The outdoor air is sampled for the previous 24 hour period using a device called a Burkard air sampler. The Burkard samples the air constantly over a 24 hour period and the airborne mold and pollen granules are captured on glass slide. The glass slide is then retrieved daily and stained before being placed under a microscope. The individual pollens and molds are then counted and classified.
The count is reported as the number of pollen granules per cubic millimeter of air/24 hours and interpreted as absent, low, moderate, or high. The mold spores are similarly counted and reported also as absent, low, moderate, or high.
These counts are also reported to a national data base.
You are welcome to view pictures of this device at http://pollencountingstation.shutterfly.com.