2. Oxidants

Contemporary Issues in Environmental Assessment


Deforestation is intimately linked to carbon dioxide release, climate change, acid rain, and extinction of plant and animal species. Deforestation not only contributes to the greenhouse effect, but also destroys the long-term ability of the land and forest resources to meet human needs, and inhibits the development of viable local economies. The loss of plant and animal species due to loss of forest habitat is hard to quantify, but this represents an irreversible loss of resources to the world, as well as to the local peoples.

Forests are an important economic resource for many regions of the United States. Any reduction in timber harvesting, no matter how justified on environmental grounds or overall economic grounds, is going to meet with strong opposition. Often, the very livelihood of a large proportion of the regional population depends upon the timber-harvesting industry; in many cases forests are tended as slow-growing crops on small or large tree farms. In areas that are relatively scenic or pristine the ultimate losses due to reduced aesthetic and tourism values also may be serious.

Industrialized Countries

Deforestation in industrialized countries is not usually a major economic problem. Impacts on forests due to acid rain, photochemical oxidants, overharvesting, and changed land use are not well understood. Most industrialized countries are able to make the necessary economic tradeoffs and manage forest resources effectively. Regional problems related to efforts to control timber harvesting to a renewable level are manageable, and some long-term policy options (as discussed later in this section) can be developed to balance economic and environmental requirements.

Newly Industrializing Countries

Problems of deforestation in newly industrializing countries are serious and environmental and economic consequences significant. Of the world’s annual carbon dioxide release due to deforestation, 40 percent was contributed by tropical South America, 37 percent by tropical Asia, and 23 percent by tropical Africa. Five countries accounted for half of the total: Brazil, Colombia, Indonesia, Côte d’Ivoire, and Thailand (Postel, 1988). By the late 1980s, 45 countries around the equator that were practicing aggressive forest clearing destroyed 20 to 40 hectares every minute (Gradwohl & Greenberg, 1988). Clearing for crops, fuel wood, cattle ranching, and commercial timber harvest destroyed 39 million acres annually (Postel, 1988). Fortunately, the pace of deforestation seems to be slowing at the global level as well as in newly industrializing regions: The estimate of forest cover change globally indicates an annual loss of 5.21 million hectares (Mha) between 2000 and 2010, compared with 8.33 Mha between 1990 and 2000 and 15.5 Mha between 1980 and 1990 (Marcoux, 2000; Food and Agriculture Organization, 2010).

Tropical deforestation is driven by poverty, traditional practices, national development policies, and foreign debts. Much of this cleared land is unsuitable for the monocrop agricultural practices being adopted, and becomes barren within one to two crop seasons. Lands cleared for pasture may support livestock for only 5 to 10 years. Once forests are removed, rural people are unable to meet their pressing need for fuel wood, and soil erosion, floods, and drought become more severe. Forest regeneration on cleared lands is largely unsuccessful due to the lack of natural seed, predators that feed on the seeds and seedlings, and the hot, dry conditions of tropical pasture land compared to the forest environment.

Brazil, the site of 30 percent of the world’s tropical forest area, alone contributes one-fifth of the total carbon dioxide emissions from deforestation. Although the annual release has been estimated at 336 million tons, 500 million tons of carbon dioxide were released in 1987 (Postel, 1988). Government programs are unfortunately responsible for most of this deforestation. Brazil’s current problems have roots in the decision of the 1960s to provide overland access to Amazonia before there was adequate understanding of the resources available and how they could be developed in a sustainable manner.

Beginning in the 1960s, the Brazilian government undertook major road-building programs to open the Amazon, followed by subsidized settlement. In the 1970s subsidized programs for large-scale export projects in livestock, timber, and mining were initiated; 72 percent of the tropical forests altered up to 1980 were due to cattle-ranching efforts. Despite subsidies and tax incentives, the supported livestock projects have performed at only 16 percent of what was expected because cattle ranching in this environment is intrinsically uneconomic (Mahar, 1990). The government also supports a policy that considers deforestation as evidence of land improvement and thus gives the tenant rights of possession, which can both then be sold. In 1989, a program was initiated to end subsidies for new livestock projects, and may support agro-ecological zoning for the country.

Policy Options

As discussed earlier, forestry management in developed countries, where serious and ongoing deforestation is not a major problem, need only to focus on some long-term market-economy-based policies. Increasing total forested area and ending subsidies that support logging are two policy options. For example, the 13 million hectares of marginal U.S. cropland that have been set aside in the Conservation Reserve Program, if reforested, could absorb 65 million tons of carbon annually until the trees mature, reducing U.S. carbon emissions by 5 percent (OTA, 1991). Federal subsidies of below-cost timber sales in remote areas of national forests promote excessive timber cutting and cost billions in the early 1990s (Wirth & Heinz, 1991). Efforts to increase the productivity of forests and to plant and manage trees as a renewable biomass energy source are other possibilities for U.S. policy. Some regional issues related to economic impacts of reduced timber harvesting are important and would require creative, region-specific policy options.

Slowing deforestation in Third World countries will require the financial and technical support of industrial nations to ease their international debt burden and to assist them in developing sustainable economies. Developing countries are encouraged by their debt burden to exploit forests for quick economic gain. “Debt-for-nature swaps” were devised by the World Wildlife Fund science director Thomas Lovejoy in 1984 as an innovative approach to this problem: a nongovernmental organization (e.g., The Nature Conservancy) purchases a portion of the debt and then donates the debt instrument to the country’s bank in exchange for environmentally appropriate action. Swaps of millions of dollars contribute much-needed funds for environmental programs, but have little impact on national debts measured in tens or hundreds of billions of dollars.

A policy option for developed countries may be to require industry to make equal investment in reforestation whenever a carbon-emitting project is undertaken. A joint venture between Applied Energy Services, World Resources Institute, and CARE was planned to offset carbon emissions from a coal-fired power plant in Connecticut with forestation in Guatemala. Twelve thousand hectares of woodlot and 60,000 hectares of combined trees and crops were to be planted, to be harvested on a sustainable basis. Large-scale forestation programs face the difficulties of locating and financing the purchase of suitable land and gaining cooperation from local governments. However, as an example, this project was relatively inexpensive ($16.3 million) because land would not be purchased, and workers and families to benefit from the planting would not be paid (Flavin, 1990).

2. Oxidants

Something in the Air

Lessons Learned

A principal lesson learned from the twentieth century air pollution episodes is the importance of meteorological conditions. The same emissions in terms of absolute mass released in the same time period can result in very different air quality, depending on winds, ambient temperature, atmospheric pressure, and, most importantly, inversion conditions. This is particularly important when conditions lead to elevated inversions above valleys, such as the Meuse River valley in Belgium and the Los Angeles Basin. Elevated inversions can cap pollutants in a stagnant air stratum when vertical air circulation occurs above the valley, so that pollution levels build up over days (see Figure 3.4). This indicates that pollution controls need to be targeted toward worst-case scenarios, rather than average meteorological conditions.

Another lesson is the importance of paying attention to evolving knowledge. The increasing awareness of the importance of certain conventional pollutants, like PM, SOx, and NOx, and their links to health effects should have been heeded and incorporated into city planning and pollution control decision making processes. Also, the toxic cloud episodes could have been expected from these more conventional episodes, given that toxic substances like methylsiocyanate are much more toxic than oxides of sulfur and nitrogen. Thus, a release even at low concentrations should have been provided for in contingency plans. Even worse, shortly after the Bhopal disaster, engineers and regulators were given a test on what they had learned about toxic releases and communicating risk when a release of another toxic gas (this time, aldicarb oxime) occurred at the Institute, West Virginia, pesticide plant. The same company implicated at Bhopal, Union Carbide, owned the Institute plant and, in the minds of many, once again failed the test. There should have been significant progress made, since in many ways the two plants and situations were so very similar, but some of the same weaknesses remained even after the horrible consequences of Bhopal.

The Bhopal disaster again reminds us that forgetting the past can be deadly.

Contaminant of Concern: Photochemical Oxidant Smog

The term smog is a shorthand combination of “smoke-fog.” However, it is really the code word for photochemical oxidant smog, the brown haze that can be seen when flying into Los Angeles, St. Louis, Denver, and other metropolitan areas around the world. Smog is made up of at least three ingredients: light, hydrocarbons, and radical sources, such as the oxides of nitrogen. Therefore, smog is found most often in the warmer months of the year, not because of temperature, but because these are the months with greater amounts of sunlight. More sunlight is available for two reasons, both attributed to the earth’s tilt on its axis. In the summer, the earth is tilted toward the sun, so the angle of inclination of sunlight is greater than when the sun is tipped away from the earth leading to more intensity of light per earth surface area. Also, the days are longer in the summer, so these two factors increase the light budget.

Hydrocarbons come from many sources, but the fact that internal combustion engines burn gasoline, diesel fuel, and other mixtures of hydrocarbons makes them a ready source. Complete combustion results in carbon dioxide and water, but anything short of combustion will be a source of hydrocarbons, including some of the original ones found in the fuels, as well as new ones formed during combustion. The compounds that become free radicals, like the oxides of nitrogen, are also readily available from internal combustion engines, since the ambient air is more than three-quarters molecular nitrogen (N2). Although N2 is relatively not chemically reactive, with the high temperature and pressure conditions in the engine, it does combine with the O2 from the fuel/air mix and generates oxides that can provide electrons to the photochemical reactions.

The pollutant most closely associated with smog is ozone (O3), which forms from the photochemical reactions just mentioned. In the early days of smog control efforts, O3 was used more as a surrogate or marker for smog, since one could not really take a sample of smog. Later, O3 became recognized as a pollutant in its own right since it was increasingly linked to respiratory diseases.

Cities that failed to achieve human health standards as required by the Clean Air Act’s National Ambient Air Quality Standards (NAAQS) were required to reach attainment within six years of passage, although Los Angeles was given 20 years, since it was dealing with major challenges in reducing ozone concentrations. Almost 100 cities failed to achieve ozone standards and were ranked from marginal to extreme. The more severe the pollution, the more rigorous controls required, although additional time was given to those extreme cities to achieve the standard. Measures included new or enhanced inspection/maintenance (I/M) programs for autos; installation of vapor recovery systems at gas stations and other controls of hydrocarbon emissions from small sources; and new transportation controls to offset increases in the number of miles traveled by vehicles. Major stationary sources of nitrogen oxides also have to reduce emissions.

The ozone threshold value is 0.12 parts per million (ppm), measured as a one-hour average concentration. An area meets the ozone NAAQS if there is no more than one day per year when the highest hourly value exceeds the threshold. (If monitoring did not take place every day because of equipment malfunction or other operational problems, actual measurements are prorated for the missing days. The estimated total number of above-threshold days must be 1.0 or less.) To be in attainment, an area must meet the ozone NAAQS for three consecutive years.

Calculating compliance can be tricky. Air quality ozone value is estimated using a calculation usually based on the fourth highest monitored value with three complete years of data and is selected as the updated air quality value because the standard allows one exceedance for each year. It is important to note that the 1990 Clean Air Act Amendments required that ozone nonattainment areas be classified on the basis of the design value at the time the Amendments were passed; generally the 1987–1989 period was used.

The strong seasonality of O3 levels makes it possible for areas to limit their O3 monitoring to a certain portion of the year, termed the O3 season. Peak O3 concentrations typically occur during hot, dry, stagnant summertime conditions; that is, high temperature and strong solar insolation (i.e., incoming solar radiation). The length of the O3 season varies from one area of the country to another. The months of May through October are typical, but states in the south and southwest may monitor the entire year. Northern states have shorter O3 seasons, for example, May through September for North Dakota. This analysis uses these O3 seasons to ensure that the data completeness requirements apply to the relevant portions of the year.

Children have higher health risks associated with exposure to ozone than do most adults. The average adult breathes 13,000 liters of air per day, but on an air-per-kilogram basis, children breathe even more air than do adults. Because children’s respiratory systems are prolific and still developing, they are more susceptible than adults to many environmental threats. Children are outside playing and exercising during the summer months more frequently than in the less warm months. Unfortunately, this is also the time of year with elevated O3. In addition, asthma is a growing threat to children and adults. Children make up 25 percent of the U.S. population, but comprise 40 percent of the asthma cases. The asthma death rate has increased three-fold in the past 20 years, and African Americans die at a rate six times that of Caucasions. Even moderately exercising healthy adults can experience 20% or greater reductions in lung function from exposure to low levels of ozone over several hours. These factors make smog an important public health concern.

The principal lesson from the history of air pollution episodes, as from every case in this book, is that the atmosphere is not infinite in its ability to absorb wastes. Although this appears obvious to the twenty-first century scientist, it is actually a fairly recent realization.

2. Oxidants

Pollution, Air in Encyclopedia of Toxicology


Exposure to ozone, a potent photochemical oxidant in the ambient air, is a major health concern in urban and rural communities throughout the United States. Nationally, average ozone levels have decreased by 14% from 1990 to 2008. In the Eastern United States, much of the improvement in ozone levels has occurred since 2002 due largely to reductions in emissions of oxides of nitrogen (an ozone precursor). Still, in 2009, 61.5 million people in the United States resided in counties where the ozone NAAQS was exceeded.

Based on the evidence integrated across human controlled exposure and epidemiological and toxicological studies, there is clear, consistent evidence of a causal relationship between short-term exposure (i.e., from days to weeks) to ozone and respiratory effects. Human clinical and toxicological studies show that short-term exposures to ozone cause lung function decrements, respiratory symptoms, lung inflammation, epithelial damage, and permeability, and increases in airway responsiveness (a condition in which the conducting airways have an enhanced bronchoconstriction following exposure to variety of stimuli, e.g., allergens, cold air, sulfur dioxide). Collectively, these findings provide biological plausibility for associations in epidemiological studies between short-term ambient exposure to ozone and asthma exacerbation, respiratory-related hospitalizations, and emergency department visits.

The magnitude of respiratory effects (e.g., decrements in pulmonary function and symptomatic responses) is generally a function of ozone concentration, minute ventilation rate (volume of air inhaled per minute), and exposure duration. Any physical activity will increase minute ventilation and therefore the dose of inhaled ozone. For healthy young adults exposed in a controlled clinical study at rest for 2 h, 500 ppb is the lowest ozone concentration reported to produce a statistically significant group mean decrements in lung function. With longer, 7-h exposures to as low as 60 ppb ozone, during a moderate level of exercise, statistically significant decrements in lung function, increases in respiratory symptoms, and pulmonary inflammation have been reported. Although there is a relatively rapid recovery in pulmonary function and respiratory symptoms over a few hours following exposure, the inflammatory response occurs shortly after exposure and persists for at least one day. An influx of neutrophils and an increase in a number of mediators including eicosanoids, neutrophil elastase, and cytokines have been measured in bronchoalveolar lavage fluid recovered from subjects exposed to near ambient concentrations of ozone.

In otherwise young healthy adults exposed for 2–8 h to ozone, controlled clinical studies have demonstrated a large degree of intersubject variability in lung function decrements, respiratory symptom responses, inflammation, airway responsiveness, and altered epithelial permeability. The magnitude of increases in inflammation, airway responsiveness, and epithelial permeability, in response to ozone exposure, do not appear to be correlated, nor are these responses correlated with changes in lung function. However, these responses of healthy individuals to ozone tend to be reproducible within a given person over a period of several months indicating differences in the intrinsic responsiveness of individuals. It should be noted that even when group mean responses are small and seem physiologically insignificant, some intrinsically more responsive individuals experience distinctly larger effects under the same exposure conditions. For example, small group mean changes (e.g., 2–3%) in forced expiratory volume in 1 s (FEV1) have been observed in healthy young adults exposed to 60 ppb ozone for 7 h. However, 10% of the group may experience FEV1 decrements in excess of 10% under these conditions, even with group mean decrements of less than 3%. Therefore, within the general population, a proportion of otherwise healthy individuals experience greater than average health effects and may be at increased risk of more adverse responses.

With repeated ozone exposures over several days, lung function and respiratory symptom responses become attenuated in both healthy individuals and asthmatics, but this tolerance is lost after about a week without exposure. Airway responsiveness also appears to be somewhat attenuated with repeated ozone exposures in healthy individuals, but becomes increased in individuals with preexisting allergic airway disease. Some indicators of pulmonary inflammation are attenuated with repeated ozone exposures, whereas other markers, such as epithelial integrity and damage, do not show attenuation, suggesting continued tissue damage during repeated ozone exposure. Both respiratory symptoms and decrements in pulmonary function have neural components and these responses decrease with increasing age beyond young adulthood. However, other cell and molecular responses to ozone exposure likely exist and may even increase as antioxidant defenses change with increasing age.

The adverse functional effects observed in controlled clinical studies are similar to those reported during exposure to ambient air. Decrements in lung function have been noted in a series of camp studies in which children were exposed to ambient ozone during normal outdoor play activity. Compared to controlled chamber studies, greater decrements in lung function were observed in the camp studies when the data were normalized for ozone concentration. A number of factors may explain the greater response in the camp study, but the most likely reason is the simultaneous exposure to ambient co-pollutants such as acid aerosols. Epidemiological studies have found strong correlations between respiratory symptoms such as cough, throat irritation, and chest discomfort and ambient ozone levels. Exacerbation of asthma increases in hospital admissions for respiratory infections, and excess mortality has also been reported to be associated with oxidant air pollution episodes. Older age, female sex, ethnicity, atrial fibrillation, socioeconomic status indicators (i.e., educational attainment, income level, employment status), and diabetes modify ozone mortality associations and may increase susceptibility to ozone-related mortality. Thus, a number of epidemiological, field, and clinical studies provide evidence that adverse respiratory effects occur after acute exposure to ozone at or below the current US NAAQS.

Population-based epidemiological and animal toxicology studies examining the effects of long-term ozone exposure (i.e., from months to years) show associations with long-term reductions in lung function, development of asthma, pathological changes, and premature mortality. Animal studies using concentrations well above the current NAAQS reveal that the centriacinar region of the airways and the nasal cavity are the most sensitive to pathological changes induced by chronic ozone. Studies of infant primates show changes in immune responses similar to asthma and the development of irreversible changes to the structure of the distal (deep) lung that decrease pulmonary function. Epidemiological studies have demonstrated that chronic exposure to ozone is associated with decrements in lung function and increases in the incidence and severity of asthma. Limited new epidemiological evidence also suggests that long-term exposure to ozone increases the risk of premature mortality. The ability of these epidemiological studies to establish cause and effect is hampered by confounding factors such as co-pollutants.

Overall, evidence shows that both short- and long-term exposure to ozone at ambient concentrations is associated with respiratory morbidity and premature mortality.

2. Oxidants

Vulnerability of Ecosystems to Climate

The photochemical oxidant ozone (O3) and pollutants such as sulfur dioxide (SO2) have been shown to damage plants (Kita et al. 2000; Potter et al. 2002; Ashmore 2005). However, the combined effects of pollutants, CO2 levels, temperature, and changes in precipitation are not mechanistically well understood (Kirschbaum 2004; Ashmore 2005; DeLucia et al. 2000, 1994). In localized studies, higher levels of O3 and other pollutants were associated with insect-related disturbances (Jones et al. 2004), suggesting an increase in pollutant-related stress increases the likelihood of occurrence of other stressors. O3 was also found to interact with frost (Oksanen et al. 2005), increasing the negative effects of frost on pigment loss and stomatal conductance. Integrating O3 with CO2, temperature, and precipitation changes within models, results in varying productivity predictions (Hanson et al. 2005). Single factor studies may help us understand mechanisms but they may not be useful in determining the magnitude of long-term ecosystem responses, since combined factors may cancel or compound each other (we discuss this concept in depth in a subsequent section). There is no doubt that the increase in atmospheric O3 will modify the response of forest to elevated CO2, temperature, precipitation, and radiation. But few multifactor experiments currently exist and our modeling efforts are bound by our understanding of the interactions.

2. Oxidants

Photochemical Oxidants

Much improvement in the levels of smog and, more importantly, photochemical oxidants has been made in the United States and other countries since the late 1960s. However, room for improvement persists.

The oxidant of critical importance in the photochemical atmosphere is ozone (O3). Several miles above the Earth’s surface, in the troposphere, there is sufficient shortwave ultraviolet (UV) light to directly split molecular O2 to atomic O to combine with O2 to form O3. These UV wavelengths do not reach the Earth’s surface. In this region, nitrogen dioxide efficiently absorbs longer wavelength UV light, which leads to the following simplified series of reactions:NO2 + UVO + NOO + O2O3O3 + NONO2

This process is cyclic, with NO2 regenerated by the reaction of NO and O. In the absence of hydrocarbons, this series of reactions would approach a steady state with no excess or buildup of O3. However, near the Earth’s surface, the hydrocarbons, especially olefins and substituted aromatics, are attacked by the free atomic O, which, with NO, produces more NO2. Thus, the balance of the reactions shown in the above reactions is upset so that O3 levels build up, particularly when the Sun’s intensity is greatest at midday. The reactions with hydrocarbons are very complex and involve the formation of unstable intermediate free radicals that undergo a series of changes. Aldehydes are major products in these reactions. Formaldehyde and acrolein account for ∼50 and 5%, respectively, of the total aldehyde in urban atmospheres. Peroxyacetyl nitrate (CH3COONO2), often referred to as PAN, and its homologs, also arise in urban air, most likely from the reaction of the peroxyacyl radicals with NO2.