The pollen count measures the concentration of aeroallergens suspended in the air over a specific time period. This measurement is reported as the number of pollen grains found per cubic meter of air (grains/m³). This data provides an objective metric for airborne plant matter, primarily from trees, grasses, and weeds, that can trigger allergic rhinitis (hay fever) and asthma. Monitoring the pollen count allows allergy sufferers to manage their symptoms by taking preventative medications or limiting outdoor exposure when concentrations are high. Unlike a pollen forecast, which is a prediction, the pollen count reflects an actual, measured concentration from a recent 24-hour period.
Air Sample Collection Techniques
The process of determining the pollen count begins with the physical collection of airborne particles using specialized sampling devices. The Burkard volumetric spore trap, often referred to as a Hirst-type sampler, is a widely used instrument that operates by suction. Air is drawn into a small orifice at a controlled rate (typically 10 liters per minute), and particles are impacted onto a revolving surface. This surface is a transparent tape or drum coated with an adhesive substance, such as silicone grease, that captures the pollen grains over a continuous seven-day cycle.
The rotation of the drum is synchronized with a clock mechanism, ensuring the tape moves slowly and continuously, which makes it possible to determine the specific time of day a particle was collected. An alternative sampling method uses a Rotorod sampler, which employs two rotating rods coated in a sticky substance. The rods spin rapidly on a motor, effectively sweeping particles out of the air onto their adhesive surface.
The Burkard trap provides a continuous, volumetric sample over an extended period. The Rotorod, in contrast, is often used for shorter, intermittent sampling periods. Samplers are typically placed on rooftops or elevated platforms to measure concentrations representative of the air breathed at ground level.
Laboratory Analysis and Pollen Identification
Once the collection period is complete, the adhesive-coated sample is transported to a laboratory for microscopic analysis. The continuous tape from a Burkard sampler is removed and cut into segments corresponding to specific time intervals, often 24 one-day sections. These segments are then mounted onto glass microscope slides using a specialized mounting medium, frequently a gelatin stained with a dye like fuchsine.
The staining process selectively colors the plant material, making the otherwise translucent pollen grains highly visible under a light microscope. Trained aerobiologists or technicians systematically examine the slide, scanning the tape along a specific longitudinal track, or traverse, using high-powered magnification. The technician identifies and counts the different types of pollen (tree, grass, and weed) based on their unique size, shape, and surface features, or morphology.
The identification process is meticulous, requiring the technician to accurately recognize subtle morphological differences among hundreds of species. For a typical 24-hour sample, a predetermined number of sections of the tape are examined, and the raw count of each pollen type is recorded. This raw number is the foundation for the final reported count, which must undergo a mathematical conversion to standardize the data.
Calculating the Daily Pollen Count
The raw count obtained from microscopic examination must be converted into a standardized concentration value to be meaningful. This conversion uses a mathematical formula that accounts for the specific operating parameters of the collection device. The final number is always expressed as the number of pollen grains per cubic meter of air (grains/m³).
The formula standardizes the count by incorporating the area of the slide traverse examined, the total area of the tape, and the known volume of air that passed through the sampler during the 24-hour collection period. This calculation extrapolates the pollen grains counted to represent the total number of grains in a full cubic meter of sampled air. This standardization allows for meaningful comparisons of pollen levels across different geographic regions and time periods.
The standardized numerical counts are then categorized into public health risk levels for easy interpretation. For example, the National Allergy Bureau (NAB) uses categories like Low, Moderate, High, and Very High, with specific numerical thresholds for each pollen type. Tree pollen counts above 90 grains/m³ are classified as High, while grass pollen may reach the High category at only 20 grains/m³.