Pollen is a fine, powdery substance released by plants, trees, and grasses as part of their reproductive cycle. For millions of people, this airborne material acts as an allergen, triggering uncomfortable seasonal symptoms. The purpose of a pollen count is to quantify the concentration of these microscopic grains in the air at a specific location and time. This daily metric serves as a practical tool for allergy sufferers, allowing them to anticipate potential symptom severity and manage their exposure accordingly.
How Pollen Counts are Measured
Pollen levels are determined using specialized equipment, such as the Burkard volumetric spore trap or the Rotorod Sampler. These devices are typically placed on rooftops and operate continuously, drawing in a measured volume of air over a 24-hour period. Airborne particles, including pollen grains, are collected onto a sticky surface or tape within the sampler.
A trained aerobiologist or certified technician retrieves the collection surface and examines it under a high-powered microscope. They identify, count, and categorize the different types of pollen present. The final measurement is calculated and reported as the number of pollen grains per cubic meter of air, abbreviated as grains/m³. This 24-hour collection period ensures the reported count represents the day’s average atmospheric concentration.
Standardized Categories: What Defines “High”
Pollen count measurements are translated into four risk categories—Low, Moderate, High, and Very High—to help the public understand the severity of the air quality. The “High” category signifies a significant concentration of airborne pollen that is likely to cause noticeable symptoms for most individuals with pollen sensitivities. This classification indicates that the average person with an allergy will almost certainly experience discomfort.
For tree pollen, a count is considered High when it reaches 90 grains/m³ up to 1,499 grains/m³. Grass pollen has a lower threshold, with a High count beginning at 20 grains/m³ and extending up to 199 grains/m³. Weed pollen levels are classified as High when the concentration is 50 grains/m³ to 499 grains/m³.
The “Very High” category represents an even greater risk, where almost all sensitive individuals will experience severe and disruptive symptoms. For tree pollen, a Very High count is defined as 1,500 grains/m³ or more. Grass pollen is considered Very High at 200 grains/m³ or greater, while weed pollen reaches this level at 500 grains/m³ or more.
The Impact of Pollen Type on Severity Thresholds
Not all pollen is equally potent, which is why the numerical thresholds for a High count vary significantly between the three main types: trees, grasses, and weeds. Tree pollen, which is typically present in the spring, often requires a much higher numerical count to be classified as High compared to grass pollen. This difference reflects the relative allergenicity of the pollen type, meaning that a lower concentration of grass pollen is needed to trigger a reaction.
Grass pollen, prevalent during the late spring and summer months, is notoriously allergenic and can trigger symptoms in sensitive people even at modest concentrations. The lower numerical threshold for a High grass pollen count reflects this increased potency. Weed pollen, commonly released in the late summer and fall, has a High threshold starting at 50 grains/m³.
Monitoring the specific type of pollen that is peaking is often more informative than relying solely on a total pollen count. For example, a total count might be labeled “Moderate,” but if that count is composed primarily of the highly allergenic grass pollen, a person with a grass allergy may still experience severe symptoms.
Actionable Steps When Counts Are High
When pollen counts are reported as High or Very High, several behavioral modifications can reduce exposure. Limiting time outdoors is the primary recommendation, particularly between mid-morning and early afternoon when pollen levels often peak. Outdoor activities should also be avoided on dry, windy days, as these conditions maximize pollen dispersal.
To maintain an indoor sanctuary, keep windows and doors closed, relying on air conditioning to cool the home and car. Using a high-efficiency particulate air (HEPA) filter in living spaces helps capture airborne pollen. Individuals should change clothes immediately after returning indoors and shower before bed to remove pollen collected on hair and skin.
Medical strategies should be employed proactively on High pollen count days. Antihistamines and nasal corticosteroid sprays are most effective when taken before symptoms begin, so treatment should start as soon as High counts are forecasted. Wearing sunglasses or a wide-brimmed hat outdoors can help shield the eyes from direct pollen exposure.