EPA (US Environmental Protection Agency) is the federal agency responsible for air quality.

EPA Air Pollutant Information page about specific air pollutants.

AIRNow Local Air Quality Conditions and Forecasts of the Air Quality Index (AQI) air quality forecasts and current conditions for all of the U.S.

National Ambient Air Quality Standards (NAAQS) outlines the laws for air pollution.

Toxics Release Inventory (TRI)  Contains data on certain toxic chemicals released by U.S. facilities.

CARB (California Air Resources Board), CalEPA (California Environmental Protection Agency): The California state agency responsible for air quality.

Diesel & Health Research by the CARB.

California AB 2588 Air Toxics "Hot Spots" Program by CARB. Emissions and facilities data for California. This law requires stationary sources to report the types and quantities of substances released into the air.

SC-AQMD (South Coast Air Quality Management District)  The local agency responsible for air quality.  The site includes data on air pollution and programs in Southern California.

SC-AQMD MATES IV (Mulitple Air Toxics Exposure Study) Report on air toxics in Southern California, including an interactive map of cancer risk.

WHO (World Health Organization) has information on indoor and outdoor air pollution, and global air quality guidelines.