Outdoor Air Quality

Acid rain: Precipitation (rain, snow, sleet, or hail) that contains high levels of dilute sulfuric or nitric acids. Acid rain is produced when water combines with sulfur dioxide and various nitrogen oxides in the atmosphere. The burning of fossil fuels (especially gasoline and high-sulfur industrial fuels) is thought to be the main cause, but some natural sources (volcanic gases and forest fires) may also contribute to acid rain. Acid rain can contaminate drinking water, endanger wildlife and vegetation, and damage buildings and monuments.

Acid: A water-soluble and sour chemical compound that produces positive ions in solution. An acid is the opposite of a base; together, an acid and a base neutralize each other and react to form water and a salt. Common vinegar, for example, is a weak acid.

Acute: Occurring over a short period of time, usually a few minutes or hours. An acute exposure can result in short-term or long-term health effects.

Agency for Toxic Substances and Disease Registry (ATSDR): An agency of the U.S. Department of Health and Human Services. ATSDR's mission is to prevent exposure and adverse human health effects and diminished quality of life associated with exposure to hazardous substances from waste sites, unplanned releases, and other sources of pollution in the environment. The agency's functions include public health assessments of waste sites, health consultations concerning specific hazardous substances, health surveillance and registries, response to emergency releases of hazardous substances, applied research in support of public health assessments, information development and dissemination, and education and training concerning hazardous substances.

Air pollution: Unhealthy particles and gases in the air that harm people, animals, plants, and even objects such as buildings and statues. Air pollution can be present as a solid, liquid, or gas. Acid rain is an example of gases and liquids mixing with otherwise clean air.

Air toxic: Also Hazardous air pollutant or Toxic air pollutant. Any air pollutant that is likely to cause serious or irreversible long-term health effects in humans. Air toxics may cause cancer, developmental effects, reproductive problems, neurological disorders, and genetic mutations. They include pollutants for which a national ambient air quality standard does not exist.

Ambient air: Open air or outdoor air. Ambient air is a blanket of gases surrounding the earth. At ground level, air is a mixture of invisible and odorless gases, mostly nitrogen and oxygen, with smaller amounts of water vapor, argon, carbon dioxide, neon, helium, and hydrogen.

Area source: Also Nonpoint source. Any small source of human-generated air pollution that is released over a relatively small area but that cannot be classified as a point source. Area sources may include vehicles and other small engines, small manufacturing companies, dry cleaners, gas stations, and household activities such as wood burning. Individual area sources may not release much pollution, but the cumulative effect of many area sources may be significant.

Asthma: A serious, chronic lung disease that causes the airways (bronchial tubes) to become narrow and makes it hard to breathe. Asthma attacks are often caused by environmental triggers, such as molds, dust mites, and tobacco smoke.

Carbon monoxide (CO): A criteria air pollutant. Carbon monoxide is a colorless, poisonous gas formed by burning, especially fuels such as gasoline, oil, and wood. Breathing too much carbon monoxide interferes with the body's absorption of oxygen and therefore is especially dangerous for people with respiratory and heart disease.

Carcinogen: Any substance that causes or aggravates cancer.

Chronic: Occurring over a long period of time (more than 1 year).

Coarse particles: See Particulate matter.

Combustion: The process of burning. Many important air pollutants, such as sulfur dioxide, nitrogen oxides, and small particles, are the products the combustion of fuels such as coal, oil, gasoline, and wood.

Concentration: The amount of one substance dissolved or contained in a unit amount of another substance, or the relative proportions of two or more quantities in a mixture. Sea water, for example, contains a higher concentration of salt than does fresh water. Concentrations, which may be expressed in various ways, are often described in terms of a component's percentage by weight or volume. Very low concentrations, such as those of various substances in the atmosphere, are commonly expressed in parts per million (ppm).

Consumption advisory: A governmental warning advising people not to eat, or to eat only in limited quantities, certain foods, such as fish contaminated with mercury.

Contaminant: Any substance or material that enters a system (such as the environment) where it is not normally found.

Criteria pollutant: Criteria Pollutants - EPA uses six "criteria pollutants" as indicators of air quality, and has established for each of them a maximum concentration above which adverse effects on human health may occur. These six pollutants are: Ozone, Nitrogen Dioxide, Sulfur Dioxide, Particulate Matter (less than 10 microns), Lead, Carbon Monoxide.

Cumulative exposure: The sum of exposures of an organism to a chemical over a period of time.

Dioxin: Any of a family of compounds known chemically as dibenzo-p-dioxins. Dioxin is considered highly toxic and may cause cancer or birth defects. Dioxins are created as by-products in many industrial processes.

Ecosystem: The interacting system of living things and their nonliving environment.

Emphysema: A chronic and progressive disease in which the small air sacs in the lungs become abnormally large. Emphysema causes difficulty in breathing, coughing, and an increased susceptibility to infection; it can also increase the likelihood of heart disease. It is often associated with long-term cigarette smoking.

Exposure: Contact with a chemical by swallowing, breathing, or direct contact (such as through the skin or eyes). Exposure may be short term (acute) or long term (chronic).

Fine particles: See Particulate matter.

Hazardous air pollutant (HAP): See Air toxic.

Hydrocarbon: A chemical compound consisting only of hydrogen and carbon. Hydrocarbons, which occur naturally in petroleum, natural gas, coal, and wood, are often used as fuels. They are emitted into the air when the fuel does not burn or burns only partially. Hydrocarbons react in the presence of nitrogen oxides and sunlight to form ground-level ozone, a major component of smog. The most commonly tracked hydrocarbons are volatile organic compounds (VOCs).

Inflammation: A local reaction when tissue becomes irritated, injured, or infected. Inflammation is usually characterized by pain, redness, swelling, heat, and sometimes loss of function.

Lead (Pb): A heavy metal that is hazardous to human health. Lead occurs in the air as small particles, often from emissions from lead smelters and other metal-processing plants.

Metal: A chemical element that usually has a shiny surface, is generally a good conductor of heat and electricity, and can be melted or fused, hammered into thin sheets, or drawn into wires.,. The metals comprise about two-thirds of all known elements. Unlike nonmetals, metals form positive ions and basic oxides and hydroxides. Upon exposure to moist air, many metals react chemically with the oxygen of the atmosphere to form a metallic oxide, such asrust on exposed iron.

Microgram: A unit of mass equal to one millionth of a gram.

Micron: A unit of length equal to one millionth of a meter.

Milligram: A unit of mass equal to one thousandth of a gram.

Mobile source: A moving object that is a source of pollution. Mobile sources include road vehicles, such as cars, trucks, buses, and motorcycles, and nonroad vehicles, such as trains, planes, and gasoline-powered lawn mowers. A mobile source is distinguished from a stationary source or point source.

National Ambient Air Quality Standards (NAAQS): Standards set by the EPA for all criteria pollutants. EPA has established primary standards to protect public health and secondary standards to protect other aspects of public welfare, such as preventing materials damage, preventing crop and vegetation damage, or assuring visibility.

Nitrogen oxide: A criteria air pollutant. Nitrogen oxides (collectively known as NOx) are produced from burning fossil fuels, including gasoline and coal. Nitrogen oxides react with volatile organic compounds to form smog. Nitrogen oxides are also major components of acid rain. They may be harmful to the lungs and aggravate asthmatic symptoms.

Nonattainment area: A geographic area of the United States in which the level of a criteria air pollutant persistently exceeds the level allowed by the National Ambient Air Quality Standards (NAAQS). An area may have acceptable levels of one criteria air pollutant but unacceptable levels of one or more other criteria air pollutants; thus, an area can be both attainment and nonattainment at the same time. It has been estimated that 60% of Americans live in nonattainment areas.

Nonpoint source: See Area source.

Ozone (O3): A criteria air pollutant. Ozone is a variety of the oxygen molecule that consists of three atoms of oxygen instead of the more common two atoms. It occurs in nature, for example, when lightning strikes. Ozone is also formed by a chemical reaction of nitrogen oxides and volatile organic compounds together with sunlight and warm temperatures. In the upper atmosphere, high concentrations of ozone act to shield the Earth from harmful ultraviolet radiation from the sun. Holes in the ozone layer allow excess radiation to reach the Earth's surface, which increases the risk of skin cancer in humans. At ground-level, ozone is often referred to as smog. Breathing too much ozone can damage lung tissue and aggravate respiratory problems like asthma and emphysema. High ozone levels are also harmful to plants, crops, and trees, as well as to materials like rubber and paints. Ozone is a widespread and serious urban air pollution problem.

Part per billion (ppb): Unit of measurement commonly used to express a contamination ratio, as in establishing the maximum permissible amount of a contaminant in water, land, or air.

Part per million (ppm): Unit of measurement commonly used to express a contamination ratio, as in establishing the maximum permissible amount of a contaminant in water, land, or air.

Particulates, particulate matter (PM-2.5 and PM-10): A criteria air pollutant. Particulate matter includes dust, soot, smoke, and other small solid particles found in the air or in emissions. Particulates are produced by many sources, including the burning of diesel fuels by trucks and buses, incineration of garbage, mixing and application of fertilizers and pesticides, road construction, industrial processes such as steel manufacture, mining operations, agricultural burning, and operation of fireplaces and wood stoves. Particulates include both fine particles (PM-2.5), which come primarily from fuel combustion in cars and trucks, and coarse particles (PM-10), which result largely from vehicles traveling on unpaved roads. Particulate pollution can cause eye, nose, and throat irritation and other health problems. Exposure to fine particles is associated with heart and lung disease, increased respiratory symptoms and disease, decreased lung function, and even premature death. Exposure to coarse particles is primarily associated with the aggravation of respiratory conditions, such as asthma.

Point source: Also Stationary source. A source of pollution that is stationary, or stays in one place. Some 24% of air toxics come from large stationary sources, such as chemical plants, oil refineries, and pulp and paper mills.

Primary standard: A pollution limit set by the EPA for a criteria pollutant and based on health effects.

Secondary standard: A pollution limit set by the EPA for a criteria pollutant and based on environmental effects such as damage to property, plants, visibility, etc.

Smog: A mixture of pollutants, principally ground-level ozone, produced by chemical reactions in the air. Smog can harm health, damage the environment, and hinder visibility. Major smog occurrences are often linked to heavy motor vehicle traffic, sunshine, high temperatures, and certain other weather conditions. Smog may develop far from the source of smog-forming chemicals (such as volatile organic compounds), since the chemical reactions that cause smog occur high in the air, as the prevailing winds carry the reacting chemicals from their sources.

Stationary source: See Point source.

Sulfur dioxide (SO2): A colorless gas formed during the burning of fuels containing sulfur, such as coal. Breathing SO2 may irritate the respiratory system and aggravate asthmatic symptoms.

Toxic air pollutant: See Air toxic.

Volatile organic compounds (VOCs): Substances containing carbon and various proportions of other elements, such as hydrogen, oxygen, fluorine, chlorine, bromine, sulfur, and nitrogen; these substances easily become vapors or gases. VOCs are commonly found in gasoline, solvents (paint thinners, lacquer thinner, degreasers, and dry cleaning fluids), oil-based paints and inks, and consumer products, such as aerosol spray products. VOCs react with nitrogen oxides, sunlight, and heat to form ozone. Many VOCs are considered air toxics.