Pollen is a fine, powdery substance produced by seed plants as part of their reproductive cycle. It contains male gametes needed for fertilization. When airborne pollen grains are inhaled by humans, the immune system of allergy sufferers (those with hay fever / allergic rhinitis) mistakenly identifies them as harmful invaders. This triggers the release of histamines, causing sneezing, itchy eyes, runny nose, and congestion. The most allergenic pollens come from wind-pollinated plants (anemophilous), as they produce vast quantities of lightweight grains designed to travel long distances.

Pollen season generally spans from late January through November, with three distinct overlapping waves:

Tree pollen (Feb–May): The earliest, often starting with cedar/cypress in late winter (southern US as early as January), followed by birch, oak, maple, elm, and pine in spring.
Grass pollen (May–Aug): Peaks in late spring and summer. Timothy, Bermuda, orchard, and rye grasses are among the most common culprits.
Weed pollen (Aug–Oct): Ragweed season dominates late summer and fall, with a single plant capable of releasing one billion pollen grains. Sagebrush, nettle, and pigweed also contribute.

In northern latitudes, the season is compressed; in southern/warmer regions, it may extend year-round. December and January are typically the lowest-pollen months for most areas.

Pollen count measures the concentration of pollen grains in a cubic meter of air over a 24-hour period. The interpretation scale used by most weather services and allergy organizations:

Low 0–30 grains/m³ — Minimal risk; most sufferers are unaffected.
Moderate 31–60 grains/m³ — Noticeable symptoms for sensitive individuals.
High 61–120 grains/m³ — Widespread symptoms; limit outdoor exposure.
Very High 120+ grains/m³ — Severe reactions likely; take full precautions.

Pollen counts are typically highest on warm, dry, windy days and lowest after heavy rain. Local counts vary by vegetation, urbanization, and weather patterns. Always check your local pollen forecast for the most accurate information.

The most notorious allergenic plants by category:

Trees: Birch (especially in northern Europe and Canada), Oak, Cedar/Juniper (the "Cedar Fever" in Texas), Ash, Elm, and Plane trees. A single birch tree can produce up to 5 million pollen grains per season.
Grasses: Timothy grass, Kentucky Bluegrass, Bermuda grass, Orchard grass, and Rye grass. Grass pollen is the most common cause of hay fever worldwide.
Weeds: Ragweed is the undisputed champion — it affects over 23 million Americans annually. Other significant weed allergens include English Plantain, Lamb's Quarters, Pigweed, and Mugwort/Sagebrush.

Note: Brightly colored, insect-pollinated flowers (roses, sunflowers, daisies) rarely cause allergies — their pollen is heavy and sticky, not airborne.

Weather plays a critical role in daily pollen concentrations:

Warm & Dry → High pollen. Heat and low humidity encourage pollen release and keep grains airborne longer.
Windy → Very high spread. Wind disperses pollen over hundreds of miles; birch pollen from continental Europe has been detected in the UK.
Rain → Washes pollen away. Steady rain cleans the air effectively. However, brief thunderstorms can rupture pollen grains into smaller, more penetrating particles — a phenomenon called "thunderstorm asthma."
Cold & Frost → Low pollen. Freezing temperatures suppress plant activity and pollen production.
Time of day: Pollen counts tend to peak between 5 AM and 10 AM, so allergy sufferers should limit outdoor activity during morning hours.