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This chapter describes the roles played by free radicals and other reactive species (RS) in toxicology, and whether antioxidants can protect. It begins with an overview of the metabolism of xenobiotics and the various mechanisms by which RS can contribute to their toxicity. Toxins whose mechanisms of action in relation to RS-induced damage are described include carbon tetrachloride (whose hepatotoxicity is due to trichloromethyl radical formation and consequent lipid peroxidation), chloroform, halothane, bromobenzene, bromotrichloromethane, pentachlorophenol, TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), hexachlorobenzene, paraquat, diquat, menadione, dicoumarol, juglone, plumbagin, lawsone, manganese, methcathione, isoproterenol, α‎-methylDOPA, benzene, aniline, pyocanin, alloxan, nitro- and azo-compounds, streptozotocin, ethanol, acrolein, allyl alcohol, cocaine, cannabis, Ecstasy, amphetamines, paracetamol, phenacetin, chlorine, ozone, sulphur dioxide, nitrogen dioxide, cigarette smoke, fire smoke, PM_2.5, welding fumes, fly ashes, asbestos (including the role of iron in its carcinogenicity), silica and coal dust, arsenic, lead, cobalt, mercury, nickel, cadmium, titanium, vanadium, chromium, aluminium, and zinc. The ability of antibiotics to be pro-oxidant (e.g. the polyenes, gentamicin) or antioxidant (e.g. the tetracyclines, especially minocycline) is described and related to their anti-bacterial actions. There is a discussion of the mechanisms by which ionizing radiation damages cells, and how radioprotective agents (especially GSH and other thiols) work. This is further discussed in relation to food irradiation, and radioresistant organisms (e.g. rotifers, Deinococcus radiodurans). The mechanism by which hypoxia decreases radiation damage is presented and related to radiotherapy for cancer treatment. The history of the toxic oil syndrome, which led to formation of the global Societies for Free Radical Research, is presented.

The potential for human activities to adversely affect the environment has become of increasing concern during the past three decades. As a result, the transport and fate of contaminants in subsurface systems has become one of the major research areas in the environmental/hydrological/earth sciences. An understanding of how contaminants move in the subsurface is required to address problems of characterizing and remediating soil and groundwater contaminated by chemicals associated with industrial and commercial operations, waste-disposal facilities, and agricultural production. Furthermore, such knowledge is needed for accurate risk assessments; for example, to evaluate the probability that contaminants associated with a chemical spill will reach an aquifer. Just as importantly, knowledge of contaminant transport and fate is necessary to design “pollution-prevention” strategies. A tremendous amount of research on the transport of solutes in porous media has been generated by several disciplines, including analytical chemistry (chromatography), chemical engineering, civil/environmental engineering, geology, hydrology, petroleum engineering, and soil science. This research includes the results of theoretical studies designed to pose and evaluate hypotheses, the results of experiments designed to test hypotheses and investigate processes, and the development and application of mathematical models useful for integrating theoretical and experimental results and for evaluating complex systems. While much of the previous research has focused on transport of nonreactive solutes, it is the transport of “reactive” solutes that is currently receiving increased attention. Reactive solutes are those subject to phase-transfer processes (e.g., sorption, precipitation/dissolution) and transformation reactions (e.g., biodegradalion). Of special interest in the field of contaminant transport is so-called nonideal transport. In the most general sense, nonideal transport can be described as transport behavior that deviates from the behavior that is predicted using a given set of assumptions. A homogeneous porous medium and linear, instantaneous phase transfers and transformation reactions are the most basic set of assumptions for ideal solute transport in porous media. As discussed in a recent review, transport of reactive contaminants is often nonideal (Brusseau, 1994). The potential causes of nonideal transport include rate-limited and nonlinear mass transfer and transformation reactions, as well as spatial (and temporal) variability of material properties.

Reactive (or “odd”) nitrogen is emitted into the atmosphere in a variety of forms, with the most important being NOx (NO and NO2), ammonia (NH3), and nitrous oxide (N2O). Emissions of these species into the atmosphere have been summarized, for example, by the IPCC Fourth Assessment Report (the AR4; IPCC, 2007). Some discussion of NOx emissions and trends has also been presented in Chapter I. Emissions of NOx are mainly the result of anthropogenic activity associated with fossil fuel combustion and industrial activity. For the 1990s, the AR4 estimates total anthropogenic NOx emissions of 33.4 TgN yr−1, with natural emissions (mostly from soil and lightning) accounting for an additional 8.4–13.7 TgN yr−1. Ammonia emissions are comparable in magnitude to those for NOx, with anthropogenic emissions (45.5 TgN yr−1) again exceeding natural emissions (10.6 TgN yr−1). Although the majority of the ammonia produces aerosols or is scavenged by aerosol and is subsequently lost from the atmosphere, some gas phase oxidation does occur, which can in part lead to NOx production. The N2O source strength is about 17.7 TgN yr−1, with natural sources outweighing anthropogenic ones (IPCC, 2007). However, N2O is essentially inert in the troposphere, and thus the vast majority of its photooxidation and concomitant NOx release occurs in the stratosphere. The major NOx − related reactions occurring in the Earth’s troposphere are summarized in Figure III-A-1. As just alluded to, the species NO and NO2 are jointly referred to as NOx and are often treated collectively. This is because, under daytime conditions, these two species are rapidly interconverted, with the interconversion occurring on a much shorter timescale than the loss of either species.