Criteria Pollutants

Criteria Pollutants: Definitions and Health Effects

Lead (Pb)

Lead is a naturally occurring element found in the earth's crust. While it has some beneficial uses, it can be toxic to humans and animals. With EPA's regulatory efforts, including the removal of lead from motor vehicle gasoline, levels of lead in the air decreased by 98 percent between 1980 and 2014. The remaining sources of lead emissions vary from one area to another. At the national level, major sources of lead in the air are ore and metals processing and piston-engine aircraft operating on leaded aviation fuel. Other sources are waste incinerators, utilities, and lead-acid battery manufacturers. The highest air concentrations of lead are usually found near lead smelters. Lead is also persistent in the environment and can be added to soils and sediments through deposition from sources of lead air pollution.

Elevated lead in the environment can result in decreased growth and reproductive rates in plants and animals, and neurological effects in vertebrates. Lead distributes throughout the body in the blood and is accumulated in the bones. For humans, lead can adversely affect the nervous system, kidney function, immune system, reproductive and developmental systems and the cardiovascular system. Lead exposure also affects the oxygen carrying capacity of the blood. The lead effects most commonly encountered in current populations are neurological effects in children and cardiovascular effects (e.g., high blood pressure and heart disease) in adults. Infants and young children are especially sensitive to even low levels of lead, which may contribute to behavioral problems, learning deficits and lowered IQ.

Sulfur Dioxide (SO2)

Sulfur dioxide (SO2) is a colorless gas that has a pungent odor. Sulfur dioxide belongs to the family of sulfur oxide gases. Sulfur is prevalent in raw materials such as crude oil, coal, and metal ores. Sulfur oxide gases are formed when fuel containing sulfur is burned; when gasoline is extracted from oil; and, when metals are extracted from ore. Sulfur dioxide is also used in bleaching, as a refrigerant and for fumigating fruit. The majority of SO2 released into the air comes from electric utilities, particularly those that burn coal. Other sources that release SO2 include petroleum refineries, cement manufacturing, metal processing facilities, locomotives, large ships, and some off-road diesel equipment. Sulfur dioxide dissolves in water vapor to form sulfuric acid, and interacts with other gases and particles in the air to form sulfates and other products that can be harmful to people and the environment.

Sulfur dioxide contributes to respiratory illness and aggravates existing heart and lung diseases, especially in children and the elderly. People with asthma are affected by peak levels of SO2. It also contributes to the formation of acid rain, which damages trees, crops, historic buildings, and monuments, and makes soils, lakes, and streams acidic. SO2 is not confined to areas where it is emitted. Sulfur dioxide and sulfate particles can be transported and deposited far from the point of origin. It also contributes to the formation of atmospheric particles that cause visibility impairment.

Carbon Monoxide (CO)

Carbon monoxide (CO), is a colorless, odorless, tasteless, and poisonous gas that is formed when carbon in fuel is not completely burned. It is a component of motor vehicle exhaust, which contributes approximately 56 percent of all CO emissions nationwide. Higher levels of CO generally occur in areas with heavy traffic congestion. Other sources of CO emissions include industrial processes (metals processing and chemical manufacturing), residential wood burning, and natural sources such as forest fires. The highest levels of CO in outside air typically occur during the colder months of the year when the pollutant becomes trapped near the ground beneath a layer of warm air.


Carbon monoxide reduces oxygen delivery to our bodies’ organs and tissues. The health threat from higher levels of CO is most serious for those who suffer from heart diseases such as angina, clogged arteries, or congestive heart failure. For such persons, a single exposure to CO at low levels may cause chest pain and reduce that person’s ability to exercise; repeated exposures may contribute to other cardiovascular effects. Even healthy people exposed to high levels of CO can develop vision problems, dizziness, headache, reduced manual dexterity, and experience difficulty performing complex tasks.

Nitrogen Oxides (NOx)

Nitrogen oxides (NOx), is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2), along with particles in the air can often be seen as a yellowish-brown layer over many urban areas. Nitrogen oxides form when fuel is burned at high temperatures, such as in a combustion process. The primary sources of NOx are motor vehicles, boilers, furnaces, and other industrial, commercial, and residential sources that burn fuels.

NOx and sulfur dioxide react with other substances in the air to form acids which fall to earth as rain, fog, snow or dry particles. Acid rain causes deterioration of car finishes, buildings and historical monuments, and causes lakes and streams to become acidic and unsuitable for many species of fish. NOx reactions can also cause the formation of small particles that penetrate deep into the lungs and can cause or worsen respiratory disease such as emphysema and bronchitis, and aggravate existing heart disease. Nitrate particles and nitrogen dioxide can block the transmission of light, reducing visibility in urban areas and on a regional scale in our national parks and wilderness areas.

Ozone (O3)

Ozone (O3) is a gas that occurs in the Earth’s upper atmosphere and at ground-level. In the upper atmosphere ozone acts as a protective layer against ultra-violet (UV) radiation. Ground-level ozone, or “bad” ozone, is created by a chemical reaction between nitrogen dioxide and volatile organic compounds (VOCs) in the presence of heat and sunlight. Formation of ground-level ozone is dependent on weather. Ground-level ozone levels are typically the highest on hot summer days, with little or no cloud cover and very little wind.


Motor vehicle exhaust, industrial emissions, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC emissions that help to form ground-level ozone. Peak ozone levels typically occur during hot, stagnant summertime conditions. Large urban areas tend to have the highest ozone levels, but even rural areas can experience increased ozone levels when wind carries ozone hundreds of miles from their original sources.

Even at low levels, ground-level ozone triggers a variety of health problems including asthma attacks, reduced lung capacity, and increased susceptibility to respiratory illnesses like pneumonia and bronchitis. Ozone can cause permanent lung damage after long-term exposure. Ozone can irritate lung airways and cause inflammation. Other symptoms include wheezing, coughing, pain when taking a deep breath, and breathing difficulties during exercise or outdoor activities. People with respiratory problems are most vulnerable to elevated ozone levels. Ground-level ozone also damages the leaves of trees and other plants, ruining the appearance of cities, national parks, and recreation areas. Ozone reduces crop and forest yields, and increases plant vulnerability to disease, pests, and harsh weather.

Particulate Matter (PM)

Particulate matter is a broad classification of non-gaseous pollutants that consist of very fine solid particles and liquid droplets or aerosols. Examples of these solid particles can include dust, dirt, soot, and particles in smoke. Some particles are directly emitted into the air from sources such as vehicles, factories, construction sites, tilled fields, unpaved roads, stone crushing, and burning of wood. Other particles may be formed in the air when gases from burning fuels react with sunlight and water vapor such as fuel combustion in motor vehicles or at power plants. Particles can be suspended in the air for long periods of time and vary in size.

PM particle

Some particles are large or dark enough to be seen as soot or smoke. Others are so small that they can only be detected with an electron microscope. The photo above shows a particle with an aerodynamic diameter of less than or equal to 2.5 microns, also called PM2.5. Particulate matter can be transported great distances in the atmosphere. Particulate matter with an aerodynamic diameter of less than or equal to 10 microns is designated as PM10. Burning of wood, diesel and other fuels, and open burning contribute particulate matter to the atmosphere, generally in the form of smoke. In addition, dust from agricultural operations, unpaved roads, and dust storms contain a significant proportion of PM10. Some areas within the state of Kansas experience occasional severe episodes of blowing dust or dust storms. In the 1930s, dust clouds originating in Kansas and neighboring states were observed on the East Coast of the United States.

Particulate Matter causes a wide variety of health and environmental issues. Health issues include: aggravated asthma; respiratory symptoms such as coughing and difficult breathing; chronic bronchitis; decreased lung function and premature death. Elevated PM concentrations result in increased hospital admissions and emergency room visits for people with heart and lung disease. Health problems for sensitive people increase if they are exposed to high levels of PM for several days in a row.

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