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A pollen count is a measure of the number of grains of pollen in a cubic meter of air. The more pollen grains present, the more someone that is allergic to the pollen will suffer. Counts are usually taken at a local level by news and weather stations. They are often taken for specific local plants such as trees, grasses, and weeds.
The most common method is by using a collection device called a Rotorod Sampler that is often located on the rooftop of a building to gather pollen particles and other allergens.
Sticky "collector rods" are coated with a thin layer of Silicone Grease and exposed to the outdoor air from anywhere between 15 minutes to 24 hours. The pollen, allergens, and other particles in the air come in contact with the rods and adhere to them.
These rods are then brought in and examined under a microscope at 400x magnification. If there are less than 400 grains of pollen on a rod, then every single grain on the rod is counted by hand!
If there are more than 400 grains on a rod, then only a small portion is counted, and the total amount is estimated using statistics. The counts are then reported as the number of pollen grains in a cubic meter of air at a given time.
There are many different resources for finding out an accurate pollen and allergen count where you live.
Although these sits can give you a good overview of pollen conditions in your state or region, they are not always the best sources of detailed information for your specific city or allergens of concern. Usually, the best resources will be your local weather or new station's website.