Aftertreatment Devices:Devices that remove pollutants from exhaust gases after they are discharged from the combustion chamber e.g., catalytic converters, diesel particulate filters and diesel oxidation catalysts. They are also known as emissions control devices or exhaust aftertreatment systems. (Source: Donaldson Company, Inc.)

Air pollution is the contamination of air by the discharge of harmful substances. Air pollution can cause health problems, including burning eyes and nose, itchy irritated throat, and difficulty breathing. Some contaminants found in polluted air (e.g., benzene, carbon dioxide, carbon monoxide, lead, nitrogen oxide, particulate matter, and sulfur dioxide) can cause cancer, birth defects, brain and nerve damage, and long-term injury to the lungs and breathing passages. Above certain concentrations and durations, air pollutants can be extremely dangerous and can cause severe injury or death. (Source: National Safety Council)

Air Toxics: Chemicals in the air that are known or suspected to cause cancer or other serious health effects, such as reproductive problems or birth defects. Air toxics are also known as "hazardous air pollutants." Mobile sources emit a number of air toxics associated with both long-term and short-term health effects in people, including heart problems, asthma symptoms, eye and lung irritation, cancer, and premature death. Examples of substances classified as air toxics as toxics by the U.S. Clean Air Act include: acetaldehyde, benzene, 1,3-butadiene, formaldehyde, and polycyclic organic matter (POM). California air toxics regulations also classify diesel exhaust particulates as a toxic air contaminant.(Source: USEPA, Donaldson Company, Inc.)

Carbon Monoxide (CO): A colorless, odorless gas that forms when carbon in fuel is not burned completely. Carbon monoxide is a component of exhaust from motor vehicles and engines. Carbon monoxide emissions increase when conditions are poor for combustion; thus, the highest carbon monoxide levels tend to occur when the weather is very c old or at high elevations where there is less oxygen in the air to burn the fuel. (Source: USEPA)

Emissions Releases of pollutants into the air from a source, such as a motor vehicle or a factory. (Source: USEPA)

Ozone: A gaseous molecule that contains three oxygen atoms (O3). Ozone can exist either high in the atmosphere, where it shields the Earth against harmful ultraviolet rays from the sun, or close to the ground, where it is the main component of smog. Ground-level ozone is a product of reactions involving hydrocarbons and nitrogen oxides in the presence of sunlight. Ozone is a potent irritant that causes lung damage and a variety of respiratory problems. (Source: USEPA)

Pollutants (Pollution): Unwanted chemicals or other materials found in the environment. Pollutants can harm human health, the environment, and property. Air pollutants occur as gases, liquid droplets, and solids. Once released into the environment, many pollutants can persist, travel long distances, and move from one environmental medium (e.g., air, water, land) to another. (Source: USEPA)

Smog: A commonly used term for pollution caused by complex chemical reactions involving nitrogen oxides and hydrocarbons in the presence of sunlight. Ozone is a key component of smog. Smog-forming chemicals come from a wide variety of combustion sources and are also found in products such as paints and solvents. Smog can harm human health, damage the environment, and cause poor visibility. Major smog occurrences are often linked to heavy motor vehicle traffic. (Source: USEPA)