Published: 2007-05-06 12:17:59
Updated: 2007-05-06 12:17:59
Posted May 6, 2007 12:17 p.m. EDT
MIKE MOSS SAYS: David, Pollen counts and concentrations are generally taken using a sampling system called the "rotorod," which uses silicone grease coated clear rods that are exposed to the air on a set schedule, usually over a 24-hour sampling period at a 10% duty cycle (i.e. collecting pollen for one minute followed by 9 minutes of no sampling), then stained and examined microscopically to determine the number of pollen grains covering either the entire rod or some portion therof. This "count" can then be converted to a concentration, usually given in units of grains per cubic meter of air, based on the calculated amount of air the rods were exposed to. I don't have information on the specific placement of the samplers in our area relative to trees and so forth, except that the NC Division of Air Quality sampler that provides the counts that we and the New&Observer use is located in Raleigh near the intersection of Capital Boulevard and I-440. The samples are measured only on weekdays for 24-hour periods ending at 9 am, and so provide an average over a one day period and do not track shorter-term peaks and lulls in pollen concentration.
Here are addresses for a couple of more detailed descriptions of how some labs conduct their count using the rotorod system
In addition to the counts listed in the newspaper from the NC DAQ, you can find counts for Durham and Charlotte online at
The Raleigh counts taken by DAQ are not available online, but if you would like to have them e-mailed to you automatically you can send an email to email@example.com, and put Subscribe Pollen Report in the subject line.
As for the discrepancy you noted in the paper, that may arise because for some reason DAQ chooses to report the individual pollen levels (for trees, grasses and weeds) that are printed in the paper as direct rotorod counts rather than in concentration units. Typically, the "count" numbers are substantially higher than the equivalent concentration given in grains per cubic meter.