Below is information on the current background air quality in some of North America's major cities based on data taken from http://www.airnow.gov recorded at 12:30 pm GMT on May 23.
City Quality Atlanta (Georgia) [Good] Chicago (Illinois) [Good] Dallas (Texas) [Moderate] Denver (Colorado) [Good] New Orleans (Louisiana) [Good] New York City (New York) N/A Miami (Florida) [Good] San Francisco (California) [Moderate] Washington DC (District of Columbia) [Good] Montreal (Quebec) [Good] Toronto (Ontario) [Good] Vancouver (British Columbia) [Good]
Air Quality Good Moderate Unhealthy for sensitive groups [UHSG] Unhealthy Very unhealthy Hazardous N/A N/A
The above data was collected from http://www.airnow.gov, an interactive website developed by the US EPA, NOAA, NPS, tribal, state and local agencies to provide public information on national air quality.
The data used is collected using either federal reference or equivalent monitoring techniques or techniques approved by the state, local or tribal monitoring agencies and is displayed on a hourly basis and as such is not fully verified and is therefore only for the purposes of reporting the Air Quality Index.
The indices have six levels from ‘Good' through to ‘Hazardous' based upon five major pollutants regulated by the Clean Air Act -- ground-level ozone, particulate pollution, carbon monoxide, sulfur dioxide and nitrogen dioxide and the potential health risk posed by these pollutants.
For more information on the index calculation and the health risks posed by pollutants see: http://www.epa.gov/airnow/aqi_brochure_08-09.pdf