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A pollen count is simply that: a count of the average number of pollen grains contained in a cubic yard of air during the collection time, usually 24 hours.
Special devices have been developed to collect pollen and ensure an accurate count of the amount of pollen in a unit of air. Currently, the most commonly used method is that of specially coated glass rods. During predetermined time intervals, the rods are rotated in the air for specified intervals. As the rods are rotated, pollen grains from the air stick to the coated surface.
After 24 hours of such intermittent sampling (usually from
8 AM one morning until 8 AM the next morning) the rods are collected and stained. As the staining fluids are absorbed by the pollen grains, the grains distend and assume a highly characteristic appearance, one generally very different from that of the natural pollen. This change in appearance on staining permits most pollens to be easily identified. The pollen grains are then counted. The average number of grains of each pollen, as well as the average total number of pollen grains per cubic yard of air sampled, is then calculated as the pollen count for the last 24 hours.
When you hear about or read about today’s pollen count, remember that that sample was taken from 8 AM yesterday until 8 AM today.
The amount of pollen in the air is affected by several factors. If you are following the pollen counts provided by the television, radio and/or print media in your area, it will be helpful for you to be aware of these variants:
1. The pollen count you hear or read about today is always at least 24 hours old.
2. “Rain tends to wash pollen out of the air” is generally a true statement: large droplets are not efficient pollen removers, but small droplets are very efficient. Therefore, brief thunderstorms are less efficient pollen removers than are prolonged gentle rains. In fact, thunderstorms may actually re-disperse settled pollen in the air.
3. Humidity affects the pollination process. During times of high relative humidity, pollen grains tend to absorb moisture. This moisture adds weight and fills the tiny air pockets in the rains’ outer walls, making them less aerodynamic. During periods of low relative humidity, water evaporates from the pollen grains walls. This opens the air pockets and lightens the pollen, making it more buoyant. This is why the combination of low humidity and a windy day increases the amount of pollen in the air – and increases the misery of allergy sufferers.
4. Temperature affects pollen counts. Warm air encourages the process of pollination, whereas cool temperatures reduce pollen production.
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