reports does not mean that no e
铿
ects 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 e
铿
ects 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 Labora-
tories). 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 environ-
ment. The more than 50-year use of these nanosilver products
presents a unique opportunity for environmental scientists to
study the e
铿
ects of the discharge of nanosilver into sewer
systems, wastewater treatment plants, and natural waters, espe-
cially 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.
鈥
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
Table 2. Details about Some Selected EPA-Registered Nanosilver Products
product type
product name
company
EPA registration number
date registered
reference
water
铿
lter
989 Bacteriostatic Water Filter Media
Barnebey & Sutcli
铿
e Corp
(Now owned by Calgon)
58295-1
1-Dec-1988
a
water
铿
lter
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,
鈥
Silver Impregnated Carbon
鈥
(1968), Pittsburgh Activated Carbon Company, now owned by Calgon (
鈥
activated carbon
impregnated with a metallic silver having a crystallite size of
not over 250 A
[25 nm].
鈥
...
鈥
[T]he silver must be dispersed as particles of
colloidal size (less than
250 A
鈥
) (emphasis added).
b
US Patent 6,165,358,
鈥
Water Puri
铿
er for a Spa
鈥
(2000). Zodiac Pool Care, Inc.
鈥
puri
铿
cation materials are described, for
example, in U.S. Pat. No. 5,352,369
鈥
. US Patent 5,352,369,
鈥
Method of Treating Water
鈥
(1994). Fountainhead Technologies Inc.:
鈥
the elemental silver
preferably includes at least 2% of silver crystals having crystal sizes between approximately
3 nm
and
10 nm
鈥
(emphasis added).
c
鈥
These engineered silver
particles currently vary in size between about
10
-
50 nm in diameter
鈥
. (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.foreigna
铿
airs.house.gov/archives/109/20915.pdf] (p.38)].
d
SNWG
鈥
Evaluation of Hazard and Exposure
Associated with Nanosilver and Other Nanometal Oxide Pesticide Products
鈥
, Presentation to Scienti
铿
c 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)
鈥
Ciba Specialty Chemicals forms marketing cooperation with Bio-Gate for silver anti-
microbial technology
鈥
(December 14, 2005, Basel, Switzerland) [http://cibasc.com/index/med-index.htm?reference=41794&checkSum=C441-
84952B5155A13ECC5E419C8F7310] Note
铿
gure caption
鈥
Scanning electron microscopy showing the high porosity HyGate 4000 powder:
primary
particle size 50
-
200 nm
...
鈥
(emphasis added).
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POLICY ANALYSIS
argyria were documented during this period. Argyria is a condi-
tion characterized by a bluish-gray discoloration of the skin.
33
The toxicity of silver is considered to be relatively low
3
and toxic
e
铿
ects 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
signi
铿
cant portion of the historical toxicological research on the
e
铿
ects of silver on humans can thus be considered early examples of
鈥
nanotoxicology
鈥
;
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 expo-
sure limits based on the threshold value above which develop-
ment 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
渭
g/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 Indus-
trial Hygienists has established separate threshold limit values for
metallic silver (0.1 mg/m
3
) and soluble compounds of silver
(0.01 mg/m
3
).
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 su
铿
cient to protect
consumers also from novel nanosilver, at least in forms equiva-
lent to those available in the 1930s.
If we turn our attention to environmental risk assessment of
silver we see a di
铿
erent picture. The data about e
铿
ects 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-sul
铿
de
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 sul
铿
de, 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 sul
铿
de 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
Figure 3.
TEM analysis of silver-impregnated carbon
铿
lter, Zodiac Nature2 G (EPA registration no. 67712-1). Top left: larger particle with silver
particles, discernible by their brightness; top right: magni
铿
cation 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.
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2011, 45,
1177鈥1183
Environmental Science & Technology
POLICY ANALYSIS
(with lethal concentrations for some sensitive aquatic organisms
of only 1
-
5
渭
g/L
3
), the question whether nanosilver has a
di
铿
erent toxicity than dissolved silver is of eminent importance.
However, this should be discussed in the light of the recent
铿
ndings 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 concentra-
tions 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 sur-
face waters.
42
Nanosilver thus contributes only a small extent to
the total silver
铿
ow in the environment.
鈥
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
speci
铿
c material
. Applying the general pre
铿
x
鈥
nano
鈥
does not
in itself automatically render a material harmful. Although today
鈥
s
nanosilver has many alternative nomenclatures and historical
aliases including
鈥
colloidal silver
鈥
, 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
铿
rst exposure limits for silver and
subsequent regulations on its use. Clearly nanosilver is a material
that does not
铿
t the paradigm of a
鈥
new
鈥
chemical with new and
unknown risks. To consider otherwise is to confuse nomencla-
ture (nano) instead of considering the material itself.
Regulators rightly state that policy needs to be made on the
basis of sound science. A
铿
rst principle of science is that
assumptions need to be tested. For example nanosilver is
assumed to be a new material because of the term
鈥
nano
鈥
.
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 manufac-
tured 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
铿
lter systems) bringing bene
铿
t 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 scienti
铿
c under-
standing about nanosilver (e.g., with respect to its environmental
behavior and e
铿
ects), it would be a mistake for regulators to
ignore the accumulated knowledge of our scienti
铿
c and regula-
tory 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
鈥
nano
鈥
. However, this does also not mean that nanosilver
should be treated as harmless without testing its e
铿
ect, but rather
that it is su
铿
cient 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.
鈥
ASSOCIATED CONTENT
b
S
Supporting Information.
TEM analysis of water
铿
lters
and TEM analysis of Algaedyn. This material is available free of
charge via the Internet at http://pubs.acs.org.
鈥
AUTHOR INFORMATION
Corresponding Author
*E-mail: nowack@empa.ch.
鈥
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.
鈥
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