The pollen count measures airborne pollen grains, a significant factor for individuals who experience seasonal allergies (hay fever or allergic rhinitis). This measurement provides the concentration of these microscopic particles in the atmosphere over a 24-hour period. Monitoring this concentration allows the public to receive advance notice of high pollen exposure days, enabling them to take preventative measures. The final reported number is derived through a standardized, scientific process involving specialized equipment and laboratory analysis.
The Standardized Collection Process
The method for capturing airborne pollen relies on volumetric air sampling. Monitoring stations use specialized instruments, such as the Burkard or Hirst-type spore trap, to collect the daily sample. These devices draw a precise, standardized volume of air into the sampler, typically at a rate of ten liters per minute, over a full day.
The sampler uses a pump to draw air through an inlet slit, often protected by a wind vane. As the air passes through the device, suspended particles, including pollen grains, are impacted onto a sticky surface. This surface is typically a clear plastic tape or drum coated with an adhesive material.
The tape moves slowly past the inlet, completing a full advancement over the 24-hour collection cycle. This ensures the captured pollen represents a continuous, twenty-four-hour concentration. After the collection period is complete, the tape is removed from the sampler and prepared for laboratory analysis.
Analysis and Calculation of the Daily Count
Once the sticky tape is retrieved from the volumetric sampler, a trained aerobiologist prepares the sample for microscopic examination. A specific section of the tape, representing the 24-hour collection, is trimmed and mounted onto a glass slide. The sample is often stained using a dye, such as Calberla’s stain, which helps the colorless pollen grains become visible under the microscope.
The aerobiologist examines the slide at high magnification to identify and count the captured pollen grains. This process requires expertise to differentiate between pollen types, categorizing them into broad groups like tree, grass, and weed pollen. Because counting every single grain on the entire tape is impractical, the analyst counts the pollen only in a predetermined number of narrow, longitudinal tracks across the slide.
The count from this small, examined fraction is then extrapolated using a mathematical formula to determine the total concentration in the sampled air volume. This calculation converts the number of grains counted on the slide section into the final daily figure, which is reported as the number of pollen grains per cubic meter of air (grains/m³). This final number represents an average concentration of the airborne pollen over the previous 24 hours.
Interpreting the Pollen Index
The raw number of grains per cubic meter is translated into a simple, standardized index for public communication. This index uses descriptive categories—Low, Moderate, High, and Very High—to make the data understandable and actionable for allergy sufferers. The index allows individuals to quickly assess the potential risk of exposure and adjust their outdoor activities or medication use accordingly.
The numerical thresholds defining these categories are not uniform across all pollen types. For example, a count of 90 grains/m³ for tree pollen may qualify as “High,” while a count of 20 grains/m³ for grass pollen might also be categorized as “High.” This difference exists because various pollen types have different sizes, atmospheric behaviors, and allergenic potencies.
The use of these indices helps standardize reporting, ensuring that a “Moderate” rating in one region corresponds to a similar level of potential symptom severity in another. This public health information becomes a practical tool for managing seasonal allergy symptoms.