We would love to have air quality monitors every square mile or so, but it's just not practical. So, monitors are placed in locations that can be used to represent a larger area. In your area, there is most likely a monitor to represent the quality of the air you are breathing.
To learn what criteria are used to place monitors, please see http://www.epa.gov/air/oaqps/qa/monprog.html. For more information about the monitors in your area, please contact your local air quality office.