Assessments of air quality can be made at several levels, ranging from the personal to the scientific. In many cases individuals assess air quality based on their own experiences. A steel worker in 1915 Pittsburgh would judge our air quality today in North Carolina to be relatively clean. That does not mean that residents of heavily air polluted places are able to identify unhealthful air quality. Because pollution from industrialization during the 19th and 20th centuries was associated with economic opportunity and wealth and opportunity, health risk and environmental hazards of pollution were ignored. In The City, a 1909 play by Clyde Fitch, one character asks, "Who wants to smell new-mown hay, if he can breathe gasoline on Fifth Avenue instead?" The times have changed and our perception of what constitutes pollution has changed. We have clean air by 19th century standards, but that does not mean the air is clean.
Figure 1 illustrates the same choking air pollution recently found in China that once fumigated valleys of the Monogahela, the Allegheny, and the Ohio Rivers in the United States.
As regions around the world industrialize without careful controls on air pollutants,
they tend to pass
through a stage where health risks from air pollution are high. Mexico City is
a good example because its location within a topographic basin hinders the removal
of pollutants from the air and results in high concentrations of ozone and other
air pollutants. Mexico City frequently experiences ozone levels greater than
300 ppb (parts per billion) with peaks between 400 and 450 ppb. These levels
are two and three times greater than anything ever recorded in North Carolina.
Attention to environmental issues often coincides with or follows increased wealth. Historically, many regions have witnessed high levels of pollution during the initial period of industrialization, but as manufacturing generates wealth, some attention and economic resources are directed toward cleaning up the environment in general and the air in particular (Figure 2). Attention to anthropogenic environmental issues often coincides with or follows industrialization and increased wealth. The major cities of Western Europe, North America, and Japan have gone through this process and enjoy much cleaner air today than 50 to 75 year ago.
 Air Quality In the News |
 Excessive Auto Emmissions |
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Identification of Air Pollutants The Toxic Release Inventory Indoor Air Pollution Criteria Pollutants Emission and Fate of Air Pollutants Emission, Dispersion, (Transformation), Removal, Receptor Response Effects of Fine Particle and Ozone Pollution on Health Ozone Pollution National Progress Toward Cleaner Air Regional and Local Effects Future Prospects |
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There are many way to classify air pollutants. The shortest list is generated by considering only those contaminants that directly threaten human health and safety. By that standard carbon dioxide, despite being implicated as an agent of global change, is not considered an air pollutant because there are no known health effects from breathing atmospheric levels of carbon dioxide. Levels of carbon dioxide in the atmosphere result mostly from natural sources.
Some air pollutants are biological and are studied by aerobiologists. Examples include terpenes from pines and isoprenes from deciduous trees, tree pollen, spores, even blue-mold spores that reduced yields for tobacco farmers in 2001.
One of the frequently overlooked air pollutants is noise. Noise is often overlooked as a serious problem and often considered merely a nuisance. The long term effect can be noise induced hearing loss. Many common machines from leaf blowers and motorcycles to cars and trucks can generate noise levels that threaten hearing loss. Former President Reagan suffered hearing loss from a movie pistol fired too close to one of his ears. Although most people try to protect their hearing from such extreme sound pressure and the instantaneous damage it can cause, many fail to protect themselves from loud noise. Exposure to merely loud sound pressure for extended periods of time is just a damaging as much louder sound for a brief period of time. Exposure to a 102 db noise for 8 hours a day, five days a week for 10 years results in a 25 percent chance of serious hearing damage, for example. Passengers in some trucks, cars, or motorcycles may experience noise levels approaching 85-95 db mainly from tire and wind noise. Research is underway to find surface textures for pavement that greatly reduce the noise generated by highway traffic. Although the benefits of these new surfaces lie many years in the future, they will better protect the hearing of motorists and nearby residents alike. In general, when noise makes conversation difficult, sound levels may represent a threat to hearing.
Approximately 667 chemicals listed in the Toxic Release Inventory. These are chemicals that industries are permitted to release into the air, water, or subsurface. The quantity of the permitted release is limited by the potential threat to the environment and to human safety. The permitted amount released into the air is a matter of public record and emissions are monitored to insure each facility operates within permitted emission levels.
Examples of chemicals included in the toxic release inventory include: benzene, lead, ammonia, and toluene. Potential health effects include cancer and central nervous system damage. Nationally the trend has been mostly downward over the past several years. For examples, the releases of benzene has declined by 40 percent and lead declined 47 percent during the past decade.
In North Carolina, the greatest quantity of toxics are released within the Piedmont region and Mecklenburg County is a good example. In 2001 industries in Mecklenburg County emitted 271 tons of hazardous air pollutants (HAP). The leading emissions by volume were dichloromethane, n-hexane, toluene, and methyl ethyl ketone.
Some toxics may actually be a greater threat in the home than outside. In January of 1998, Wayne R. Ott and John W. Roberts published “Everyday Exposure to Toxic Pollutants” in Scientific American. The routines of daily life bring many people in contact with a variety of chemicals that are easily ignored, but reduce air quality within the well sealed environment of the modern insulated home. Off-gassing from recently dry cleaned clothes, gasses from cigarette smoke, moth balls, and toilet cleaners and fresheners, for example, contribute impurities to enclosed air space within the home. Herbicides and pesticides from lawn and garden care as well as oils and solvents from parking and garages are frequently tracked into the home where they become embedded in the carpet. Many of these chemicals can remain in the carpet far longer that they would survive outside where sunlight and bacteria would accelerate their decomposition. The combined effect is that benzene, chloroform, carbon monoxide, pesticides, and other toxics degrade indoor air substantially. Because the source for some of these indoor air toxics is the carpet, they affect infants, toddlers, and pets disproportionately. Several techniques are useful in reducing exposure to these air toxics. Don’t walk through motor oil and grease or other chemicals. Leave your outdoor shoes and any contaminated work clothes outside the living space. Keep the carpet clean.
Criteria Pollutants are particulate matter, sulfur dioxide, nitrogen dioxide, carbon monoxide, ozone, and lead. They are constantly measured by monitors located at selected sites around the state. Criteria pollutants are limited to those listed below (Table 1). The national primary standard for each pollutant is designed to protect human health and the secondary standard is designed to protect the environment, including agriculture and forestry. Two adjustments have been made in recent years. One is that particulate matter that is less than or equal to 2.5 micrometers has been added to the list of criteria pollutants and the other is that a new 8-hour standard for ozone has been adopted. The ultrafine particles represented by the PM-2.5 category are thought to be among the most unhealthful air pollutants because they are not expelled by coughing or sneezing and typically remain in the lungs. The new 8-hour ozone standard improves upon the previous 1-hour standard by better measuring oxidant exposure among those who work outdoors for most of the day during the summer.
Sources of air pollutants are grouped into three categories to simplify the complex ways they can enter and disperse through the atmosphere. One is point sources. They are relatively large emission sources that would appear as a single point in map view. Smokestacks are good examples of point sources. Once exhaust is emitted from the smokestack the plume rises for a short distance and is carried downwind (Figure 3). As pollution is transported downwind, it disperses or diffuses both horizontally and vertically. How rapidly the pollution disperses depends upon atmospheric conditions. In some cases the plume of pollution might travel downwind in narrow cone shaped plume, while during other weather conditions the plume might disperse quickly. There are various types of models that estimate the anticipated effects of a proposed facility and aid in decisions to permit or disallow construction.
The second is line sources. Line source plumes drift downwind from roads, runways, and crop-dusting operations. Line sources are slightly more complex to model than point sources because emissions from line sources often have greater variability than those from point sources.
The third, area sources, are the most complex, because they include all the sources that are too small to be considered individually. Many devices powered by small engines such as lawn care equipment contribute to area source emissions. Other area emissions include fugitive emissions that escape from many diverse activities. The aroma of freshly baked bread from a bakery and the odor from automobile body shop are both examples of area source hydrocarbon emissions.
The search to better understand how air pollution occurs and interacts within the atmosphere is analyzed by specialists in several related fields. The rate and type of point source emissions are managed to a large extent by the state and local governments which manage a permitting procedure and maintain emission inventories. These local extensions of the U.S. Environmental Protection Agency work with businesses and industries to ensure that the rate of emissions from individual facilities remain within acceptable limits.
As pollutants drift and disperse downwind, weather conditions may exacerbate or attenuate the concentration of pollutants at ground level. North Carolina and much of the southeastern United States are particularly vulnerable because warm core anticyclones frequently stagnate and create episodes when ground level pollutant concentrations increase (see Figure 12.11, p. 293 in The North Carolina Atlas for information about this regional phenomenon). For example, drought conditions between 1999 and 2002 contributed to worsening air quality across much of North Carolina. Conversely, the rains of 2003 (along with somewhat reduced economic activity) helped to attenuate levels of pollution by washing particles and other contaminants from the air.
The effects of air pollution include corrosion and soiling of buildings and statuary; damage to natural vegetation, agriculture, and forestry; and health effects. There are many questions about the health effects of air pollution and the following section focuses on the two most problematic and pervasive air pollutants in North Carolina.
The human respiratory system can tolerate some types and levels of air pollution
without any measurable ill effect. How much air pollution is too much for people?
The answer to that question cannot be found by direct experimentation as it
is when natural vegetation, forestry, or crop tolerances are established. In
recent years the pivotal air quality debate has centered upon uncertainties
in defining the air quality threshold necessary to protect public health. The
difficulty in establishing a proper health threshold lies with two problems.
For one, scientists prefer to use deterministic models based on direct experimentation
that clearly identify cause and effect linked by process, but most investigations
that seek to identify health thresholds are epidemiologic studies. On the other
hand, epidemiologic studies are probabilistic studies that draw conclusions
from effects observed within groups of people over a period of time.
Although there are many short term or acute effects associated with air pollution,
the more difficult question addresses the long term effects on health and lifespan.
Some of the more influential epidemiologic studies have been published by Dockery,
Pope, Moogavkar, Luebeck, and Cody. Dockery and others established a statistical
link between the amount of air pollution and mortality in six cities.
Although several different criteria pollutants were examined, the strongest link was established between particles and mortality. In the years since this study was published, the U.S. Environmental Protection Agency has established primary standards for ultra- fine particles and many states, including North Carolina, have begun to monitor these particle concentrations. (See: Dockery, D. W. et al. 1993. “An Association Between Air Pollution and Mortality in Six U.S. Cities,” The New England Journal of Medicine 329: 1753-1759.)
Pope and others found similar evidence that particulates are associated with reduced lifespan. (See: Pope CA, Thun MJ, Namboodiri MM, Dockery DW, Evans JS, Speizer FE, and Heath CW, 1995. “Particulate Air Pollution as a Predictor of Mortality in a Prospective Study of U.S. Adults”, American Journal of Respiratory and Critical Care Medicine 151:669-674.)
Moolgavkar and others presented evidence that the association between sulfur dioxide pollution and disease is complex and potentially ambiguous. (See: Moolgavkar, S. H. et al. 1995. “Particulate Air Pollution, Sulfur Dioxide, and Daily Mortality: A Reanalysis of the Steubenville Data”. Inhalation Toxicology 7: 35-44.)
Later, Moolgavkar and others conceded that air pollution is unhealthy, but withdrew from establishing a definite threshold for each offending pollutant. As the authors state, “Air pollution, which is a complex mixture, appears to be associated with mortality even at the generally low levels of pollution in U.S. cities, but currently neither the statistical tools nor the biological understanding of mechanisms exists to tease out the contribution made by each component of this mixture. We conclude that it is not possible with the present evidence to show a convincing correlation between particulate air pollution and mortality.” (See: Moolgavkar SH, Luebeck EG, 1996. “A Critical Review of the Evidence on Particulate Air Pollution and Mortality,” Epidemiology, 7 (4):420-428.)
Premature death from air pollution is not the only health metric. Significant associations have been found between ozone concentrations and the rate of hospital admissions for respiratory-related hospital admissions. Stronger relationships tend to occur in areas with higher levels of ozone. Representative publications include: Moolgavkar SH, Luebeck EG, Anderson EL, 1997. “Air Pollution and Hospital Admissions for Respiratory Causes in Minneapolis-St. Paul and Birmingham”, Epidemiology 8(4):364-370
Moolgavkar and others investigated the association between air pollution and hospital admissions for chronic obstructive pulmonary disease and pneumonia among the elderly in Minneapolis-St. Paul, MN, and Birmingham, AL. Pollutants included in the analyses were PM10, SO2, NO2, O3, and CO in Minneapolis-St. Paul, and PM10, O3, and CO in Birmingham. After adjusting for temperature, day of week, season, and temporal trends, they found little evidence of association between air pollution and hospital admissions for respiratory causes in Birmingham. In contrast, they found that air pollution was associated with hospital admissions for respiratory causes in Minneapolis-St. Paul. Among the individual pollutants, ozone was most strongly associated with admissions (estimated increase in hospital admissions associated with a 15-parts-per-billion increase in O3 on the previous day).
Similar effects were found by Cody and others. (Cody RP, Weisel CP, Birnbaum G, and Lioy PJ, 1992. “The Effect of Ozone Associated with Summertime Photochemical Smog on the Frequency of Asthma Visits to Hospital Emergency Departments”, Environmental Research 58(2): 184-194.)
Elguindi examined the statistical relationship between asthma emergency department visits and air pollutants in Charlotte and found that at the relatively low pollutant concentrations prevalent in Charlotte, no unambiguous association between asthma visits and air pollution could be discerned. This finding does not preclude the expectation that a statistically significant relationship would exist at higher pollutant levels or that these levels would not prompt asthma attacks unassociated with hospital visits. (Elguindi, Nelli, 1997,” Assessing the Contribution of Air Pollutants to Asthma Emergency Department Visits: A Case Study of Charlotte, NC”, Unpublished M.A. Thesis, Department of Geography and Earth Sciences, UNC Charlotte) There is no question that high concentrations of fine particles and ozone cause respiratory distress and reduce lifespan. Defining the precise threshold (acute and chronic) where these effects begin to occur and defining the margin of safety necessary to protect public health remains stubbornly elusive.
Since the Clean Air Act was enacted in 1970, many incremental steps have led to a massive improvements in air quality in the United States. Despite some uncertainty about the precise health threshold for fine particles and ozone, ambient air quality in North Carolina remains among the best in the nation. Consider the following maps (Figures 4 and 5) that delimit areas where the primary standard for ozone and fine particles is unsatisfactory and classified non-attainment.
Because the health threshold has been difficult to define precisely, some fluctuation in the standard has taken place. Prior to 1980 the primary standard for ozone was based on a one-hour average of less than or equal to 80 parts per billion. Areas with higher levels of ozone were designated as non-attainment. Between 1980 and 2000 the standard was relaxed to less than or equal to a one-hour average of less than or equal to 120 parts per billion. Today the standard has been tightened to an eight-hour average of 85 parts per billion. If the history of primary standards is any guide, the prospect that either the ultra-fine particulate standard or the ozone standard might be relaxed in the future is real.
Figures 4 and 5 define the non-attainment areas based upon a one-hour standard that was accepted between 1980 and 2001. Today the more stringent standard will put 11 North Carolina counties and parts of 24 others in non-attainment (Figure 6). (See: http://daq.state.nc.us/news/pr/2003/nonattain_0715.shtml)
Just as uncertainty about the health threshold for ozone has led to changes in the primary standard, new knowledge about the benefits of lower fine particle concentrations have lead to standards for PM2.5. In North Carolina there are currently 15 counties with PM2.5 concentrations above the new national standard (Figure 7).
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| Two Photos taken at
the same location on different days on the Blue Ridge Parkway near
Linville Falls showing clean air (left) and haze (right). (Hugh Morton) |
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In North Carolina, there seems to be a substantial weekend effect. Air quality on weekends is better than during the week. Charlotte, for example, between 1990 and 1995 had better air quality on Saturdays and Sundays than during the rest of the week. When plotted against the average for each pollutant, Saturday and Sunday were below the average while weekdays were generally greater than the 7-day average (Figure 8). In North Carolina there are currently 15 counties with PM 2.5 concentrations above the new 15 microgram per cubic meter national standard. Most of these counties are Piedmont counties (Figure 9).
To gain a regional perspective, view the daily sequence of air quality maps for the southeastern United States below. The sequence begins in the spring and ends in fall because ozone which represents one of the important health threats is primarily a summertime pollutant. Notice that the region is depicted as having healthy air on most days, but the pattern is punctuated occasionally with episodes of poor air quality shown as shades of orange and red. Notice also that these episodes are spatially associated with the major Piedmont cities.
Time Sequence of Air Quality Across Southeast, Summer 2001 |
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As transport winds blow pollutants such as ozone across an urban area, the city adds to the burden of the transported chemicals. How much do cities degrade air quality within the metropolitan region? Evidence from Charlotte indicates that as ozone is transported along the prevailing wind vector from southwest to northeast, air leaving the metropolitan regions in mid-afternoon contains approximately 27 percent more ozone than does the air entering the urban region (Figure 10). Ambient levels entering the city average 62 ppb while those for air leaving the city averaged 79 ppb. Emission controls within the city can only affect the Charlotte-induced 17 ppb increase. This is not to say that emission controls within Charlotte are not worthwhile, they are. This example highlights one the paradoxes of ozone pollution control: local emission controls may benefit downwind residents more than local residents.
The list of pollution control strategies that hold promise for North Carolina is long and varied. Reduced nitrogen oxide emissions from older coal-fired power plants remains an attainable goal. Nitrogen oxides are precursors to ground level ozone pollution and for that reason remain problematic within the state. Advanced automotive catalyst systems, light duty diesel emission controls, and possibly fuel cells will all contribute to cleaner vehicle miles. Within the next several years, sulfur will be phased out of gasoline. Sulfur-free gasoline will substantially reduce the remaining factions of sulfur oxide emissions.
Aside from technological innovations, one of the most cost effective methods of reducing highway transportation-related emissions is to live close to the workplace. When feasible, this simple practice reduces transportation related pollution, congestion, saves money, and best of all, it saves time and frustration.
