Nanotechnology and the Environment - Chapter 8

8 The Potential Ecological Hazard of Nanomaterials Stephen R. Clough Haley & Aldrich CONTENTS 8.1 Underlying Principles of Ecological Exposure, Effects, and “Risk” ..........170 8.1.1 Terrestrial vs. Aquatic Ecosystems ..................................................170 8.1.2 Risk and Hazard............................................................................... 171 8.1.3 Toxicity ............................................................................................ 171 8.1.4 Exposure ..........................................................................................173 8.2 Factors That Can Affect the Toxicology of Nanomaterials ........................ 174 8.2.1 Toxicity of Nanomaterials ................................................................ 174 8.2.2 Exposure to Nanomaterials ..............................................................177 8.2.2.1 Sources and Routes of Exposure ........................................177 8.2.2.2 Exposure and Dose .............................................................178 8.2.3 Summary ..........................................................................................179 8.3 Anticipated Hazards To Terrestrial Ecosystems .........................................179 8.4 Anticipated Hazards to Aquatic Ecosystems ..............................................180 8.4.1 Methodologies for Evaluating Hazards and their Limitations.........188 8.4.2 Discussion of Results .......................................................................189 8.5 Recommendations for Managing the Risks of Future Nanomaterials and their Production ....................................................................................190 References ..............................................................................................................190 Puzzles eventually have answers; mysteries, however, cannot. Unknowns or uncer-tainties preclude a def initive answer to a mystery [1]. A mystery “can only be framed, by identifying the critical factors and applying some sense of how they have inter-acted in the past and might interact in the future. A mystery is an attempt to define ambiguities” [1]. In its infancy, nanotechnology can seem mysterious to both the layperson and the scientist. Science now enables us to construct nanomaterials but, paradoxically, some generally accepted scientif ic principles do not appear to apply to their inherent biological activity. For example, a substance like gold that is physi-ologically inert at the microscale has been shown to have biological activity at the nanoscale [2]. This change, in effect, can result from the fact that a particle that is less than 100 nanometers (nm) in size can behave more according to the laws of 169 © 2009 by Taylor & Francis Group, LLC 170 Nanotechnology and the Environment quantum physics than Newtonian physics. As the science emerges, the mysteries of nanomaterials will become puzzles that will be solved. The scientif ic paradigms for nanotechnology may take much longer to decipher because conventional scientif ic methodologies, instrumentation, or principles may not apply in some of the upcom-ing studies. Many fear that regulations put into place to protect both the workplace and the environment will be too little, too late. Thischapterdiscussesoneofthemysteriessurroundingnanotechnologyandpres-ents data that scientists will ultimately use to solve the puzzle. It faces the question: “If a nanomaterial were to be released into the general environment, would it pose a significant risk to ecological organisms such as fish or wildlife?” The answer begins with some background information on how toxicologists assess impacts to fish and wildlife, referred to in ecological assessments as “ecological receptors.” 8.1 UNDERLYING PRINCIPLES OF ECOLOGICAL EXPOSURE, EFFECTS, AND “RISK” This section provides a brief primer on ecological risk assessment, to provide the reader with the context for discussing the potential hazards of nanomaterials. 8.1.1 TERRESTRIAL VS. AQUATIC ECOSYSTEMS Because of obvious differences in habitat, ecotoxicology comprises two main cat-egories of environmental assessment: (1) terrestrial and (2) aquatic. The former category addresses the impacts of chemicals released into the environment on ter-restrial species. Examples include invertebrates such as earthworms, bees, beetles, and grubs; birds, including doves, quail, robins, and hawks; reptiles, such as lizards and snakes; and mammals, such as shrews, mice, foxes, or bears. The latter category includes aquatic species, such as phytoplankton (e.g., single or multicellular algae), zooplankton (e.g., rotifers, cladocercans, paramecia), benthic invertebrates and insect larvae (e.g., mayflies, caddisflies, stoneflies) and fish (e.g., embryos, fry, juve-niles, or adults). Of course, some animals — for example, amphibians such as frogs, toads, and salamanders — may spend portions of their life cycle in both the aquatic and terrestrial environment. Organisms in a third category, semiaquatic receptors, strongly depend on waterbodies for food or sustenance. These semiaquatic recep-tors include fish-eating birds (e.g., kingfisher, heron, osprey, and eagle) or mammals whose habitat is primarily aquatic (e.g., beaver, muskrat, and otter). With the possible exception of some deserts, these different types of habitat are not mutually exclusive. The forces of the water cycle will strongly affect both the fate and the transport of contaminants within a terrestrial ecosystem. In addition, animal activity can affect markedly the landscape of a terrestrial ecosystem. The leg-trap-ping of beavers, for example, was once an accepted method in the United States to obtain their thick pelts. Many states, however, now view these traps as inhumane and have banned their use. Consequently, their populations are back on the rise and, as a © 2009 by Taylor & Francis Group, LLC The Potential Ecological Hazard of Nanomaterials 171 result, their natural impoundments are transforming once-dry forest land into large, productive wetlands. Because of the limited data available regarding the effects of nanomaterials on ecological receptors in the wild, this chapter first examines the underlying principles that must be in place for there to be a valid supposition that nanomaterials may even-tually pose a risk to any terrestrial, aquatic, or semiaquatic organisms/receptors. 8.1.2 RISK AND HAZARD Risk is generally def ined as the probability that a hazard will occur in a given time and space. It is virtually impossible to determine the probability that a chemical may pose a risk to an organism, population, or community in the wild. Thus, the term “ecological risk” is something of a misnomer. The term “hazard,” which is the likeli-hood that an adverse event can take place, better expresses the degree of harm to an ecological receptor. However, these terms often are used interchangeably. Risk (or hazard) is a function of toxicity and exposure. Unless an ecological receptor is exposed to a chemical or nanomaterial, there can be no risk or hazard. If exposure is great enough, substances that have a low inherent toxicity can still result in a toxic response. Paracelsus, known as the Father of Modern Toxicology, stated that “[a]ll substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy.” Thus, if enough of a substance of known (but low) toxicity is ingested, a hazard may exist. Although table sugar is classified as virtually non-toxic, eating too much cake or candy will result in nausea and/or vom-iting, a toxic response elicited by the over-consumption of sugar. The potential for harm also depends on the duration of exposure. Short-, medium-, and long-term contact with the material in question are referred to, respec-tively, as acute (single dose), subchronic (multiple exposures over 2 to 3 months), and chronic (greater than 3 months to a lifetime) exposures. Over time, some animals can become tolerant to some materials, or cross-tolerant to similar materials. A good example is the highly toxic metal cadmium. An acute exposure of an organism to the metal will impart tolerance or resistance to subsequent exposures due to the induc-tion of metal binding proteins by various tissues. 8.1.3 TOXICITY Ecological hazard assessments can focus on individuals or populations. Individual organisms can be exposed to nanomaterials via inhalation, dermal contact, and ingestion. Exposure pathways historically have been framed in the context of food webs that embody many different types of autotrophic and heterotrophic interac-tions. Persistent, bioaccumulative, and/or toxic substances (PBTs) will bioconcen-trate, bioaccumulate, and/or biomagnify in a food web. Scientists generally divide the evidence of ecological harm into two classes of effects criteria: (1) Assessment Endpoints and (2) Measurement Endpoints. They generally ascribe Assessment Endpoints to a less-tangible (or more subjective) value, such as “Will Chemical X, if released into the environment at Concentration Y, have an adverse effect on the population of predatory fish?” A Measurement Endpoint is a more specific, objective measurement at the individual or community level that © 2009 by Taylor & Francis Group, LLC 172 Nanotechnology and the Environment supports the evaluation of the Assessment Endpoint, such as: “What is the Concen-tration Y of Chemical X that will adversely affect 20% of a known population of rainbow trout in the laboratory?” The main endpoint for measuring ecological toxicology is the LD50, or the lethal dose required to kill 50% of the organisms under controlled laboratory testing conditions. For aquatic organisms, the LC50 and EC50(or the respective lethal con-centration and effect concentration required to kill or affect 50% of the organisms) are the more appropriate terms used for a toxicity endpoint. When dose is plotted versus response, the slope of the curve is a general indication of the potency of the toxicant: the steeper the slope, the more potent the toxicant relative to chemicals of a similar class. One can generalize about how these criteria will ref lect the relative toxicity of a substance based on its structure and the principle that like dissolves like. Because cellmembranesprimarilycomprisealipidbilayer,lipophilicorfat-lovingsubstances are, as a general rule of thumb, more toxic than hydrophilic or water-loving (soluble) substances. Lipophilic substances are more easily absorbed by inhalation, ingestion, or dermal exposure, and tend to have a longer half-life (i.e., the time required to reduce the body burden of a toxicant by one-half, either by metabolism or excre-tion), while water-soluble substances are more easily metabolized by the liver and/or excreted by the kidney and thus tend to have a shorter residence time in the body. In the f ield of inhalation toxicology, foreign matter is generally categorized as gas, vapor, or particulate (or fibrous) matter. The latter can affect physically the elas-ticity of the lung. Examples include silicosis in concrete and quarry workers, asbes-tosis in shipyard workers, and pneumoconiosis in coal miners. Nanoparticles would be classified as particulate matter, but because these particulates are so extraordi-narily small, they fall in a toxicological gray area. Some comprise potentially toxic elements that, if dissociated or dissolved, may cause adverse effects inside a cell. Therefore, they may cause adverse extracellular physical effects similar to those caused by larger fibers such as asbestos or fiberglass insulation, or may be actively or passively internalized by cells and cause toxic effects by interfering with cellular processes. Data from a battery of both in vitro and in vivo bioassays may be needed to reveal to the investigator the inherent toxicity of the various elements and com-pounds that comprise nanomaterials (for some of which there are little to no toxico-logical data). The difficulty will lie in separating whether an adverse effect reflects a physical effect induced by the nanomaterial or a direct toxic effect resulting from the composition of the material itself. For example, carbon black, a common nanomaterial in commercial use for decades, is considered biologically inert. Although it may remain in the body in a sequestered form, it is expected to have a low inherent toxicity [3]. In contrast, a unique nanomaterial constructed from one (or more) elements may be inherently toxic. Consider cadmium, a highly toxic metal used to make quantum dot alloys of cadmium selenide or cadmium telluride. Toxic effects on the reproductive sys-tem or the nervous system are of particular concern. The response of these sys-tems, in general, will take a longer time to unravel than other biological endpoints, because the endpoints take a long time to achieve, are expensive to characterize, or © 2009 by Taylor & Francis Group, LLC The Potential Ecological Hazard of Nanomaterials 173 the results are characteristically subtle, requiring innovative and/or very sensitive testing methodologies. The natural physiological variability within a population means that individuals may react differently upon exposure. The reasons given for this variability often are physiological, such as internal genetic differences, or environmental. The gender of an animal, the species, or its age can make a very signif icant difference in the response following exposure to a chemical or nanomaterial. Younger animals are generally more susceptible to toxicants than older animals, partly due to the fact that they weigh less and therefore, pound for pound, will get a larger dose than would an adult animal. Similarly, there are some strains of mice that are very resistant to the toxic effects of heavy metals, whereas other strains are overly sensitive. The results of these variations in sensitivity can be observed in the classic dose/response curve, which is typically an S-shaped function. Plotted on a graph, with the dose on the x-axis and the percent of organisms affected on the y-axis, the cause of the inf lections in the S-shaped curve are due to the presence of sensitive individuals in the low dose ranges and tolerant individuals in the high dose ranges. 8.1.4 EXPOSURE A complete exposure pathway must exist for an animal to be affected by a chemical or nanomaterial. This means that a mechanism must exist to transfer the compound or nanomaterial in question from the source in air, water, soil, or sediment to the receptor organism in question. Without exposure, there can be no risk. Therefore, and this is a critical factor as nanotechnology evolves, as long as nanomaterials are properly handled and/or contained, risk and/or hazard(s) will be negligible. Scientists use the term “fate and transport” to refer to processes that affect a sub-stance as it travels from the source to a potential receptor. As described in Chapter 6, various processes can change the nature and concentration of a nanomaterial, which, in turn, can change its potential to induce toxicity. Partitioning from one phase of media to another is an extremely important phe-nomenon that can affect the properties (and often the quantities) of a nanomaterial within an environmental medium. Partitioning typically is expressed in terms of a ratio or partition coefficient (e.g., water-to-sediment, soil-to-water, water-to-air, water-to-biota, etc.). For example, a bioconcentration factor (BCF) is the ratio of the concentration of a substance in f ish tissue to the concentration in a waterbody. Weathering, which includes the variety of chemical reactions and physical atten-uation processes that occur after a chemical is released into the environment, will generally decrease exposure, bioavailability, and/or toxicity. The exceptions to this are compounds or materials that resist degradation, such as mercurials or arsenicals, some types of commercial pesticides, polychlorinated dioxins and furans, and poly-chlorinated biphenyls, to name just a few examples. Another important underlying principle in ecological toxicology is the differ-ence between exposure and dose. An exposure is the sum total of a compound or nanomaterial thatreachesan ecological receptor,but the dose isa smallerpercentage of the total material that actually enters the body. Bioaccessibility and bioavailability © 2009 by Taylor & Francis Group, LLC ... - tailieumienphi.vn