What Is Pollen Count — And Why Does It Matter?

Pollen count is a measurement of how many pollen grains are present in one cubic meter of air over a 24-hour collection period. Scientists use air sampling devices called rotorod or Burkard traps to capture airborne particles, which are then identified and counted under a microscope. The count is reported in grains per cubic meter and categorized as Low, Moderate, High, or Very High.

The scale matters. A count of 14 grains/m³ is Low — most people with allergies feel little to nothing. A count of 1,500+ grains/m³ is Very High — the level where even people without diagnosed allergies often notice eye irritation and nasal congestion. Atlanta recorded a count of 14,801 grains/m³ in March 2025, setting a US record that sent millions of residents indoors for days.

Tree, Grass, and Weed Pollen — What's the Difference?

Not all pollen triggers the same reactions, or arrives at the same time of year. Tree pollen is the first wave — starting as early as January in southern states and peaking through May across most of the country. Oak, birch, cedar, maple, and pine are the primary offenders. Tree pollen grains are large and travel short distances, but concentrations near wooded areas can be extreme.

Grass pollen takes over in late spring and early summer. Timothy grass, Kentucky bluegrass, Bermuda, and ryegrass are the main triggers. Grass pollen is smaller than tree pollen and penetrates deeper into the airways — which is why grass allergies often produce more respiratory symptoms than tree allergies do.

Weed pollen — dominated by ragweed — arrives in August and runs through the first hard frost. A single ragweed plant produces up to one billion pollen grains per season. Those grains travel up to 400 miles on the wind, meaning cities far from any visible ragweed still register high weed pollen counts throughout fall.

How to Use Pollen Count Data Practically

Checking your city's pollen count each morning takes 30 seconds and can meaningfully change how you plan your day. On Very High days, taking antihistamines before going outside — not after symptoms start — is far more effective. Keeping windows closed, showering after outdoor activity to remove pollen from hair and skin, and timing exercise for late afternoon rather than early morning are all evidence-based strategies that reduce total exposure. Check your city's current pollen count here.