Basically it’s acid rain causing ammonium nitrates and sulfates (can make the moisture in the air have a pH commonly near 3 - coke is about 4.2), elemental carbon, organometallic carbon (gives me a killer headache), and of course a varied assortment of the usual hydrocarbon combustion products such as benzene/mercury/arsenic…. you name it.

Another good place to start is with VIEWS main page. Click ’spacial patterns” on the left, and then under “parameters” select “FM” which stands for Fine Mass (pm2.5). The resulting US map shows average, worst, or best 20% days for all monitors within the IMPROVE system which are found in mostly parks and forests. Here:
http://vista.cira.colostate.edu/views/Web/AnnualSummary/ContourMaps.aspx

The EPA’s site Airdata can give you your area or cities average level. Select “reports and maps”, then ’select geographic area”, then your state and “go”, then “monitor locator” and select pm2.5 and the year (i suggest 2007) to find the id number of your nearest monitor, then back to page with monitor locator on it and instead select “monitor value” and you will see the annual average. Santa Fe, NM averages 4.8 and the worst parts of Los Angeles or DC (or shreveport, LA during the summer low wind months) can avg 18ug’s/m3. A very good feel good day in Vermont (when the jet stream driven cold front “cleans” the regional haze away) commonly measures 2 or 3 ug’s. Airdata here:
http://www.epa.gov/air/data/index.html

Get a true HEPA floor standing filter from SEARS or Lowe’s, let it run 24/7, and you’ll get a lot out (99.97% effective for particles 300 nanometers and larger). You’ll need a “clean room” ULPA for the most harmful nanoparticles. ULPA’s get 99.997% of 0.1 micron (100nanometers) and larger. Heart disease and cancer are all proven results of breathing 6,000 liters of this toxic air per day. Sort of like smoking cigs but less concentrated and all day long.

Buy a true hybrid (97% reduced emmissions !!!!), like the Prius or Civic/Insight, and breathe easier !