From the government's AIRNow website. Click for latest report.
The weather forecasts for summer’s hot, steamy days usually come with an air-quality rating – “code yellow, code orange, etc.” But what does that really mean?
The federal Environmental Protection Agency, National Oceanic and Atmospheric Administration and other authorities have developed AIRNow, a website where you can get the latest air quality information and learn more about the system.
According to the AIRNow site, the air-quality index you see and hear in the news tells us how clean or polluted the air is, and how it can affect health. It’s based on the EPA’s calculations for five major air pollutants regulated by the federal Clean Air Act: ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide.
LINK
AIRNow website, http://airnow.gov/
by 
