Natural and artificial substances in the atmosphere that affect human health or well-being, or the well-being of any other organisms in the environment. Air pollution also applies to situations in which contaminants affect structures and artifacts or esthetic sensibilities (such as visibility or smell). Most artificial impurities are injected into the atmosphere at or near the Earth's surface (see illustration). The lower atmosphere (troposphere) cleanses itself of some of these pollutants in a few hours or days as the larger particles settle to the surface and soluble gases and particles encounter precipitation or are removed through contact with surface objects. Removal of some particles (such as sulfates and nitrates) by precipitation and dry deposition results in acid deposition, which may cause serious environmental damage. Also, mixing of the pollutants into the upper atmosphere may dilute the concentrations near the Earth's surface, but can cause long-term changes in the chemistry of the upper atmosphere, including the ozone layer. See also: Atmosphere; Troposphere

The process for selecting an air-pollution monitoring site. Air pollutants including particulate matter, sulfur dioxide, nitrogen oxides, volatile organic compounds (precursors of ground-level ozone), carbon monoxide, and lead, and toxic air pollutants pose global concerns in terms of health. This article will examine air-pollution monitoring site selection from a United States' perspective with broad application to the process around the world. Historically in the United States, the Clean Air Act passed in 1963 was concerned only with "criteria" air pollutants, the six common air pollutants for which the U.S. Environmental Protection Agency (EPA) had set standards (listed above as the first six examples of air pollutants). In 1990, the Clean Air Act was amended to address 188 chemical classes of air toxics. Mobile source air toxics (MSATs) are emitted by vehicles either directly from exhaust systems or indirectly, such as from re-entrainment of particle matter from roads (Fig. 1). Addressing MSATs requires the combined expertise of engineering (for example, civil and mechanical engineering as applied to highway design and vehicle performance, respectively), the physical sciences (for example, particulate and gas phase partitioning of chemical compounds), and the social sciences (for example, decision theory as applied to selecting sites for representative samples from which to infer possible exposures). See also: Air pollution

A natural process in which harmful or beneficial effects are caused by secondary metabolites that spread from a donor organism to a recipient and are produced by plants, algae, bacteria, and fungi. Allelopathy is a key ecological process and has been studied predominantly in plants (Fig. 1). The chemical compounds involved in allelopathy are referred to as allelochemicals and comprise almost all classes of organic chemical substances. Hans Molisch (1937) coined the term allelopathy from the Greek words allelon for mutual and pathos for harm or affection, based on his observation of the premature ripening of apples and pears that were stored together with fruits from early ripening varieties. Depending on the purpose, the early ripening effect could be regarded as beneficial or harmful. Historically, detrimental effects have made botanists aware of allelopathy. One of the first accounts of an allelopathic effect that is commonly observed, namely the zone of growth inhibition around walnut trees (Juglans species), was reported by Pliny (23–79 CE). Even earlier, Theophrastus (372–287 BCE) described allelopathic effects of weeds on crop plants, including the inhibition of growth of alfalfa (Medicago sativa) by pigweed (Amaranthus retroflexus). See also: Agricultural science (plant); Agricultural soil and crop practices; Agriculture; Botany; Chemical ecology; Plant growth

A chemical element, chemical symbol As, atomic number 33, and atomic weight 74.922. Arsenic is one of the 21 chemical elements represented in the periodic table (Fig. 1) that is composed of only one stable nuclide. There are 32 other radioactive arsenic nuclides known. Arsenic and its compounds are highly toxic, which has led to regulations limiting exposure; for example, in the United States, the maximum permitted level of arsenic in drinking water is 10 parts per billion. Exposure to arsenic causes symptoms including stomach pain, nausea, vomiting, diarrhea, numbness, partial paralysis (particuarly in hands and feet), blindness, and thickening and discoloration of the skin. Long-term low-level arsenic exposure can contribute to the development of cancer, heart disease, lung disease, and diabetes. See also: Atomic number; Atomic mass; Cancer; Chemical element; Diabetes; Heart disorders; Isotope; Lung; Medical geology; Nuclide; Toxicology

Frameworks for assessing risk prior to a complete understanding of nanomaterial behavior. Nanotechnology has been and is being used in exciting and potentially revolutionary applications, ranging from sunscreens and drug delivery to more efficient solar cells and the production of stronger and lighter tires. As nanomaterials are produced and incorporated into products and processes, the materials will inevitably enter the environment and come in contact with living organisms (Fig. 1). Nanomaterials are advantageous for new or improved applications; however, the mayerials may interact with organisms and the environment in unexpected ways. Some of these unpredictable impacts will be positive, and some will be potentially harmful. Regulators, companies, researchers, and the public agree that methods for assessing the relative safety of nanomaterials are needed immediately. The potential for negative impacts must be addressed if positive gains are to be realized. See also: Nanotechnology

The study of behavior change in response to drugs, chemicals, or other environmental conditions or agents. Behavioral toxicology investigates the effect of toxic exposure on organismal behavior. The toxic exposure can involve various chemicals, drugs, or other environmental conditions or agents, which are collectively referred to as toxicants or toxins. Importantly, changes in behavior, as well as changes in sensation, mood, intellectual function, and motor coordination, are used by behavioral toxicologists to identify risks associated with exposure to potential toxicants and to determine mechanisms by which toxicants can affect the central nervous system (see figure). The information gained through behavioral toxicology can be used by scientists and government agencies to set limits in permissible levels of exposure to an environmental toxicant. Techniques employed in behavioral toxicology include epidemiologic surveys, field-based research, and laboratory-based experiments using human or nonhuman subjects. See also: Central nervous system; Environmental toxicology; Forensic toxicology; Nervous system disorders; Toxicology; Toxin

Identifying exposure sources, routes, frequency, duration, and magnitude using biomarker data combined with other approaches. The goal of environmental health science is to understand the interplay between the environment and humans to evaluate the effects of human activities on the public health and environment and, conversely, to evaluate the effects of various aspects of the environment on human health. When investigating the effects that exposures to chemicals have on human health, the major challenge lies in establishing the causal relationship between the magnitude of exposure to these chemicals and the incidence of adverse outcomes (such as cancer and irritation) at various biological endpoints. This causal relationship can be established only when all elements on the source–exposure–dose–effect continuum are linked (Fig. 1).

The analysis of contact between humans and chemical substances, such as through ingestion, dermal exposure, injection, or inhalation, and the study of how these interactions affect human health. Chemical exposure is an inherent part of modern life, as people encounter a vast array of chemicals in their everyday activities. Chemicals are pervasive in air, water, and products humans use (Fig. 1). Although many different chemicals are essential for technological advancement and improving quality of life, concerns about potential consequences of chemical exposure associated with human health are valid. Understanding the science of chemical exposure involves studying the sources, routes, and mechanisms by which chemicals enter and interact with living organisms—particularly the human body—and how these interactions affect health over a lifetime.

The application of computational biology, using mathematical and computer models, to assess the risk chemicals pose to human health and the environment and to better understand the mechanism through which given chemicals induce harm. Hundreds of thousands of chemicals in current or past use are present in the environment, leaving human populations and ecosystems potentially at risk of exposure to them. The large number and various forms of chemicals preclude regulators from evaluating every chemical with the most rigorous testing strategies. Instead, standard toxicity tests have been limited to a relatively small percentage of chemicals, with the hope that the "worst" chemicals have received specific attention. The chemicals that are tested may represent large classes of compounds, such as certain types of pesticides. Advances in computational biology have helped scientists develop a more detailed understanding of the risks posed by a larger number of chemicals. See also: Environmental toxicology; Toxicology

The use of data analytics and informatics-based approaches to discriminate between potential environmental contaminants and noncontaminants. New and emerging environmental contaminants are chemicals that have not been previously detected or that are being detected at levels significantly different from those expected in both biological and ecological arenas (that is, human, wildlife, and environment). Many chemicals can originate from a variety of sources, including consumer, agriculture, and industry as well as natural and/or anthropogenic disaster scenarios. For example, endocrine-disrupting chemicals (EDCs), pharmaceuticals, and personal care products (such as therapeutic, nontherapeutic, and veterinary drugs, as well as cosmetics and fragrances) are known to be present in many of the world's water bodies and thought to originate from a variety of sources, including improper disposal into municipal sewage, agribusiness, and veterinary practices. The detection and quantification of these chemicals from a toxicology and exposure perspective is paramount to understanding their effects on both the ecosystem and human health. EDCs act on the endocrine system and are known to alter sexual development and fertility in many vertebrate species. It is suspected that they may play a role in species population decline as well as public health issues. See also: Environmental toxicology; Toxicology