Recording
the Pollen Count
What is the Pollen Count?
The pollen count is a measure of the number of pollen grains of a certain type per cubic metre of air sampled, averaged over 24 hours. The pollen forecasts are produced by the National Pollen and Aerobiology Research Unit using the pollen counts measured each day at the Pollen UK network sites, together with information about the weather, growing seasons and the flowering times of the plants.
How do the grass pollen forecast categories relate to the pollen count?
The pollen forecast is usually given as low, moderate, high or very high.
Low is less than 30 pollen grains per cubic metre of air.
Moderate is 30 to 49 pollen grains per cubic metre of air.
High is 50 to 149 pollen grains per cubic metre of air.
Very High is 150 or more pollen grains per cubic metre of air.
Most sufferers will start to experience symptoms when the count reaches the moderate category, although counts will be higher near to large sources of grass pollen, such as hay meadows.
The categories for tree and weed pollen types are different due to their different sizes and allergenicity.
How is the pollen count measured?
Each of the pollen monitoring sites around the country has a Burkard volumetric spore trap. Traps are located usually on the roof of a suitably accessible building about two or three storeys high. Traps are located at this height to enable the general ambient airflow to be monitored which contains a good mix of the local and further distant pollen sources gathered on the wind. If the trap was at ground level then it would mainly collect pollen from the immediate vicinity and results between sites would not be comparable.
The air is sucked into the trap through a slit at a rate of 10 litres per minute and the pollen and other particles are captured on a prepared adhesive tape passing the slit at a set rate. Some sites have the adhesive tape mounted onto a rotating drum, others use a microscope slide. The tape, after exposure, is put onto a microscope slide and covered with a gel mountant containing a stain to aid identification and the pollen is then counted. Counting is done using a standard sampling procedure.
Twelve transects are counted across the width of the slide representing two hourly intervals. A daily total is obtained by summing the counts found in the twelve transects and these figures are then converted to grains per cubic metre of air using an equation called the correction (or conversion) factor. Counting each slide can take an hour or even longer depending on the amount of pollen on it. Nobody has yet developed a satisfactory automatic method for counting pollen.
Above: The pollen monitoring site at Worcester. The instruments are volumetric Burkard pollen and spore traps
The pollen monitoring network sites are all on exposed rooftops. The sites have continuous sampling. The daily pollen count is an avarage of 24 hours. It is given as the number of pollen grains of one type (usually grass for the media ) per cubic metre of air.
The trap samples air at 10 l/min through a critical orifice. The inlet is kept facing the airflow because the top part of the trap can move around, driven by the wind vane