Paradigms to assess the environmental impact of manufactured nanomaterials

Visualize printing all 24 volumes of the Encyclopaedia Britannica on the head of a pin. In 1959, Richard Feynman articulated this reality in an insightful address at the annual meeting of the American Physical Society. In what became a prophetic speech, “There's plenty of room at the bottom” 1, Feynman discussed manipulating and controlling matter on a small scale. Back then, forward thinking conjured images of going to the moon in an era when computers occupied entire floors of buildings. Fifty years later, we no longer have to imagine. We are actively manipulating and controlling materials and devices on the scale of nanometers. With these advances, researchers have heralded both positive and negative effects of nanotechnology. Advocates point to efficient energy consumption, a cleaner environment, and eradicating health problems. Others have noted we do not know enough about how nanomaterials function, how they add potential stressors to the environment, or what chemical reactions may result when nanomaterials meet other particles. Both groups have called for further debate and advanced research into nanotechnology to determine the balance between risks and benefits. The ability to build products inexpensively with almost every atom in the right place holds tremendous promise for advances in virtually every sector of society. For example, smart drugs will deliver medicine only to the cells that need it; strong yet light materials will be used for automobile bumpers, airplanes, and tennis racquets; and tiny reactive particles will clean water at a fraction of previous costs. Although these benefits are exciting to scientists, they are often mysterious to the general public. One unknown involves the safety of these emerging materials, often called engineered nanomaterials. Scientists characterizing the environmental, health, and safety of nanomaterials (Nano EHS) are obligated to produce reliable data on which quantitative risk assessments can be built. In the past two decades, scientists have advanced this area, learned from mistakes, and realized that tools developed for working with substances dissolved in a solution cannot a priori be applied to particles. Although life on earth evolved in the presence of natural nanomaterials (including carbon, cellulose, and nanosilver), engineered nanomaterials—those produced for a specific purpose—may be identical to natural nanomaterials, but pose potential hazards due to significantly elevated environmental concentrations. In many instances, the question may be how engineered nanomaterials differ from natural nanomaterials. It is precisely this difference that may define their potential for adverse effects. Against this background, this article seeks to answer several questions: Where does the science need to provide reliable data that will assist policymakers and regulators develop strategies to manage nanomaterials and instill public confidence regarding the safety of these materials? What are the critical needs that will move us forward safely and intelligently in this promising field? Are the paradigms generally developed to assess the fate and effects of solute contaminants applicable to nanomaterials? We propose a way to answer these questions and move Nano EHS forward, creating a new framework for detecting, determining the fate, characterizing the hazards, and assessing the risk of engineered nanomaterials. To understand why and how these frameworks are relevant, we must first look at what nanotechnology is, examine the current state of the field, and highlight the issues inherent in studying nanotechnology. Nanomaterials Among Us: Their Risks and Benefits Tipping the scale: The relative size of materials in our world. Left to right: Water molecule (H20); Citrate stabilized gold sphere; Buckminster fullerene (C60); DNA; multiwalled carbon nanotube; red blood cells; penny; tennis ball. Nanomaterials, the physical structures that make nanotechnology possible, include all materials that contain nanoscale structures. The nanoscale is 1 to 100 nm—one billionth of a meter. At this size, the characteristics of materials change: their strength, conductivity, and reactivity differ substantially from macro- or micron-sized materials, shifting the ordinary rules of physics and chemistry to the sidelines. Although it is difficult to imagine something one cannot actually see, nanomaterials are the smallest increments of the physical world and are at a similar scale to the biomolecules key to crucial life functions. The typical distance between two carbon atoms in a molecule is 0.12 to 0.15 nm; the diameter of a DNA double helix is about 2 nm; the diameter of Buckminster fullerene, also known as C60, is about 5 nm. The similarity in size between these natural biomolecules and manufactured nanomaterials translates into an increased potential for manufactured materials to interfere with biological processes. This could include the behavior of cell membranes, biochemical pathways in cells, or even the genetic code itself. Let us put nanoscale dimensions into perspective. Imagine shrinking the moon to the size of a tennis ball. This is the same as shrinking the tennis ball to the size of a Buckminster fullerene molecule (Fig. 1). As materials are “nano-sized,” physical phenomena change from those of the bulk materials (adding as little as 2% by volume of silicate [common clay mineral] nanoparticles to a polyimide resin increases its strength by 100%). These phenomena do not appear as macro materials are reduced to microns, but begin to show up when they shrink to the nanometer range. In nanometers, the surface-to-volume ratios become huge (see Particle Size Versus Surface Area). As a result, unexpected mechanical properties may emerge, and catalytic activity may become apparent. For example, carbon nanotubes are elongated structures composed entirely of carbon that behave in a completely different way than do traditional pure carbon materials such as graphite. Carbon nanotubes have extraordinary tensile strength, with carbon nanotube fibers spun from individual nanotubes being strong yet lightweight enough to be used in bulletproof materials. New generation carbon nanotube fibers have a strength-to-weight ratio about 117 times that of steel 3, making them one of the strongest materials known to humankind. The unique characteristics of nanomaterials bring a new dimension to environmental effects testing. The physical and chemical properties that make nanomaterials unique also equip them with the potential to affect humans and the environment adversely. The physical form of the material matters, which is equally as important as its chemical reactivity. Indeed, the structure and reactivity of nanomaterials are linked inextricably. For example, mechanistic logic on reactive oxygen chemistry would suggest that modifying the surface of a nanomaterial to make it photoreactive to facilitate the oxidation of microorganisms might also increase the potential for oxidative injury to organisms (such as TiO2 nanoparticles in fish 4). Despite existing laboratory research that helps us understand nanomaterial fate, our ability to estimate exposure concentrations is poor, which results in high levels of uncertainty in estimating risk. Without trustworthy information from scientific or regulatory sources, the public may be more likely to believe the emerging alarmist literature on nanomaterial hazards (http://www.foe.org/pdf/Nano_Sunscreens.pdf). The two components of risk, hazard and exposure, seek to identify the innate harm a particular substance may present and the potential for an organism to encounter that material. In traditional scientific inquiry, assessing risk requires linking physical and chemical behavior to effects on the human body, other organisms, and the environment 5. Our inability to develop quantitative risk assessments for nanomaterials with acceptable levels of uncertainty stems from the uncertainty inherent in our current estimates of exposure and toxicity. Furthermore, few toxicity studies have quantified chronic effects or the consequences of bioaccumulation. Unfortunately, assessing the hazard associated with nanomaterial exposure and characterizing risks have not kept pace with nanotechnology advances. One reason for this gap is that research generally requires limited physical and chemical characterization, and perhaps only modest effort on toxicological assessment to answer the immediate research question related to the innovation. Regulatory bodies, in comparison, often need large and robust datasets. Specifically, a primary driver of this information gap is funding. According to the National Nanotechnology Initiative (www.nano.gov), between 2005 and 2011, almost $10 billion will have been spent in the United States on nanotechnology research. During that same period, only $480 million (less than 5%) will be spent to understand the environmental, health, and safety issues related to developed nanotechnologies. In addition, new materials that are not used as pesticides or food additives fall under the regulatory purview of the Toxic Substances Control Act. This act does not require a robust toxicological dataset for materials registration. Hence, emerging materials such as these are being used within society with a minimal hazard characterization. This lack of investment and lack of regulatory incentive has resulted in both humans and ecosystems being exposed to nanomaterials without an accurate picture of the probability that the nanomaterial used may cause harm. In the last few years, the urgency to understand these effects has produced a growing body of research on human and environmental hazard and exposure 6-8. While research studies to characterize the risk of nanomaterials to humans have generally focused on occupational exposure and inhalation toxicology, risk to the ecosystem presents a more diverse challenge: to engage physicists and material scientists in understanding particle structure, environmental engineers to quantify particle release, chemists to understand behavior, and biologists and toxicologists to quantify bioavailability and toxicity in different trophic levels. Ultimately, quantifying ecological risk will also complement assessing the effort to quantify human risk. The very real possibility exists that nanomaterials may move through foodwebs and culminate in human exposure 9. The infancy of this research and the unique physical and chemical characteristics of nanomaterials compound the challenge regulators and industry face in understanding and managing potential risk from nanomaterials. Although some may believe human health is more important than environmental effects, the intimate link between the two and the intrinsic protection value of ecosystems and their functions call for including both in assessing risk. To that end, ecotoxicologists have used standardized testing methodology and standard test organisms in the rush to understand the effects of nanomaterial exposure. The result? Ecotoxicologists overwhelmingly treat nanomaterials in the same way they have considered traditional chemicals for more than 40 years. They use the same bioassays and endpoints and assume that nanomaterials behave the same as soluble contaminants. Quantifying the behavior and effect of nanomaterials in organisms and environmental media, however, has proven to be more difficult than previously experienced with soluble chemicals. Many of those methods are proving inadequate and do not adapt well to the unique properties of nanomaterials. The scale of the task to completely characterize hundreds of materials through dozens of methods is as vast and growing as rapidly as the number and types of nanomaterials increase. Moreover, the diffuse disciplinary areas and the rapid pace of nanomaterial risk research create challenges in providing a clear snapshot of the current state of science and paths for revising methodologies. In a nutshell, here is the problem: assessing the risks of nanotech advancements is way behind developing those advancements. This gap exists for three reasons. First, the scientific research required to develop nanotechnology does not yield adequate data to assess the risks of those products. Second, scientific research data are not adequate for risk assessments because the regulatory agencies require much more robust data sets (50 endpoints vs 2 lab-model endpoints). Third and finally, research on the environmental health and safety of nanomaterials receives less than 5% of the funding spent to develop new nanomaterials. So, this raises the question: What can we do? We can start to address the above situation by understanding the environmental effects of nanomaterials, which requires detailed knowledge of both the fate and behavior of the materials in the environment and biological effects (hazards). Critically, it is vital to understand the interplay between environmental chemistry and ecotoxicology so that effects on the ecosystem can be understood or predicted. We must not lose sight of the overall goal of sustaining biodiversity and ecosystem functions while allowing for innovations in nanotechnology. During the past decade, nanomaterial fate and transport have become a major focus of research. Several review papers have highlighted the way environmental factors such as calcium concentration, pH, and the effects of organic ligands will alter the colloid behavior of nanomaterials and influence their ultimate fate 6-10. The scientific community is on a steep learning curve, however, but has the benefit of hindsight. Much of the peer-reviewed literature published in the early 2000s lacked adequate experimental controls or appropriate methodology simply because it was not clear what facets of experimental design would be the most important for studying nanomaterials. Nanomaterial characterization—describing the physical and chemical characteristics that drive and affect particle behavior—has been inadequate or incomplete. Quantitative exposure metrics that describe precisely how experiments were performed to isolate the causal effects were insufficient. More recent publications have attempted to overcome these inadequacies. While progress in this area is evident, research in this field is hampered by little experience with particles as contaminants. For example, until now, ecotoxicologists considered only soluble forms to be bioavailable, whereas insoluble forms were considered nonbioavailable and therefore nontoxic. The lack of standardized approaches to characterize the ecological effects of particles, uncertainty regarding how information regarding exposure and effects can be incorporated into ecological risk assessment, and concerns about the inability to tie adverse effects causally to specific nanomaterial properties will result in the need to test each of the exponentially increasing number of nanomaterials. Even under the most optimistic assumptions, the cost and time to evaluate the risk associated with just nanomaterials currently being used is huge (see discussion in Choi et al. 11). Particle Size Versus Surface Area A key characteristic surrounding the concern of nanomaterials with respect to ecological and human health is the extremely high surface area to volume ratio. This illustration and table depict changes in the physical characteristics of gold particles at a constant mass of 1 µg as they are subdivided to approach, and reach, the nanoscale: (A) 1 particle, (B) 10 particles, (C) 100 particles, and (D) 1,000 particles all of which have a combined mass of 1 µg. At a constant mass, there is a three-order-of-magnitude change in particle diameter for this conceptual model to reach a nanomaterial. More important, however, is that the total number of particles that represent this decrease in particle diameter increase nine orders of magnitude. As a result, total surface area of the depicted mass of particles increases three orders of magnitude. Concern about this phenomenon emanates from the fact that while the same amount of elemental material may enter a given system, contaminants on the nanoscale are greater in number, may be translocated to more and different locations, and have the potential to deliver a higher exposure of complex material attached to their surface. Clearing this hurdle is not simply a matter of understanding and solving the new scientific problems that engineered nanomaterials pose. The infrastructure for environmental science also needs to be adapted. Clearly nanoscience is a worldwide concern and interdisciplinary collaboration. Several countries have national initiatives investing in nanotechnology environmental health and safety. These include the National Nanotechnology Initiative and the European Union's Seventh Framework Programme on nanotechnology (http://cordis.europa.eu/fp7/home_en.html). Both of these efforts are multidisciplinary. In the United States, the National Nanotechnology Initiative is coordinated by the White House and involves 25 federal agencies. Although more than 60 nations have similar initiatives, coordination is needed worldwide to invest limited resources and make the best use of the data generated by subsequent research. Many nations are cooperating to build scientific understanding and develop these materials responsibly through international forums such as the Organisation for Economic Co-operation and Development and the International Organization for Standardization. These international activities are focused on developing common global standards, definitions, and test methods, and on providing a better understanding of the physical and chemical properties of nanomaterials that could affect human health or the environment. At the institutional level, it is no longer sufficient for toxicologists to work with chemists and biochemists to explore toxicant bioavailability and modes of action. Rather, to understand the impacts of nanomaterials this team must include, among others, material scientists who understand the synthesis, structure, and innate properties of each material; physicists who characterize nanomaterial behavior; and specialist areas of life sciences to understand how biomolecules can coat or modify nanomaterial surfaces (e.g., protein corona 12) (Fig. 2). Together, such an interdisciplinary team can develop the scientific foundation for quantitative models that reliably predict the repercussions of exposure to nanomaterials. Science overlay map for Nano EHS research. This map represents the scientific fields involved with Nano EHS research (for Methods, see Rafols et al. 19). Disciplines are positioned in the map near others with related or similar characteristics, whereas those disciplines that are dissimilar are positioned at distant locations. Sizes of the spheres are proportional to the percentage of publications from that particular scientific field. From this figure, we observe major subject concentrations in and we this map with a science overlay map for all of nanotechnology we that while both concentrations in and the of Science and Science become much more in Nano EHS research. from Rafols et al. research is also that the field of Nano EHS is a and field. et al. developed an that the among a given body of The more diverse or the subject within a given of the higher its This which from to 1, is in assessing the interdisciplinary of a field. While the for most science disciplines the for Nano EHS This that this science on the work of several of the of this science is the increasing in the number of and number of working in Nano EHS et al. that both the and for Nano EHS publications increases Furthermore, that the number of is increasing at a than the number of The need also exists to and a new generation of environmental scientists who have in physics and material not just and is crucial for understanding and developing every of material to and hazard and risk we understand these the scientific and are for managing the and its potential With increased scientists can paths To reach the ultimate goal of developing quantitative risk assessments that regulators can use with a high of several areas of Nano EHS must be The current for characterizing the fate and effects of contaminants be and to the unique challenges of nanomaterials. Nano EHS research including these materials in biological and environmental the environmental fate of assessing the hazard of these and developing quantitative risk assessments 1). chemistry is well to concentrations of traditional such as or organic For some of the organic their environmental concentrations are and chemists have developed complex methods to such organic chemicals. These however, have also the focus from the water to the organic chemicals such as or the of This has environmental chemists with tools to explore the fate and behavior of soluble chemicals in the environment. some of these approaches could also be to work with nanomaterials To and characterize nanomaterials, however, several challenges have from the properties of nanomaterials. First, the chemical used in the chemistry of traditional soluble needs to be to include physical This will chemists to for forms that are most in the of assessing the risk assessment of nanomaterials. many have been from colloid These however, are well for pure but not yet for nanomaterials in complex environmental natural or laboratory bring specific These challenges include the fact that natural are not at the potential of a natural environment is difficult to in a laboratory and nanomaterial state changes bioassays engineered nanomaterials from of similar but natural materials is also Furthermore, in often require methods to challenge or to nanomaterials in environmental It be however, that assessing effect concentrations may be sufficient at million and the of nanomaterials or their environmental life is a these challenges by the number of nanomaterials, nanomaterial and environmental and and the research effort to develop adequate chemistry the of toxicologists is crucial to efforts to the For example, one questions is nanomaterial surface particle be as an in nanomaterial effect that nanomaterials will behave than soluble chemicals in the environment is Although transport is almost entirely water or to a particle transport needs to for and and both as well as As with most contaminants in the environment, nanomaterials can be to up in ecosystems or from release, from or as a material within products. The fate of each of these nanomaterials will The and and nanomaterials are most to a among the research on environmental risks of nanomaterials is the use of nanomaterials. however, is growing that nanomaterials from a are often in the the environmental behavior of the nanomaterial is to a large by the properties of the between nanomaterials and those actually or are for fate in the environment, but information on this is More we have knowledge on the of nanomaterials that are actually the fate and effects of and nanomaterials, nanomaterial fate and time for nanomaterial or transport processes. These issues in the knowledge we have on the of nanomaterials use and into their products. further research needs to show the and result in a similar or more diverse of nanomaterials. For example, does a surface with a material the of the do nanomaterials with different behave being and in the key question is might require a specific risk assessment because of the these questions is most because it is not the but the and the nanomaterials that are actually present in the environment. in the nanomaterial behavior will not only but also which organisms are most likely to encounter The to specific be on understanding nanomaterial behavior in the Unfortunately, current testing methods developed for materials to characterize the hazard associated with nanomaterials has been inadequate The literature many standard bioassays with organisms have been used to characterize the hazard associated with a particular nanomaterial the fact that the vast of the particles of the water within a few of test other in current toxicity testing methods to evaluate nanomaterials. These include nanomaterial and characterization, appropriate and assessing the appropriate For example, several have that the toxicity of carbon nanotubes was not due to some biochemical the toxicity was due to the and with food and The of testing methods can be as a in the current of As assessing the hazards of nanomaterials forward, recent approaches that rapid and in testing results testing is important These approaches produce large of and high data and on to of toxicological from these datasets. The between results of both in and high efforts and testing must be Without this characterization, these efforts may resources while providing minimal to assessing the hazard of nanomaterials. Quantifying nanomaterial exposure and effects are challenges environmental assessment is as determining the probability that a will cause harm given particular use This is the the in science into action. risk with the of science that they with and information to produce to manage human activities and produce and use products. The challenge here to environmental scientists is, can we use current risk assessment paradigms to quantify the risk of nanomaterials? Our lack of