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Air Quality Planning and Standards

Air Pollution Emissions Overview

What are emissions? Where do they come from?
National Trends, Criteria Pollutants, Sources, Rules and Regulations

Measuring, reporting, and using emissions data
Emissions measurement, data storage, reporting and evaluation, modeling and software

What are emissions? Where do they come from?

Emissions is the term used to describe the gases and particles which are put into the air or emitted by various sources.

National Trends
The amounts and types of emissions change every year. These changes are caused by changes in the nation's economy, industrial activity, technology improvements, traffic, and by many other factors. Air pollution regulations and emission controls also have an effect. The National Air Pollutant Emission Trends report summarizes long-term trends in emissions of air pollutants and gives in-depth analysis of emissions for the current year. The report also discusses emission evaluation and prediction methodologies.

Criteria Pollutants

The United States Environmental Protection Agency (EPA) is mainly concerned with emissions which are or could be harmful to people. EPA calls this set of principal air pollutants, criteria pollutants. The criteria pollutants are carbon monoxide (CO), lead (Pb), nitrogen dioxide(NO2), ozone (O3), particulate matter (PM), and sulfur dioxide (SO2). There are also a large number of compounds which have been determined to be hazardous which are called air toxics.


There are many sources of emissions. These have been grouped into four categories: point, mobile, biogenic, and area.

Rules and Regulations

In 1970 The United States Congress passed Clean Air Act(CAA) Amendments (the CAA was passed in 1963) which set into motion a nationwide effort to improve the country's air quality. Since then additional laws and regulations have been added including the 1990 Amendments to the Clean Air Act.

To read about these rules and regulations see:

Measuring, reporting, and using emissions data


In order to make improvements in the air quality, the amount of pollutants in the air must be measured. The Emissions Measurement Center develops standards and evaluates testing methods so that regulations can be developed and enforced.

What is an Emission Factor?

An emission factor is a relationship between the amount of emissions that are released and the activity of the producer. Emission factors are used to predict emission levels for different industries.

What are Emission Inventories?

Emission inventories are quantities of pollutants measured over time. Emission inventories can be compared with air pollutant levels in an area to determine if increased emissions decreases the air quality.

Data Storage

Once the measurements are made the information must be collected and stored so that it can be used to evaluate the air quality and effects of the regulations. The Clearinghouse for Inventories and Emissions Factors (CHIEF) is a centralized resource for emissions data.



The emissions data that is gathered is also used to create models which can help to predict what air quality will be like in the future and what effect new regulations might have on air quality.


Computer software programs have been created to help people to evaluate and sort out the pollution data that has been collected.

To find out more about emissions try this more detailed Air Pollution Emissions Projects Page.



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