When present en masse, it is possible to see pollen on surfaces of objects such as cars, patio tables, sidewalks, etc. However, it is rare to actually see pollen when it is present in the air because the number of grains in a given volume of air is too low to be seen with the unaided eye.
There are exceptions, as on those occasions when large amounts of pollen can be released at one time from trees such as junipers or pines. When this happens, the pollen cloud produced can become visible to an observer. In general, one has to sample large volumes of air and concentrate any particulates in it, in order to determine if there is pollen present.
I thought that it might be of interest to some of you to read about the process used by me and other certified pollen counters to collect and examine air samples to determine the types and numbers of pollen grains present.
How Do We Collect and Count Pollen and Mold Spores?
To count accurately the number of pollen grains and/or mold spores in the air it is necessary to sample the air over some measured period of time, and then use mathematical formulas to estimate the number of grains or spores in a given volume of air.
The collection of pollen and mold spores in the air is typically done using a machine which pulls air through a small opening at a measured volume per unit of time. Any pollen grains, mold spores, and particulates present in the air are trapped on a microscope slide (a thin piece of glass which measures approximately 25 X 75 millimeters (mm), that is, a 1 X 3 inch rectangle) which is located in close proximity to this opening.
The slide is covered with a thin layer of silicone grease to which the pollen, mold spores, and other particulates stick. (Note: other materials such as Vaseline can be used to cover the slide, but silicone grease works well and doesn’t melt and run off of the slide on hot July afternoons).
How Does the Burkhardt Volumetric Air Sampler Work?
Most NAB certified pollen collection stations use a machine called a Burkhardt Volumetric Air Sampler. This machine is basically a “weak” vacuum cleaner that pulls air through a slot which measures 2 X 14 mm. The air is pulled into the machine at a measured volume of 10 liters/minute.
The air entering the Burkhardt through this slot impacts on the microscope slide covered with silicone grease. The pollen and mold spores which adhere to the greased microscope slide can then be counted and identified.
The Burkhardt machine has a clock-like mechanism that moves the microscope slide past the 2 X 14 mm slot at a rate of 2 mm per hour. Thus a one day (24 hour) air sample collected in the Burkhardt will cover a 14 X 48 mm portion of the slide’s surface.
After 24 hours the slide is removed from the Burkhardt and brought into the lab where glycerin jelly containing a stain called basic fuschin is melted and dropped onto the portion of the slide containing the concentrated air sample. A cover slip is added and the slide is viewed with a brightfield microscope at 400 X magnification.
Pollen grains will absorb some of the fuchsin stain, a magenta colored dye, and this stains the outer wall of the pollen grains a pink to magenta color. At 400 X magnification any pollen grains are readily visible and can be counted and identified. A portion of the slide is examined and the numbers and types of pollen grains determined.
The information collected in this sample is then used to estimate the number and types of pollen grains present in the air during the 24 hours that the slide was in the Burkhardt Volumetric Air Sampler.
Pictures of the Burkhardt collector on the roof of one of the classroom buildings at BSC, and a typical slide after it has been prepared for microscopic examination are shown below.
H. Wayne Shew, Ph.D.
NAB certified counter
BSC/AAAC Collection Station—Birmingham, Alabama
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