are nanosized or dissolved metals more toxic in the...

ARE NANOSIZED OR DISSOLVED METALS MORE TOXIC INTHE ENVIRONMENT? A META-ANALYSIS

DOMINIC A. NOTTER, DENISE M. MITRANO, and BERND NOWACK*Technology & Society Laboratory, Empa-Swiss Federal Laboratories for Materials Science and Technology, St. Gallen, Switzerland

(Submitted 11 June 2014; Returned for Revision 29 July 2014; Accepted 19 August 2014)

Abstract: Recently, much has been written about the extreme urgency of elaborating the regulations for engineered nanomaterials. Suchregulations are needed both from lawmakers, to protect people from potentially adverse effects, and from industry representatives, to provethat nanoproducts are produced carefully and with caution to avoid possible lawsuits. However, developing regulations has proven to be adifficult task, and an ambiguous topic where errors can easily occur. In the present study, the authors present a meta-analysis of 3 differentnanomaterials (nano-Ag, nano-ZnO, and nano-CuO) in which data from ecotoxicity studies and published half-maximal effectiveconcentration (EC50) values are compared for both the nano form and the corresponding dissolved metal. A ratio equal to 1 means that theparticle is as toxic as the dissolved metal ion, whereas a lower ratio signifies that the nano form is less toxic than the dissolved metal basedon total metal concentrations. The results show that for 93.8% (Ag), 100% (Cu), and 81% (Zn) of the ratios considered, the nano form isless toxic than the dissolved metal in terms of total metal concentration. Very few of the studies surveyed found a ratio of EC50 values for(dissolved/nano) that was larger than 2 (Ag: 1.1%; Cu: 0%; Zn: 2.8%). Hence, a reduction in existing metal concentration thresholds by afactor of 2 in current freshwater and soil regulations for ecotoxicitymay be sufficient to protect organisms and compartments from the nanoform of these metals as well. Environ Toxicol Chem 2014;33:2733–2739. # 2014 SETAC

Keywords: Metal nanomaterial Metal oxide nanomaterial Regulation Ecotoxicity Meta-analysis

INTRODUCTION

The field of nanotoxicology has matured and developedenormously over the past decade [1–3]. A vast amount of data onthe toxicology and ecotoxicology of nanomaterials has beencollected, and although many issues are still open, some generaltrends have emerged [4–8]. A particularly interesting finding isthat the ecotoxicity of many nanomaterials is in the same order ofmagnitude as the corresponding dissolved ion [9,10].

In view of the extensive body of literature available, Hansenand Baun [11] called on the European Commission to provideregulations for nanosilver. They argued that reviewing the sameliterature time and again would not provide new information, butrather would create the unfortunate situation of “paralysis byanalysis,” and that a review usually only identifies new researchneeds and generally does not adequately correlate data into aformat that is usable for regulators and policy makers.

Because research on the risks of nanomaterials and theirunique chemical and physical characteristics is still in thedevelopment phase, both regulators and the industrial sectorface the challenge of how to understand and manage potentialrisks from nanomaterials now [12]. Beaudrie et al. [13] recentlystated that “high scientific uncertainty, a lack of environ-mental, health and safety regulations and product data,inappropriately designed exemptions and thresholds, and limitedagency resources are a challenge to both the applicability andadequacy of current regulations.” The task of finding the firstpragmatic evaluation for regulatory purposes is complex, butthat does not render it impossible.

The present study analyzes a substantial body of peer-reviewed literature on the environmental effects of 3 metal andmetal oxide nanoparticles, which are soluble enough to causetoxicity. In addition, we have developed a simple and pragmaticway to quantitatively evaluate the data foundwith respect to theirecotoxicological hazard potential. Regulatory authorities mayuse this approach when implementing first regulations based onthese values.

Nano-silver (nano-Ag), nano-zinc oxide (nano-ZnO), andnano-copper oxide (nano-CuO) are relatively soluble nano-materials for which many studies have compared the toxicity ofthe particulate form and the toxicity of its dissolved counter-part [9]. By dividing the toxicity of a nanomaterial by that of thedissolved ion, we derive a number that relates the toxicities ofboth species. This approach provides a detailed overview of themaximum toxicity that can be expected for the 3 nanomaterialsunder investigation for all biological species and environmentalcompartments under the specific conditions relevant to each pair.It does not explore the chemical and physical properties ofthe particles, the physiological mechanism of the toxicity, thebioavailability of the particles, the extent to which a metalparticle dissolves, the species sensitivity, or specific environ-mental conditions such as total organic content, or salinity. Thebasic premise is that the literature review provided in the presentstudy contains nearly all physical/chemical variations of the3 nanomaterials under investigation (with different shapes,functional groups, varying bioavailability, etc.) available todayand that these particles have been tested under exhaustiverelevant conditions for our ecosystems. The combination ofnanomaterials tested under different conditions automaticallydiscloses the harshest possible situation for ecosystems withoutidentifying the reason for the high toxicity. The toxicity of themost toxic nanomaterials can then be benchmarked to existingregulations, as regulations for dissolved silver, zinc, and copperions are readily available [14,15]. Regulators may use these new

All Supplemental Data may be found in the online version of this article.* Address correspondence to [email protected] online 26 August 2014 in Wiley Online Library

(wileyonlinelibrary.com).DOI: 10.1002/etc.2732

Environmental Toxicology and Chemistry, Vol. 33, No. 12, pp. 2733–2739, 2014# 2014 SETAC

Printed in the USA

2733

toxicity ratios to derive thresholds for nanomaterials based onexisting regulatory standards.

METHODS

Literature review

A comprehensive literature review was conducted to collectavailable terrestrial and aquatic ecotoxicity endpoints for 3engineered nanomaterials and their dissolved metallic ions. Theliterature used for our meta-analysis is based on a past literaturereview [6]. This literature was considered as being completethrough June 2012. All journals that contributed articles to ourprevious list from July 2012 until October 2013were manuallysearched. Finally, a key word search was carried out when fewdata were available (e.g., CuO or soil/sediment data). However,we do not claim that our search is complete.

The few data points that were available for AgCl and metallicCu were not included in the present study. To ensure that ourresults were based on the highest possible quality of data, weconfined ourselves to studies based on dose–response curves.Thus, all data in our evaluation are expressed in the originalpublication either as median lethal dose (LD50), median lethalconcentration (LC50), half-maximal effective concentration(EC50; end points of growth rate, population, reproduction), orhalf‐maximal inhibitory concentration (IC50). Another compul-sory criterion for data to be included in our evaluation was thatthe toxicity endpoints in the original publication had to beassessed within the same study, under exactly the sameenvironmental conditions and on the same species for boththe dissolved metal and subsequently the nanomaterial in asecond test. The literature covered the time period from 2007 to2008, when the first studies appeared comparing toxicities ofengineered nanomaterials with its soluble counterpart, toOctober 2013. We aimed to collect the greatest amount ofdata available in the literature that provided relevant informationon the ecotoxicity of nanomaterials andmetal toxicity. Hence, alldata points from a given study were taken into account whendifferent types of nanomaterials were used, when differentenvironmental conditions were tested, or when different specieswere investigated. Because we were not investigating physio-logical toxicity mechanisms, but rather aimed to derive anoverall toxicity evaluation, we included studies that did notnecessarily follow guidelines from the Organisation forEconomic Co-operation and Development (e.g., those that didnot use fully characterized nanomaterials or did not includereference materials). However, data points were excluded fromthe evaluation when the testing conditions did not representenvironmentally relevant conditions (e.g. in vitro toxicity tests,tests with genetically sensitized organisms). This emphasizes thefocus on the real-world conditions that are relevant for decision-makers in the field of nanomaterial regulation.

Some organisms tested may colonize predominantly 1 eco-system, but they can be tested in both aquatic and terrestrialsystems. In these instances, the organisms were assigned tothe test ecosystem of the study, irrespective of whether it wasthe ecosystem they predominantly colonize. This reasoningis justified in light of the fact that the bioavailability ofnanomaterials in aquatic ecosystems differs compared with thatof terrestrial systems.

Studies that used filtration of nanoparticle suspensions toderive aqueous toxicity values were not included.

For analysis of toxicity in terms of particle size, wheneverpossible the nominal particle size was used as opposed to theparticle size measured in solution during the experiment.

In a few studies, the authors indicated the nanoparticletoxicity as generally greater than the highest exposureconcentration with no negative toxic effects. In this case, thevalue used to calculate the ratio was the highest concentrationtested, which was still a useful benchmark in indicating relativetoxicity. This might have led to an overestimation of thenanoparticle-induced toxicity. However, this procedure wasapplied only when nanoparticle toxicity was less than thetoxicity of the dissolved ion. It was important to include thesevalues in our evaluation to achieve meaningful shares of toxicityratios (R) with R< 1 and R> 1, respectively. However, whetherthe toxicity ratio is slightly below 1 or orders of magnitudebelow 1 does not play a major role in interpretation of the results,because (taking a precautionary position) we suggest thresholdlimits based on the highest R values rather than the mean ormedian values or other statistical parameters.

The toxicity ratio (R)

We defined R as:

R ¼ LD; LC; IC;EC50dissolvedLD;LC; IC;EC50nano

A ratio of nanomaterial toxicity to metal toxicity was establishedfor each test system after a thorough literature search. All valuesused for the calculation of R refer to the net metal content.Reported EC50 values for metal oxide nanoparticles or metalsalts were corrected to express the net metal content.

An R value< 1 means that the nanoparticle is less toxic thanthe dissolved metal ion, although this statement is true only forthe unique nanoparticle characteristics including attachedfunctional groups, coatings, and capping agents, all of whichare potentially toxic themselves. If there was no nano-specificeffect, the R value would depend on the dissolution rate alone.For regulation purposes, R values< 1 are not relevant. Toxicityratio values of approximately 1 and above are important becausethese values determine the level at which a regulatory action canbe taken. Hence, values equal to and above 1 were analyzed inmore detail in the present study.

An R value equal to 1 indicates that a nanoparticle is as toxic asthe dissolved metal ions of the same metal. In cases in which thenanoparticle does not completely dissolve, the overall toxicitywould definitely have a nano-specific component andwould resultin a combination of nano-specific effects and metal toxicity.

Toxicity ratio values greater than 1 indicate toxicity greaterthan the toxicity of the dissolved ion alone. In addition to themetal toxicity, there may be further toxic effects attributable tothe inherent physical properties of the particle or its stabilizers orassociated surfactants caused by the particle and its chemical.

RESULTS AND DISCUSSION

The number of data points gathered from the literature searchon the physiological effects of nanomaterials and metal toxicityare summarized in Table 1. For all 3 nanomaterial/metal toxicitysystems, 72 articles were found in 25 different peer-reviewedjournals (Supplemental Data, Table S4), providing 453 datapairs (8 articles provided data for 2 metals).

By far, the largest amount of data found was for the nano-Agparticle/dissolved ion pair, and the most frequently used testcompartment was freshwater. All data points were categorizedinto 1 of 10 groups of test organisms to investigate differences inthe susceptibility of the organism to nanomaterials and metaltoxicity. All values from original publications and the calculated

2734 Environ Toxicol Chem 33, 2014 D.A. Notter et al.

R per species, compartment, and nanomaterial size are given inthe Supplemental Data (Tables S1–S3).

We took advantage of the heterogeneity of the study designswith respect to nanomaterial characteristics, environmentalconditions, and organisms studied. This provided a wholebandwidth of reported toxicities, which allowed us to formulatemeaningful results in addition to providing worst-case scenarios.By analyzing all available studies, we were able to include in ourevaluation the possible effects that varying exposure conditionshave on the fate and effects of the nanomaterial, for example,varying compositions of medium, with or without naturalorganic matter, different agglomeration states, and effects ofsedimentation. Therefore, we were able to capture the conditionsunder which the results systematically ended in a higher toxicityfrom the nano form compared with the dissolved metal. Thisapproach directly compared the effect based on the total metalconcentrations and is therefore able to decipher whether anoverall nano-specific effect played a role. Although the approachcould not identify the mode of action or exclude the possibilitythat nano-specific effects occur, our analysis allowed for robustestimations of the overall effect that matter in the nano-rangeexerts on organisms.

To a large extent, most toxicity ratios resulted in valuessmaller than 1 (Figure 1, top row). This was particularly evidentfor nano-Ag and nano-CuO, for which only 6.2% and 0% of alldata had a ratio above 1, respectively. Nano-ZnO had a slightlymore pronounced nano-specific effect, with 19.0% of R valuesbeing larger than 1. This led to the conclusion that, in most cases,the nano form of a given metal was less toxic than the dissolvedion when total metal concentration was used as the definingmetric.

The center row in Figure 1 depicts the same values found inthe top row but separated into the environmental compartmentsstudied: freshwater, terrestrial, and marine water. For the fresh-water and terrestrial compartments, only a small percentage ofthe toxicity ratios was higher than 1. This percentage includednano-Ag in freshwater (11 of 269; 4.1%) and terrestrial (1 of 8;12.5%) compartments and nano-ZnO in freshwater (12 of 72;20.8%) and terrestrial (1 of 20; 5.0%) compartments. However,the R values for nano-Ag and nano-ZnO for the terrestrialcompartment have limited significance because of the low

amount of data available. Only a few data points were availableto calculate R for the marine water compartment; 37.5% of thenano-ZnOR values (n¼ 8) were found to be greater than 1. In thecase of nano-Ag, 6 of 13 R values in the marine compartmentwere higher than 1, and some of these experiments depictedtoxicity of up to 86 times higher than the dissolved metal. Therewere no instances in which the R of nano-CuO was above 1 inany of the 3 compartments.

Toxicity ratios calculated for the categorized test organismsshowed that values larger than 1 were observed for many speciesand were not restricted to 1 specific test organism (Figure 1,bottom row). Our results failed to identify a test organism thatwas clearly more susceptible to nanomaterials (R> 1) for any ofthe metals. The same set of qualifications appears to be suitablefor all organisms under the conditions tested. Furthermore,organisms from all trophic levels are represented in this analysis.

The toxicity ratios span the ranges of 0.00026 to 86.6 for Ag,0.019 to 2.04 for ZnO, and 0.0026 to 0.89 for CuO. Therefore,Ag spans approximately 6 orders of magnitude, whereas ZnOand CuO ratios are spread over only 3 orders of magnitude. Thehigher number of data points available for nano-Ag comparedwith both nano-ZnO and nano-CuO, and the possibility of highervariability of test conditions in the experimental setup, cannot bewholly responsible for this trend because the ZnO data set alsocontains over 100 data points. We therefore propose that thefactor that most influences the variability in toxicity ratios fornano-Ag is metal dissolution kinetics and possible (particle)transformations. Nano-ZnO dissolves faster than both nano-CuO and nano-Ag [16], and thus the final Zn speciation would bemore similar for both nano-ZnO and dissolved Zn additions intothe same water (e.g., complexation with natural organic matter,carbonate, and/or phosphate precipitates). The lowest observedratio for ZnO is higher than the lowest R value for Ag. In thiscase, a low ratio signifies that a particulate form is present in thesystem but has little or no effect on toxicity. This suggests thatthe toxicity mechanism is not particle specific, but rather themetal ion is the driving source of toxicity, as suggested by manyauthors [9,17,18].

If a nano-specific effect was not occurring and the nano formof the metal was not bioavailable, a low ratio would suggestsmall amounts of particle dissolution. In these instances, a ratioof 1 would be reached after 100% of the nanoparticles had beendissolved (i.e., having the same effect as the same concentrationof the dissolved ion).

In most studies, an overall nano-specific effect was notobserved. However, in a small fraction of studies nano-specifictoxicity effects seemed to be prevalent, the most notable ofwhich are exemplified by experiments in seawater. In thesecases, the nano form is up to 100 times more toxic than the freemetal ion. However, because this effect was not observed ineither the freshwater or terrestrial compartments, we concludethat the high salt content of seawater must be responsible forthese results. In these saline waters, the speciation of the metalsis very different from that in the other compartments, and themost toxic species, the free metal ion, would be at a much lowerfraction of the total metal in solution [19,20]. Therefore, wesuggest that high ratios are not primarily the result of a highertoxicity derived from the nano form but rather because of a lowertoxicity of the dissolved metal. This relationship clearly needs tobe studied in more detail, as nanomaterials in seawater constitutea clear case in which the nano-specific effect may truly beprevalent. It has been concluded [21] that without clearknowledge of speciation of both Agþ and nano-Ag particles,the toxicity to marine organisms cannot be explained.

Table 1. Overview of the data stock acquired through literature searches

Nano-Ag Nano-ZnO Nano-CuO Total

No. of data points 290 100 63 453No. of articles 42 24 14 80No. of data points

per ecosystemFreshwater 269 72 58 399Seawater 13 8 3 24Terrestrial 8 20 2 30

No. data points per unitAlgae 22 9 12 43Annelida 3 6 0 9Arthropoda 0 11 0 11Bacteria 78 36 7 121Crustacea 91 18 32 141Fish 76 5 0 81Nematoda 8 5 0 13Plant 9 2 5 16Protozoa 3 8 5 16Rotatoria 0 0 2 2

Nano-Ag ¼ nano-silver; nano-ZnO ¼ nano-zinc oxide; nano-CuO ¼ nano-copper oxide.

Are nanosized or dissolved metals more ecotoxic? Environ Toxicol Chem 33, 2014 2735

Because of the complexity and uncertainty of transformationsin the seawater compartment, we restricted ourselves to thefreshwater and terrestrial compartments for the current evalua-tion. In these 2 systems, only a small fraction of ratios was higherthan 1, indicating that nano-specific effect may occur but thattheir influence on the overall toxicity is small compared with thedissolved metal ion. We evaluated the studies with high toxicityratios in detail to decipher any common features between them.We checked for similarities between sensitive parameters of thestudies, considering particle size, particle coating/capping agent,types of ecosystems (e.g., wastewater, river, lake), presence/

absence of natural organic matter, test organism, feeding type ofthe test organism, trophic level of the test organism, exposureduration, and the year the study was published. None of theseparameters alone was able to explain why the studies whose Rvalues were higher than 1 differed from the bulk of the examples.At present, we cannot provide any explanation as to why acertain fraction of the studies showed an overall small nano-effect in terms of toxicity.

Particle size is often discussed as one of the most importantproperties in the discussion about nanomaterial fate andbehavior [22]. Surprisingly, our results do not provide evidence

Figure 1. Ratio of the dissolved metal toxicity to the nano-form toxicity (R) based on median lethal concentration (LC50) or half-maximal effective concentration(EC50) values for Ag, ZnO, and CuO versus the number of data points sorted by increasing size. The data points for all environmental compartments are shown inthe top row, each of the environmental compartments (freshwater, marinewater, terrestrial compartment) is shown separately in the central row, and the bottom rowdepicts each group of test organisms separately. Arrows indicate the data points where R increases above 1. The associated number indicates the percentage of datapoints for which R> 1.


that there is a correlation between the calculated toxicity ratioand the reported particle size (Figure 2) for any of the metalsstudied here. In only 1 instance, for Ag particles smaller than50 nm, there was a slight tendency for R values to rise above 1;but this does not afford us enough conclusive proof to make abroader claim on particle size effects. Some studies testedtoxicity by covarying particle size and other nanomaterialproperties, such as surface coatings. In these cases, theconclusion was often that dissolution of the particles may bethe toxic mechanism, the rate of which is driven by particle sizeand surface area. However, dissolution depends not only onparticle size, but also on myriad other nanomaterial character-istics such as aggregation behavior, particle coating, and/orfunctional groups attached to the particle (or incidentalcoatings). Although we do not doubt that size is a veryimportant measure for nanomaterial characterization, thetoxicity of nanomaterials is complex and multidimensional.We explain the lack of correlation between nanosized particlesand toxicity in the present study by the fact that these othernanomaterial properties, alone or in tandem, are equally or moreimportant in determining toxic potential than particle size alone.In many cases, it appears that size is not the dominant factor

determining toxicity and, in fact, particle dissolution rates canbe strongly modified by such effects, which supports ourconclusion [23].

The results shown in Figure 2 suggest that consideration of asingle metric may not be a reliable approach for understandingnanomaterial toxicity. The combination of many differentproperties must be included, such as nanomaterial size andcoating, water chemistry, test compartment (i.e., water body orsoil), dispersion in the environment (e.g., agglomerationbehavior), and environmental conditions leading to metalspeciation. These combinations are more relevant as a groupthan a single metric alone. Our meta-analysis includes thisdynamic range of properties in the experimental setup.Consequently, our results, which cover a large span betweenextreme R values, illustrate how inmany cases the nanomaterialis much less toxic than the corresponding dissolved metal ion,but also, conversely, how in the worst-case scenarios theparticle can be much more toxic. In particular, the upper end ofthe toxicity ratios shows the worst-case scenario for nano-materials and their potential harm. Regulations based on thesevalues will ensure responsible handling of nanomaterials anddecrease the possibility of significant adverse effects. The

Figure 2. The ratio of the dissolved ion toxicity to the nano-form toxicity based on median lethal concentration (LC50) or half-maximal effective concentration(EC50) as a function of the reported particle size for Ag, ZnO, and CuO. Marine data are added for additional information.

Figure 3. The ratio of the dissolved ion toxicity to the nano-form toxicity (R) based onmedian lethal concentration (LC50) or half-maximal effective concentrationas a function of the EC/LC50 for the dissolved metal ion for Ag, ZnO, and CuO.


probability that environmental toxicity would exceed theworst-case toxicity on which the regulations could be basedis highly unlikely, essentially ensuring safety. Fortunately,the formulation of regulations based on the upper end ofthe toxicity ratios would encompass thresholds for metalemissions that are already in place. Hence, the nanotechnologyindustry may not require any special considerations comparedwith other industries that release metallic emissions to theenvironment.

Both the scientific community and regulators appear to bemoving toward an approach that would include severaldifferent safety measures based on chemical and/or physicalproperties of the material. However, it remains uncertainwhether a unique set of parameters can be identified forregulatory purposes. This is especially true in terms of thefast development of new nanomaterial production processesand new coatings or surface characteristics, all of whichmay change the dissolution rate or agglomeration behavior inthe environment. Furthermore, adopting the approach ofconsidering individual characteristics of nanomaterials on acase-by-case basis may not be pragmatic enough to translateinto regulatory action. The time and effort required for aregulation to be put in place, in addition to the precautions thatindustries would need to take to comply with the regulation,will be highly complex and demanding in the atmosphere ofrapid change and development that currently characterizes thenanotechnology field.

We now propose that the ratios derived in the present studycould be used in the context of regulation, whereby using thefraction of toxicity values above 1, along with the maximumratio, one could relate the overall toxicity of the nanomaterial tothose of the dissolved metals on which the existing thresholdsand limits are currently based. In terms of freshwaterecosystems, our silver toxicity results reveal that 3 of 269data points (1.1%) have a toxicity ratio slightly above 2, with amaximum of 2.39. Similarly, we found 2 of 72 toxicity ratios forzinc above 2 (2.8%), with amaximum of 2.04. From the 58 ratioscalculated for copper toxicity, the highest value was 0.9. Thedata available for terrestrial ecotoxicity with respect to silver,zinc, and copper are much weaker than the data for freshwatertoxicity. However, the highest ratios found for the 3 metals were1.26 (silver), 1.22 (zinc), and 0.09 (copper). Based on ouranalysis, we suggest that decision-makers could apply a nano-factor of 2 for the existing thresholds of the dissolved speciesemissions to regulate the amount of nanomaterial released. Thisproposed factor could withstand emissions of all 3 metalsinvestigated to both the freshwater and the terrestrial compart-ments considered in the detailed analysis. Applying a nano-factor of 2 for each metal and compartment, more than 98% ofthe results from the current literature were included in thethreshold. Only an extremely small fraction of studies had atoxicity ratio exceeding the suggested nano-factor of R¼ 2. Inaddition, not only the compartments in general but also all testorganisms investigated would be safeguarded by using thisnano-factor.

From the regulatory point of view, it is important that themost sensitive species be protected in an ecosystem. To ensurethat no single group of organisms—the most sensitive—isdisproportionately affected compared with other groups, wecorrelated the EC50 values of the dissolved metal with thecalculated R values (Figure 3). In case of disproportional effectsfor the most sensitive species, we would find a negativecorrelation of low EC50 with high R values. We could not findsuch a correlation. Those R values that were greater than 1 were

found over the whole range of EC50 values (�6 orders ofmagnitude for nano-Ag and 5 orders of magnitude for nano-ZnO). In contrast, it rather seems that there is a tendency for theleast sensitive organisms (high EC50) to have the highest Rvalues.

The approach we have presented is applicable for nano-materials composed of metals for which regulations for thedissolved/total metal already exist. Other materials for whichthis approach may work are quantum dots (e.g., cadmiumselenide) and metal oxides such as nickel oxide. For carbona-ceous materials (e.g., carbon nanotubes and fullerenes) that haveno dissolved counterpart, this type of analysis is not possible,and so another approach must be used such as, for example, onebased on species sensitivity distributions [6].

We believe that regulations for nanomaterial toxicity need tobe based on toxicity measures instead of specific nanomaterialcharacteristics. The goal of regulation is not to limit specificnanomaterials because of their properties but rather to protect theexposed population and ecosystems from adverse effects.Therefore, what metric would fit better than the one based ontoxicity metrics?

CONCLUSIONS

The utility of the present approach is in its simplicity andapplicability to many varied situations. However, the presentstudy does not allow one to determine whether the overalltoxicity of a nanoparticle depends solely on the metal toxicity orwhether it is a combination of several particle properties. Thisapproach is not designed to differentiate between physicalparticle properties (agglomeration, shape, etc.) or chemicalproperties (coatings, capping, etc.) causing toxicity. In addition,it is not possible to identify other determinants of toxicity such asbioavailability in the specific environment and for particularspecies, or to investigate physiological mechanisms that causetoxicity.

Ourmeta-analysis has produced 3major conclusions. First, inmost cases nanomaterials are far less toxic than the inherenttoxicity of the dissolved metal. This proves true for freshwaterand terrestrial ecosystems, irrespective of the test organisms,water chemistry, or particle properties. Second, applying thesuggested nano-factor of 2 for regulatory issues representsworst-case scenarios and ensures threshold limits that aredefensibly precautious and safe. Third, the approach we presenttakes advantage of the heterogeneity of many study designs andso in effect takes into account the most variables possible indetermining nanomaterial toxicity. This holistic approach mayprovide the most complete picture of the current state ofknowledge concerning nanomaterial toxicity and may be asuitable and pragmatic foundation on which to base the firstregulations for nanomaterial emissions.

SUPPLEMENTAL DATA

Tables S1–S4. (1.1 MB PDF)

Acknowledgment—The present study was funded by the European UnionSeventh Framework Programme (FP7/2007-2013) under grant agreement263289 (LICARA). B. Nowack developed the study design. B. Nowack, andD. Notter collected the literature. D. Notter analyzed and evaluated theliterature. All authors cowrote the manuscript.

REFERENCES

1. Kahru A, Ivask A. 2013. Mapping the dawn of nanoecotoxicologicalresearch. Acc Chem Res 46:823–833.


2. Krug HF, Wick P. 2011. Nanotoxicology: An interdisciplinarychallenge. Angew Chem Int Ed Engl 50:1260–1278.

3. Nel A, Xia T, Meng H, Wang X, Lin S, Ji Z, Zhang H. 2013.Nanomaterial toxicity testing in the 21st century: Use of a predictivetoxicological approach and high-throughput screening. Acc Chem Res46:607–621.

4. Aschberger K, Micheletti C, Sokull-Kluttgen B, Christensen FM. 2011.Analysis of currently available data for characterising the risk ofengineered nanomaterials to the environment and human health—Lessons learned from four case studies. Environ Int 37:1143–1156.

5. Batley GE, Kirby JK, McLaughlin MJ. 2013. Fate and risks ofnanomaterials in aquatic and terrestrial environments. Acc Chem Res46:854–862.

6. Gottschalk F, Kost E, Nowack B. 2013. Engineered nanomaterials inwater and soils: A risk quantification based on probabilistic exposure andeffect modeling. Environ Toxicol Chem 32:1278–1287.

7. Handy RD, Cornelis G, Fernandes T, Tsyusko O, Decho A, Sabo-Attwood T, Metcalfe C, Steevens JA, Klaine SJ, Koelmans AA, HorneN. 2012. Ecotoxicity test methods for engineered nanomaterials:Practical experiences and recommendations from the bench. EnvironToxicol Chem 31:15–31.

8. Zhu M, Nie G, Meng H, Xia T, Nel A, Zhao Y. 2013. Physicochemicalproperties determine nanomaterial cellular uptake, transport, and fate.Acc Chem Res 46:622–631.

9. Bondarenko O, Juganson K, Ivask A, Kasemets K, Mortimer M, KahruA. 2013. Toxicity of Ag, CuO and ZnO nanoparticles to selectedenvironmentally relevant test organisms andmammalian cells in vitro: Acritical review. Arch Toxicol 87:1181–1200.

10. Nowack B, Mueller NC, Krug HF, Wick P. 2014. How to considerengineered nanomaterials in major accident regulations? Environ SciEur 26:2.

11. Hansen SF, Baun A. 2012. When enough is enough. Nat Nanotechnol7:409–411.

12. Malloy TF. 2011. Nanotechnology regulation: A study in claimsmaking. ACS Nano 5:5–12.

13. Beaudrie CEH, Kandlikar M, Satterfield T. 2013. From cradle-to-graveat the nanoscale: Gaps in U.S. regulatory oversight along thenanomaterial life cycle. Environ Sci Technol 47:5524–5534.

14. United Nations. 2013. Globally Harmonized Systems of Classificationand Labelling of Chemicals (GHS), 5th ed. Geneva, Switzerland.

15. European Commission. 2008. Regulation (EC) no 1272/2008 of theEuropean Parliament and of the council of 16 December 2008 onclassification, labelling and packaging of substances and mixtures.Official J Eur Union L353:1–1355.

16. Misra SK, Dybowska A, Berhanu D, Luoma SN, Valsami-Jones E.2012. The complexity of nanoparticle dissolution and its importance innanotoxicological studies. Sci Total Environ 438:225–232.

17. Ma HB, Williams PL, Diamond SA. 2013. Ecotoxicity of manufacturedZnO nanoparticles—A review. Environ Pollut 172:76–85.

18. Shaw BJ, Handy RD. 2011. Physiological effects of nanoparticles onfish: A comparison of nanometals versus metal ions. Environ Int37:1083–1097.

19. Ferguson EA, Hogstrand C. 1998. Acute silver toxicity to seawater-acclimated rainbow trout: Influence of salinity on toxicity and silverspeciation. Environ Toxicol Chem 17:589–593.

20. Ratte HT. 1999. Bioaccumulation and toxicity of silver compounds: Areview. Environ Toxicol Chem 18:89–108.

21. Macken A, Byrne HJ, Thomas KV. 2012. Effects of salinity on thetoxicity of ionic silver and Ag-PVP nanoparticles to Tisbe battagliai andCeramium tenuicorne. Ecotox Environ Safe 86:101–110.

22. Auffan M, Rose J, Bottero J-Y, Lowry GV, Jolivet J-P, Wiesner MR.2009. Towards a definition of inorganic nanoparticles from anenvironmental, health and safety perspective. Nat Nano 4:634–641.

23. Lowry GV, Gregory KB, Apte SC, Lead JR. 2012. Transformationsof nanomaterials in the environment. Environ Sci Technol 46:6893–6899.


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