Ambient Air Monitoring Program - Pollutants of Concern
The principal ambient air pollutants, based on public health concerns, have been identified by the U.S. Environmental Protection Agency (EPA) as "criteria" pollutants. The EPA established National Ambient Air Quality Standards (NAAQS) for these criteria pollutants. One of these pollutants, lead, has not been a widespread ambient air quality concern since the removal of lead from gasoline.
• View / Download table of the ambient air quality standards 
Carbon Monoxide
Carbon monoxide (CO) is a colorless, odorless, poisonous gaseous pollutant. The primary sources of CO are motor vehicles and other combustion sources. It is formed from the combustion of hydrocarbon fuels from internal combustion engines, from home heating devices such as fireplaces, stoves and furnaces, and from industrial sources of combustion. Motor vehicle exhaust contributes about 60 percent of all CO emissions nationwide. In cities, as much as 95 percent or more of all CO emissions emanate from automobile exhaust. These emissions can result in high concentrations of CO, particularly in areas with heavy traffic congestion. Other sources of CO emissions include industrial processes, non-transportation fuel combustion, and natural sources such as wildfires.
Carbon monoxide concentrations are significantly affected by meteorological conditions, with high concentrations principally occurring during inversion periods and cold weather. A temperature inversion is present when air temperature increases with altitude, so that a warm air layer traps cooler air beneath it. The increasing levels of CO are then trapped and concentrated from the lack of vertical mixing dispersion by winds. Inversions are most frequent and have the smallest mixing depths during late fall and winter, thus contributing to elevated CO levels. This problem is compounded during periods of high pressure dominance, when atmospheric stability allows little vertical or horizontal mixing. The combination of valley basins and heavy motor vehicle traffic, with seasonal influences (temperature inversions during late fall and winter months), provides for the occurrence of elevated CO levels which may be harmful to human health and welfare.
Carbon monoxide in high concentrations can be a silent killer, as it has a strong affinity for combining with the hemoglobin of the blood. This, in turn, causes the hemoglobin to be less readily available to perform the function of carrying oxygen to the tissues. Increased automobile use through the years has been a factor in increased CO levels and health risks. People who suffer from cardiopulmonary disease, anemia, or who smoke tobacco are most likely to be affected by high CO levels. Those who may be exposed through occupational duties are also candidates for increased health risks. Lower concentrations of CO may cause such effects as headaches, diminished alertness, slower reaction time and faster blood clotting.
Lead
In the past, automotive sources were the major contributor of lead (Pb) emissions to the atmosphere. As a result of EPA's regulatory efforts to reduce the content of Pb in gasoline, the contribution from the transportation sector has declined. Today, smelters and battery plants are the major sources of Pb emissions to the atmosphere. The highest concentrations of Pb are found in the vicinity of nonferrous smelters and other stationary sources of Pb emissions.
Exposure to Pb mainly occurs through the inhalation of air and the ingestion of Pb in food, water, soil, and dust. It accumulates in the blood, bones, and soft tissues. Excessive exposure to Pb may cause mental retardation and behavioral disorders. Even at low doses, Pb exposure is associated with changes in fundamental enzymatic, energy transfer, and homeostatic mechanisms in the body. Recent studies show that Pb may be a factor in high blood pressure and subsequent heart disease.
As a result of the elimination of Pb from gasoline, Pb concentrations in the ambient air are generally so low that monitoring for Pb is not necessary. Data for lead monitoring in the Las Vegas area during the report period are not presented in this report.
Ozone
Ground level ozone (O3) is a toxic gas, one of a group of complex oxidants found in the ambient air. Unlike other pollutants, O3 is not emitted directly into the air by specific sources. Ozone is photochemical in nature, meaning that it is formed in the air by chemical reactions among nitrogen oxides, oxygen, and hydrocarbons in the presence of sunlight. Some of the more common sources are gasoline vapors, chemical solvents, combustion products of various fuels, and consumer products. These products can be frequently found in large industrial facilities, gas stations, and small businesses such as bakeries and auto body repair shops. Often these "precursor" gases are emitted in an area, but the actual chemical reaction, stimulated by sunlight and temperature, takes place in another. Combined emissions from motor vehicles and stationary sources can be carried hundreds of miles from their origins, forming high O3 concentrations over very large regions.
Ozone in different layers of the atmosphere (i.e., stratospheric O3 versus ground level O3) exhibits different effects, while the physical substance remains the same. In the upper atmosphere, O3 is produced by sunlight from oxygen in the air. Near the ground, O3 is produced primarily from man-made compounds. It is important to note that the O3 near the ground affects man adversely and therefore is considered a harmful pollutant. The stratospheric O3, however, is essential to human survival, as it plays a key role in determining the temperature of the stratosphere and prevents harmful ultraviolet solar radiation from reaching the earth's surface. Over the last two decades, growing concern has been expressed for maintaining the irreplaceable high-altitude layer of this compound, while at the same time the O3 near the ground is a harmful pollutant. Ozone is the most abundant and most reliably measured oxidant present in the air.
While O3 in the air can be related to both natural and man-made processes, measurements indicate the high concentrations in or near large urban centers are from man-made sources. An important factor in O3 occurrence is the weather. Long sunny days (spring, summer and fall) induce elevated levels of O3.
Studies have shown that continued exposure to O3 levels of 0.3 parts per million (ppm) causes nasal and throat irritation, while short exposure to concentrated levels of 0.5 to 1.0 ppm causes changes in pulmonary function, increased airway resistance, decreased vital capacity, decreased carbon monoxide diffusing capacity, and decreased forced expiratory volume. Concentrations much less than those enumerated above affect asthmatics, impair physical performance, and can result in headaches, chest discomfort, and coughs. Even moderately vigorous exercise is likely to increase the risk of health effects from O3. Individuals may be adversely affected by varying levels of O3 exposure, depending on their physical condition.
Beyond public health, vegetation and entire ecosystems are affected. Ozone has been known to reduce crop yields of citrus, cotton, potatoes, soy beans, wheat, spinach and other sensitive crops, as well as cause visible injury to a variety of plant species. As an example of O3 damage to entire ecosystems, there is evidence that conifers are affected by increased O3 levels, which, in turn, may reduce fruit and seed production, affecting diets of small mammals and subsequently altering the species composition of wildlife in the ecosystem. Ozone also cracks rubber, weakens textiles, causes dyes to fade and causes certain paints and coatings to deteriorate.
In summary, the production of photochemical oxidants has clearly been related to the exhaust pollutants discharged by automobiles and emission of hydrocarbons from gasoline handling. The presence of hydrocarbons, oxides of nitrogen, and sunlight at temperatures in excess of 68 F are ideal conditions for formation of O3 and other photochemical oxidants.
Nitrogen Dioxide
Nitric oxide (NO) and nitrogen dioxide (NO2) are the two nitrogen oxides (NOx) of primary concern to air quality control programs. Nitrogen oxides are formed by fuel combustion in automobiles, power plants, industries, homes and offices. Nitric oxide reacts with oxygen in the air to produce NO2. Motor vehicles and other fuel-burning processes are the main sources of NO and NO2 in the atmosphere.
Nitrogen dioxide has been associated with adverse effects on health more than any other nitrogen oxide. At higher exposures, NO2 causes respiratory system damage of various types, including bronchial damage. Its effects are displayed by increased susceptibility to respiratory infection and emphysematous changes.
While natural background emissions of NOx compounds are known to exist, research has shown the levels to be many times lower than those found around metropolitan and industrialized areas. Therefore, the man-made contributions to the NOx pollutant levels are of great concern.
Sulfur Dioxide
Sulfur oxides (SOx) commonly originate from burning fossil fuels. They also are produced industrially (e.g., smelting and chemical preparation). Sulfur dioxide (SO2) is the criteria pollutant of concern. Examples of highly concentrated sources of SO2 are metal smelting and oil refining industries and large oil- or coal-fired electric power plants. While fuels with lower sulfur levels have been utilized, they are more costly and less heat-efficient for industrial processes. There is an evident seasonal variation for SO2 and, because industrial consumption does not vary much throughout the year, SO2 is associated mainly with power generation and domestic heating.
In the air, SO2 reacts with oxygen, ammonia and other compounds, including water vapor, to form sulfate salts and sulfuric acid mist. It is historically the most prominent of the gaseous pollutants. Sulfur dioxide was the main suspect in such disasters as the London Killer Fog Episode of 1952. In a five-day period, fog, SO2 and a temperature inversion in the valley of the Thames River caused severe illness and an unusually large number of deaths. It is believed that SO2, in combination with particulate pollution, provided the unhealthy environment that existed.
Sulfur dioxide primarily irritates the respiratory system. At low concentrations, it causes constriction of the bronchi. While these effects are not proportional to exposure time, continuous exposure does produce irreversible degenerative changes. Generally, for short periods of exposure, the effect is seen in the first minute or two. It is more likely to affect the elderly and those people who already suffer from respiratory diseases such as asthma, chronic bronchitis and emphysema.
Particulate Matter
Particulate pollutants generally consist of a mixture of particles of dust, pollen, ash, soot, metals and other various solid and liquid chemicals found in the atmosphere. The particulate data in this report deal with particulate matter in the inhalable size range of 10 microns or smaller in aerodynamic diameter (PM10), or "coarse" particles, and the respirable size range of 2.5 microns or smaller in aerodynamic diameter (PM2.5), or "fine" particles. Ten microns is about one-seventh the diameter of human hair.
Unlike the gaseous pollutants which are continuously monitored, PM10 may be sampled every sixth day for a 24-hour period. A sampler commonly used in PM10 sampling is called a high-volume (hi-vol) sampler, which draws a known volume of air through a filter. Suspended PM10 in the surrounding air is collected on an eight-inch by 10-inch quartz fiber filter, which is weighed to indicate the quantity of the sample collected on it. By knowing the volume of air that passed through the filter and the weight of particles collected on the filter, the PM10 concentration can be calculated. The volume of air that passes through the filter in a 24-hour period is approximately equivalent to the amount of air an average adult breathes in about four months.
Meteorological conditions and other natural occurrences need to be considered when evaluating reductions in emissions for maintenance of the ambient particulate standards. While many PM10 emissions, or coarse particles, are from man-made sources (e.g., salt and sand deposited on roads to reduce driving hazards in winter, vehicles traveling on unpaved roads, construction dust, and rock processing), other PM10 pollution comes from indirect sources such as motor vehicles that carry particles which are eventually deposited on roads and subsequently agitated and suspended in the air. In addition, strong winds may cause PM10 concentrations to be high where the vegetation has been removed by man-made or natural causes.
Sources of PM2.5 emissions, or fine particles, originate from fuel combustion from a variety of sources, such as motor vehicles, power generating stations, other industrial facilities, and residential fireplaces and wood-burning stoves. Fine particles also form from the interaction of chemicals, such as sulfur dioxide, nitrogen oxides, and volatile organic compounds, with other compounds in the air.
The EPA will begin analyzing PM2.5 data collected for comparison to the new air quality standards in 2002. Some PM2.5 data collected with uncertified monitors will not be eligible for this comparison. The EPA does have PM2.5 data collected at National Parks and other rural sites, which show that rural monitoring sites in the East typically have higher annual average PM2.5 concentrations than in the West, and the Eastern sites have higher proportions of sulfate from coal-fired power generating stations and other industrial boilers than Western sites. Most of the PM2.5 particles in both the East and the West consist of sulfates and organic carbon.
Particulate matter affects a person's health through the lungs in three ways. The first is the inhalation of toxic particles. Secondly, high levels of particles overload the clearance mechanism of the lungs, lessening their ability to remove toxic particles. Thirdly, particulate matter absorbs harmful gases and enhances the effects of those pollutants on the lungs. When particle concentrations are high, asthma and other respiratory conditions can be aggravated, causing increased coughing and chest discomfort.