Tropospheric OzoneA Special Problem

The most widespread air quality problem in the United States is exceedances of the ozone standard (0.12 ppm for 1 hr per year) in urban areas. The ozone standard is based on protecting public health. Ozone is produced when its precursors, VOCs and nitrogen oxides (NOJ, combine in the presence of sunlight (Office of Technology Assessment 1989). VOCs, a broad class of pollutants encompassing hundreds of specific compounds, come from manmade sources including automobile and truck exhaust, evaporation of solvents and gasoline, chemical manufacturing, and petroleum refining. In most urban areas, such man-made sources account for the majority of VOC emissions, but in the summer in some regions, natural vegetation produces an almost equal quantity. NOx arises primarily from fossil fuel combustion. Major sources include highway vehicles and utility and industrial boilers.

About 100 nonattainment areas dot the country from coast to coast, with design values (a measure of peak ozone concentrations) ranging from 0.13 ppm to as high as 0.36 ppm. Figure 5.1.12 summarizes the data for the 3-year period 1983-85 (Office of Technology Assessment 1989). Generally, the higher the design value, the stricter the emission controls needed to meet the standard.

From one-third to one-half of all Americans live in areas that exceed the standard at least once a year. As shown in Figure 5.1.13, 130 of the 317 urban and rural areas exceeded 0.12 ppm for at least 1 hr between 1983 and 1985 (Office of Technology Assessment 1989). Sixty had concentrations that high for at least 6 hr per year. A number of areas topped the standard for 20 or more hr, with the worst, Los Angeles, averaging 275 hr per year.

Ozone's most perceptible short-term effects on human health are respiratory symptoms such as coughing and painful deep breathing (Office of Technology Assessment 1989). It also reduces people's ability to inhale and exhale normally, affecting the most commonly used measures of lung function (e.g., the maximum amount of air a person can exhale in 1 sec or the maximum a person can exhale

FIG. 5.1.12 Areas classified as nonattainment for ozone based on 1983-85 data. The shading indicates the fourth highest daily maximum one-hour average ozone concentration, or design value, for each area. (Reprinted from Office of Technology Assessment, 1989, Catching our breath—Next steps for reducing urban ozone, OTA-0-412, Washington, D.C.: U.S. Congress [July].)

FIG. 5.1.12 Areas classified as nonattainment for ozone based on 1983-85 data. The shading indicates the fourth highest daily maximum one-hour average ozone concentration, or design value, for each area. (Reprinted from Office of Technology Assessment, 1989, Catching our breath—Next steps for reducing urban ozone, OTA-0-412, Washington, D.C.: U.S. Congress [July].)

FIG. 5.1.13 Areas where ozone concentrations exceeded 0.12 ppm at least one hour per year on average, from 1983-85. Data from all monitors located in each area were averaged in the map construction. The shading indicates the number of hours that a concentration of 0.12 ppm was exceeded. The areas shown have 130 million residents. (Reprinted from Office of Technology Assessment, 1989.)

FIG. 5.1.13 Areas where ozone concentrations exceeded 0.12 ppm at least one hour per year on average, from 1983-85. Data from all monitors located in each area were averaged in the map construction. The shading indicates the number of hours that a concentration of 0.12 ppm was exceeded. The areas shown have 130 million residents. (Reprinted from Office of Technology Assessment, 1989.)

after taking a deep breath). As the intensity of exercise rises so does the amount of air drawn into the lungs and thus the dose of ozone. The more heavily a person exercises at a level of ozone concentration and the longer the exercise lasts, the larger the potential effect on lung function.

The U.S. Environmental Protection Agency (EPA) has identified two subgroups of people who may be at special risk for adverse effects: athletes and workers who exercise heavily outdoors and people with preexisting respiratory problems (Office of Technology Assessment 1989). Also problematic are children, who appear to be less susceptible to (or at least less aware of) acute symptoms and thus spend more time outdoors in high ozone concentrations. Most laboratory studies show no special effects in asthmatics, but epidemiologic evidence suggests that they suffer more frequent attacks, respiratory symptoms, and hospital admissions during periods of high ozone. In addition, about 5 to 20% of the healthy adult population appear to be responders, who for no apparent reason are more sensitive than average to a dose of ozone.

At the summertime ozone levels in many cities, some people who engage in moderate exercise for extended periods can experience adverse effects. For example, as shown in Figure 5.1.14, on a summer day when ozone concentrations average 0.14 ppm, a construction worker on an 8-hr shift can experience a temporary decrease in lung function that most scientists consider harmful (Office of Technology Assessment 1989). On those same summer days, children playing outdoors for half the day also risk the effects on lung function that some scientists consider adverse. And some heavy exercisers, such as runners and bicyclists, notice adverse effects in about 2 hr. Even higher levels of ozone, which prevail in a number of areas, have swifter and more severe impacts on health.

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