air pollution measurements were conducted over a 1-year period in 24 north american communities participating in a respiratory health study . 
 ozone , particle strong acidity , sulfate , and mass ( pm10 and pm2.1 ) were measured in all communities . in 20 of the communities , 
 sulfur dioxide , ammonia , nitric acid , nitrous acid , and particulate nitrate were measured . 
 the sampler was located centrally in the community whenever possible and samples were collected every other day . 
 concentrations of particle strong acidity , mass , sulfate , and ozone were highly correlated both in the region of the country defined as a high - sulfur source area and in the downwind transport regions . 
 these regions of the eastern united states and southern canada experienced the greatest particle strong acidity , sulfate , and particle mass concentrations during the spring and summer months ( may - september ) . 
 the particle strong acidity concentrations were highest in regions close to the high sulfur emission areas of the united states ; that is , in the area immediately to the west of the appalachian plateau and west of the allegheny mountains ( western pennsylvania , eastern ohio , and west virginia ) up through southern ontario . 
 the frequency of particle strong acidity events decreased with transport distance from the region of highest sulfur emissions . 
 low particle strong acidity and sulfates were found at the western and midwestern sites of both the united states and canada . 
 substantial concentrations of nitric acid were found in two of the california sites as well as many sites in the northeastern portion of the united states . 
 sites selected for the epidemiologic study provide a range of annual mean particle strong acidity exposures from below the limit of detection to more than 50 nmol / m3.imagesp492-afigure 1.figure 2.figure 3.figure 4.figure 4.figure 4 .