The Clean Air Act, which was last amended in 1990, required the EPA to set National Ambient Air Quality Standards (40 CFR part 50) for pollutants considered harmful to public health and the environment. The Clean Air Act established two types of national air quality standards. Primary standards set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection against decreased visibility, damage to animals, crops, vegetation, and buildings.

The EPA Office of Air Quality Planning and Standards (OAQPS) has set National Ambient Air Quality Standards for six principal pollutants, which are called "criteria" pollutants. They are listed below. Units of measure for the standards are parts per million (ppm) by volume, milligrams per cubic meter of air (mg/m³), and micrograms per cubic meter of air (µg/m³).

(1) Not to be exceeded more than once per year.
(2) Final rule signed October 15, 2008.
(3) To attain this standard, the 3-year average of the 98th percentile of the daily maximum 1-hour average at each monitor within an area mustnot exceed 0.100 ppm (effective January 22, 2010).
(4) Not to be exceeded more than once per year on average over 3 years.
(5) To attain this standard, the 3-year average of the weighted annual mean PM_2.5 concentrations from single or multiple community-oriented monitors must not exceed 15.0 µg/m³.
(6) To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations at each population-oriented monitor within an area must not exceed 35 µg/m³ (effective December 17, 2006).
(7) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.075 ppm. (effective May 27, 2008)
(8.a) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.08 ppm.
(8.b) The 1997 standard—and the implementation rules for that standard—will remain in place for implementation purposes as EPA undertakes rulemaking to address the transition from the 1997 ozone standard to the 2008 ozone standard.
(8.c) EPA is in the process of reconsidering these standards (set in March 2008).
(9.a) EPA revoked the 1-hour ozone standard in all areas, although some areas have continuing obligations under that standard ("anti-backsliding").
(9.b) The standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is < 1.

Carbon monoxide is an odorless, colorless, and toxic gas. At low concentrations CO can cause fatigue in healthy people and chest pain in those with heart disease. At moderate concentrations people can experience impaired vision and coordination, headaches, dizziness, confusion, and nausea. Acute effects are due to the formation of carboxyhemoglobin in the blood, which inhibits oxygen intake. At high concentrations, CO exposure can be fatal.

Common sources of CO are unvented kerosene and gas space heaters; leaking chimneys and furnaces; back-drafting from furnaces, gas water heaters, woodstoves, and fireplaces; gas stoves; generators and other gasoline powered equipment; and tobacco smoke. Auto, truck, or bus exhaust from attached garages, nearby roads, or parking areas can also be a source.

Lead is a toxic metal that was used for many years in products found in and around our living environments. In the past, motor vehicles were the major contributor of lead emissions to the air. As a result of the EPA’s regulatory efforts to reduce lead in gasoline, air emissions of lead from the transportation sector, and particularly the automotive sector, have greatly declined over the past two decades.

Once taken into the body, lead distributes throughout the body in the blood and is accumulated in the bones. Depending on the level of exposure, 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, heart disease) in adults. Infants and young children are especially sensitive to even low levels of lead, which may contribute to behavioral problems and learning disabilities, to seizures and even death.

Today, deteriorating lead-based paint, lead contaminated dust, lead contaminated residential soil, and industrial processes (primarily metal processing) are the major sources of lead emissions to the air. The highest air concentrations of lead are usually found near lead smelters. Other stationary sources are waste incinerators, utilities, and lead-acid battery manufacturers.

A 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. The sum of nitric oxide (NO) and NO2 is commonly called nitrogen oxides or NOx. Other oxides of nitrogen including nitrous acid and nitric acid are part of the nitrogen oxide family. While the EPA’s National Ambient Air Quality Standard (NAAQS) covers this entire family, NO2 is the component of greatest interest and the indicator for the larger group of nitrogen oxides.

Current scientific evidence links short-term NO2 exposures (ranging from 30 minutes to 24 hours) with adverse respiratory effects, including airway inflammation in healthy people and increased respiratory symptoms in people with asthma. NOx react with ammonia, moisture, and other compounds to form small particles. These small particles penetrate deeply into sensitive parts of the lungs and can cause or worsen respiratory disease (such as emphysema and bronchitis) and can aggravate existing heart disease, leading to increased hospital admissions and premature death.

The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuel. NO2 concentrations in vehicles and near roadways are appreciably higher than those measured at monitors in the current network. In fact, in-vehicle concentrations can be 2-3 times higher than measured at nearby area-wide monitors. Near-roadway (within about 50 meters) concentrations of NO2 have been measured to be approximately 30-100 percent higher than concentrations away from roadways.

"Particulate matter", also known as particle pollution or PM, is a complex mixture of extremely small particles and liquid droplets. Particle pollution is made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles.

The size of the particles is directly linked to their potential for causing health problems. EPA is concerned about particles that are 10 micrometers in diameter or smaller because those are the particles that generally pass through the throat and nose and enter the lungs; some may even get into your bloodstream. Once inhaled, these particles can affect the heart and lungs and cause serious health effects. The EPA groups particle pollution into two categories:

• "Inhalable coarse particles," such as those found near roadways and dusty industries, which are larger than 2.5 micrometers and smaller than 10 micrometers in diameter.
• "Fine particles," such as those found in smoke and haze, which are 2.5 micrometers in diameter and smaller. These particles can be directly emitted from sources such as forest fires, or they can form when gases emitted from power plants, industries, and automobiles react in the air.

Numerous scientific studies have linked particle pollution exposure to a variety of problems, including:

• increased respiratory symptoms (e.g., irritation of the airways, coughing, or difficulty breathing),
• decreased lung function,
• aggravated asthma,
• development of chronic bronchitis,
• irregular heartbeat,
• nonfatal heart attacks, and
• premature death in people with heart or lung disease.

People with heart or lung diseases, children and older adults are the most likely to be affected by particle pollution exposure. However, even if you are healthy, you may experience temporary symptoms from exposure to elevated levels of particle pollution.

The top leaders in PM_2.5 and PM₁₀ are road dust, industrial processes, and electricity generation. In 2005, road dust alone accounted for over 10 million tons of PM₁₀ and about 1 million tons of PM_2.5. Other examples of processes that emit PM are: fires, waste disposable, non-road equipment, solvents, fertilizers, and livestock.

Ozone (O3) is a gas that occurs in two layers of the atmosphere, the stratosphere and the troposphere. The stratospheric or "good" ozone layer, which extends upward from about 10 to 30 miles above the earth's surface, protects life on Earth from the sun's harmful ultraviolet rays (UV-b). However, ozone found in the troposphere, the layer of the atmosphere that extends from the earth's surface to about 10 miles up, is deemed ground level or "bad" ozone.

At ground level, ozone is an air pollutant that damages human health, vegetation, many common materials, and is a key ingredient of smog. Industry is often blamed entirely for emissions that cause ground level ozone air pollution, but, actually, private citizens are responsible for a significant percentage of the air pollutants that lead to ground level ozone production. Motor vehicle emissions are the single greatest contributor to ground level ozone pollution.

At ground level, ozone is a harmful pollutant. Ozone pollution is a particular concern during the summer months because strong sunlight and hot weather result in harmful ozone concentrations in the air we breathe. Many urban and suburban areas throughout the United States have high levels of "bad" ozone. But many rural areas of the country are also subject to high ozone levels, as winds carry emissions hundreds of miles away from their original sources.

Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma. "Bad" ozone also can reduce lung function and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue. Healthy people can also experience difficulty breathing when exposed to ozone pollution. Because ozone forms in hot weather, anyone who spends time outdoors in the summer may be affected, particularly children, outdoor workers, and people exercising. Millions of Americans live in areas where the national ozone health standards are exceeded.

In general, it is safer to be indoors on high ozone days. Outdoor ozone appears to be the major source of indoor ozone. Indoor ozone concentrations vary widely -- from less than 10 percent of the outdoor level to 80 percent of the outdoor level. This large variation is caused by many factors, such as air infiltration or exchange rate of the structure, interior air circulation, interior surface composition (e.g., rugs, draperies, furniture, etc.) and reaction with other indoor air compounds.

In situations where there is an indoor ozone source, indoor ozone concentrations have been reported to range between 0.12 to 0.80 ppm. While almost any electronic device produces ozone, most do not emit detectable levels and are harmless to one’s health. Typical indoor ozone sources that produce detectable levels are devices such as: ozone generators, electrostatic air cleaners, photocopiers, and laser printers, which can be responsible for higher concentrations of indoor ozone. When incorporating electrostatic air cleaners, or any air purification device, be sure to install devices that produce levels of ozone that are not traceable and have no accumulated effects once dissipated into the space.

Ozone has the same chemical structure (O₃), whether it occurs miles above the earth or at ground level. Ground-level or "bad" ozone is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. 

VOCs + NO_x + Sunlight = Ozone

There are other factors involved with the formation of "bad" or ground level ozone, including cloud cover, wind direction, and low wind speeds. If the weather conditions are conducive, and there are ample amounts of NO_x and VOCs, harmful concentrations of ground level ozone can form in the air.

VOCs are a broad class of chemicals that contain carbon atoms and which are either gas or liquid. Liquids tend to give off high levels of vapor at normal temperatures. Gasoline and common solvents are typical examples. VOCs are widely used as ingredients in household products including paints; varnishes; wax; fuels; cleaning, disinfecting, and degreasing products; cosmetics; and hobby products. Some VOCs are safe to handle and have little known health effects, while other VOCs are highly toxic. In addition to all of the manmade sources of VOCs, natural sources of VOCs exist. For example, trees naturally release small amounts of VOCs. 

One VOC that should be considered by itself is formaldehyde. Individuals can become sensitized to it and thereafter experience adverse reactions (mainly respiratory effects) at even extremely low levels of exposure. There are other VOCs for which this same type of effect can occur (for example, MDI, which is produced when polyurethane foam cures). Formaldehyde is given off at quite high rates by what is called "medium density particle board," which is frequently used in homes, sub-flooring, and in cabinetry and the like. Since its toxic properties have become well known, manufacturers now make the material so it emits at much lower levels. Nevertheless, this material is of concern and individuals sensitive to formaldehyde can suffer serious health effects from living in a home in which a great deal of it is present. 

The top VOC emission sources have been found to be solvent use, on-road vehicles, non-road equipment, and industrial processes. Other examples of processes that emit VOCs are fires, waste disposal, fuel combustion, electrical generation, fertilizer, and livestock.

The EPA’s National Ambient Air Quality Standard (NAAQS) for SO₂ is designed to protect against exposure to the entire group of sulfur oxides (SO_x). SO₂ is the component of greatest concern and is used as the indicator for the larger group of gaseous sulfur oxides (SO_x). Other gaseous sulfur oxides (e.g., SO₃) are found in the atmosphere at concentrations much lower than SO₂.

Sulfur dioxide (SO₂) is one of a group of highly reactive gasses known as “oxides of sulfur”. Natural processes like volcanic activity and biological decay are responsible for half of the world’s atmospheric sulfur. Of the human sulfur contribution, the largest sources of SO₂ emissions are from fossil fuel combustion at power plants (66 percent) and other industrial facilities (29 percent). Smaller sources of SO₂ emissions include industrial processes such as extracting metal from ore, and the burning of high sulfur containing fuels by locomotives, large ships, and non-road equipment. 

SO₂ is linked with a number of adverse effects on the respiratory system. Current scientific evidence links short-term exposures to SO₂ (ranging from 5 minutes to 24 hours) with an array of adverse respiratory effects, including bronchoconstriction and increased asthma symptoms. These effects are particularly severe for asthmatics at elevated ventilation rates (e.g., while exercising or playing). 

SO_x can react with other compounds in the atmosphere to form small particles. These particles penetrate deeply into sensitive parts of the lungs and can cause or worsen respiratory disease (such as emphysema and bronchitis) and can aggravate existing heart disease, leading to increased hospital admissions and premature death. The EPA’s NAAQS for particulate matter (PM) are designed to provide protection against these health effects.