Ozone exposures, discussion

Using a CO2 laser as excitation light source, applications involving ethylene demonstrate the course of events in natural processes like ripening and for species under stress conditions such as wounding, anaerobiosis, ozone exposure and flooding. This chapter ends with a discussion of a photoacoustic setup employing a CO laser instead of a CO2 laser in sensitive trace gas measurements, e.g. of methane and acetaldehyde. 

Safety, ethical, and legal considerations require that the utmost care be exercised in human experimentation. The risk inherent in this work can be minimized by the proper design of facilities for human exposure to reactive gases, such as ozone and sulfur dioxide, and reactive gas mixtures. Standards for the exposure of humans to such controlled atmospheres should be discussed by national groups and agencies, such as the American Medical Association and the National Institutes of Health. 

The major phytotoxic components of the photochemical oxidant system, discussed in Chapter 11, are ozone and peroxyacetylnitrate (PAN), but there is indirect evidence that other phytotoxicants are present. Con siderable effort has gone into controlled exposures to ozone and into field studies. Leaf stomata are the principal sites for ozone and PAN entry into plant tissue. Closed stomata will protect plants from these oxidants. Both ozone and PAN may interfere with various oxidative reactions within plant cells. Membrane sulfhydryl groups and unsaturated lipid components may be primary targets of oxidants. Young leaf tissue is more sensitive to PAN newly expanding and maturing tissue is most sensitive to ozone. Light is required before plant tissue will respond to PAN that is not the case with ozone. 

These studies, although few, suggest that exposure to photochemical oxidants can influence fertility and fecundity in animals and that the genera] health of newborn animals is much more likely to be impaired by exposure to oxidants than that of their parents. Whether the changes observed in reproduction variables can be related to mutagenic actions of ozone, discussed earlier, remains to be determined. In any event, it seems logical that effects of low concentrations of ozone and other photochemical oxidants on reproduction must be indirect and may be mediated by endocrine or ozone-biologic reaction products. 

The ozone dose responses and the specific effects on the photosynthetic activity of both herbaceous and woody plants, principally in controlled short exposures, are discussed in Chapter 11. The main aim of this section is to evaluate the effects of the chronic exposure of vegetation in natural ecosystems to total oxidants (more than 90% ozone) under field conditions or simulated field conditions. The effects of chronic exposure on agroecosystems are also discussed to a limited extent in Chapter 11. 

As discussed in detail throughout this book, there is rich and complex chemistry involving volatile organic compounds (VOCs), oxides of nitrogen, and ozone in ambient air. One might therefore anticipate similar chemistry in indoor air environments, and although there are far fewer studies, this does indeed appear to be the case. Weschler and Shields (1997b) and Wolkoff et al. (1997, 1999) review VOC-NOx chemistry that could potentially be important in indoor air enviro-ments and the implications for human exposures. 

The anomalous enrichment of and in stratospheric ozone provides a good example of the interplay of laboratory experiments, field studies and theory. The effect was first observed in the Thiemens laboratory in 1983 [4] and then in the stratosphere by Mauersberger [5]. The first truly successful explanation of the underlying mechanism did not appear for nearly two decades in a series of articles by Rudy Marcus and coworkers [6-9], a discussion which is extended in this issue. The unique distribution of oxygen isotopes in ozone has proved to be a useful tracer for diverse atmospheric phenomena, including the exposure of CO2 to excited oxygen atoms in the stratosphere [10], the productivity of the biosphere... 

The Clean Air Act recognizes a number of so-called primary air pollutants, and the EPA has established standards for these substances. Ozone, nitrogen dioxide, and sulfur dioxide are among these (the others are carbon monoxide and lead, discussed below, and total suspended particulates ). EPA s standard for ozone is 0.12 parts of the gas per million parts of air (0.12 ppm), as a one-hour exposure limit that is not to be exceeded more than once yearly. Nitrogen dioxide s limit is 0.05 ppm as an annual average. These standards are designed to prevent chronic respiratory toxicity of any type. 

As a final point we need to focus attention on a critically important risk issue that has been entirely neglected in this book, and that is only beginning to draw the attention it deserves. Our concern in this book has been focused on the effects on human health of exposures to environmental chemicals. We have not discussed how these chemicals may damage non-human life forms and even the inanimate environment (e.g., the ozone layer). This is an immense topic about which information is limited, but which could, in the long term, be more important in several respects than the topic of this book. An associate of mine has remarked that, somehow, the E has been taken out of EPA, suggesting that the agency has devoted much more attention to human health protection than to environmental protection. The lack... 

The layer of ozone that forms at an altitude of about 100,000 kilometers helps protest us from the Sun s short wave length ultraviolet radiation. Exposure to this radiation has been shown to increase the incidence of skin cancer. This ozone is chemically the same is the ozone discussed in this chapter but its production by sunlight and its value to humanity is entirely unrelated to the use of ozone described below. 

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