What counts as a high pollen count?

It depends on the pollen type. On the National Allergy Bureau scale, tree pollen is “High” at 90 grains per cubic meter or more, grass at 20 or more, and weed (ragweed) at 50 or more. Grass has the lowest threshold, so a count that would be only “Moderate” for tree can be “High” for grass.

How long does pollen season last?

It varies widely by climate. Across the 100 largest U.S. metros, the WeatherStory Allergy Index puts the median active-pollen window at about 304 days a year, ranging from 213 days in the coldest, driest metros to a near-year-round 365 days in the warm, humid South.

When does ragweed season end?

Ragweed peaks in late summer and typically runs 6 to 10 weeks, ending when the first hard freeze (around 28°F) kills the plants — usually October or November across most of the country, and later in the warm South.

How many Americans have seasonal allergies?

About 25.7% of U.S. adults and 18.9% of children had a seasonal allergy in 2021, and roughly 50 million react to ragweed each fall.

Are pollen seasons getting longer?

Yes. Research on North American pollen trends found that warming lengthened the season by about 8 days and moved its start roughly 20 days earlier between 1990 and 2018, with ragweed season lengthening up to 27 days at northern latitudes since 1995.

Where does WeatherStory get these numbers?

Pollen-count thresholds come from the AAAAI National Allergy Bureau; prevalence from CDC/NCHS; and season-trend figures from peer-reviewed studies (Anderegg 2021 and Ziska 2011, both PNAS) and the US EPA. Season-length and ranking figures come from the WeatherStory Allergy Index, a modeled seasonal pollen-load model — see the methodology page. Each stat above lists its own source and year.

Pollen & Allergy Statistics 2026

A sourced, dated reference of pollen and allergy statistics: what a high pollen count is, how long pollen season lasts, when ragweed season ends, and how common seasonal allergies are. Every figure lists its source and year.

What is a high pollen count?

A pollen count is measured in grains per cubic meter of air (grains/m³), averaged over a 24-hour air sample drawn through a volumetric spore trap — typically a Burkard or Rotorod sampler — then identified and counted under a microscope by a certified counter. The National Allergy Bureau sorts the result into four categories whose grains/m³ cut-offs differ by pollen type:

90+ grains/m³Tree pollen is rated “High” by the National Allergy Bureau at 90 grains per cubic meter of air or more, and “Very High” at 1,500+.AAAAI National Allergy Bureau (NAB) — pollen-count scale, 2026
20+ grains/m³Grass pollen is rated “High” at just 20 grains/m³ or more — a much lower bar than tree — and “Very High” at 200+.AAAAI National Allergy Bureau (NAB) — pollen-count scale, 2026
50+ grains/m³Weed pollen (mostly ragweed) is rated “High” at 50 grains/m³ or more, and “Very High” at 500+.AAAAI National Allergy Bureau (NAB) — pollen-count scale, 2026
15–89 grains/m³The NAB “Moderate” band for tree pollen runs 15–89 grains/m³; grass is 5–19 and weed is 10–49. Below those, a count is “Low.”AAAAI National Allergy Bureau (NAB) — pollen-count scale, 2026

Pollen-count ranges by type, grains/m³ (NAB scale)
Category	Tree	Grass	Weed (ragweed)
Low	0–14	0–4	0–9
Moderate	15–89	5–19	10–49
High	90–1,499	20–199	50–499
Very High	1,500+	200+	500+

Source: AAAAI National Allergy Bureau (NAB) — pollen-count scale, 2026. Grass turns “High” at the lowest count of the three types, so a reading that is only “Moderate” for tree pollen can be “High” for grass.

A separate system — the 0–12 pollen index used by many consumer forecasts (Pollen.com / IQVIA) — reports a single blended number instead of a raw grains/m³ count; on that scale a reading of 9.7–12.0 is the top “High” band.

How long is pollen season?

304 daysThe median of the 100 largest U.S. metros carries an active-pollen window — any of tree, grass, or ragweed at Low or above — of about 304 days a year; the mean is 287 days.WeatherStory Allergy Index 2026 — modeled seasonal pollen-load ranking (see /methodology)
100 of 100All 100 of the ranked metros run an active-pollen season of at least 200 days, and 19 run a near-year-round season of 330+ days.WeatherStory Allergy Index 2026 — modeled seasonal pollen-load ranking (see /methodology)
365 daysCape Coral, FL has the longest modeled season of any ranked metro at 365 days; the shortest is Spokane, WA at 213 days.WeatherStory Allergy Index 2026 — modeled seasonal pollen-load ranking (see /methodology)
152 daysLooking at a single worst allergen instead of the combined window, the median metro's dominant pollen type runs about 152 days — the longest, Cape Coral's grass, runs 365 days.WeatherStory Allergy Index 2026 — modeled seasonal pollen-load ranking (see /methodology)
+8 daysWarming has lengthened North American pollen season by roughly 8 days and pushed its start about 20 days earlier since 1990.Anderegg et al. 2021, PNAS — North American pollen trends 1990–2018 (PMC7896283)

When does ragweed season start and end?

January–AprilAcross the ranked metros, allergy season first turns active as early as January in the warmest metros (Cape Coral, FL) and as late as April in the coldest (Spokane, WA).WeatherStory Allergy Index 2026 — modeled seasonal pollen-load ranking (see /methodology)
6–10 weeksRagweed season typically runs 6 to 10 weeks in most regions, peaking in late summer and ending with the first hard freeze (about 28°F).AAFA / US EPA — Climate Change Indicators: Ragweed Pollen Season
+27 daysRagweed season has lengthened by up to 27 days at northern latitudes since 1995 — an extra 21 days in Fargo, ND and 18 days in Minneapolis, MN.Ziska et al. 2011, PNAS; US EPA — Ragweed Pollen Season
400 milesA single ragweed plant can release about 1 billion pollen grains per season, and the grains have been detected up to 400 miles out to sea and 2 miles up in the atmosphere.AAFA — Ragweed Pollen Allergy

How many people have seasonal allergies?

25.7%About 25.7% of U.S. adults — roughly 1 in 4 — had a seasonal allergy (hay fever) in 2021.CDC/NCHS Data Brief No. 460 (2021) — seasonal allergy in US adults
18.9%About 18.9% of U.S. children had a seasonal allergy in 2021; the share rises with age, from 8.5% of preschoolers to 36.3% of teens.CDC/NCHS Data Brief No. 459 (2021) — allergic conditions in US children
~50 millionRoughly 50 million Americans react to ragweed pollen each fall, the most common late-season trigger.AAFA — Ragweed Pollen Allergy

Which U.S. metros have the worst pollen?

#1 Cape Coral, FLCape Coral, Florida ranks #1 of 100 in the 2026 WeatherStory Allergy Index, with a modeled seasonal load index of 57. Cape Coral's grass pollen season runs 365 days, with a seasonal load index of 57 — #1 of 100 U.S. metros.WeatherStory Allergy Index 2026 — modeled seasonal pollen-load ranking (see /methodology)
57 tree · 42 grass · 1 ragweedTree pollen is the single worst allergen in 57 of the 100 ranked metros, grass in 42, and ragweed in 1 — grass and tree dominate because they combine high peaks with long seasons.WeatherStory Allergy Index 2026 — modeled seasonal pollen-load ranking (see /methodology)

Frequently asked

What counts as a high pollen count?: It depends on the pollen type. On the National Allergy Bureau scale, tree pollen is “High” at 90 grains per cubic meter or more, grass at 20 or more, and weed (ragweed) at 50 or more. Grass has the lowest threshold, so a count that would be only “Moderate” for tree can be “High” for grass.
How long does pollen season last?: It varies widely by climate. Across the 100 largest U.S. metros, the WeatherStory Allergy Index puts the median active-pollen window at about 304 days a year, ranging from 213 days in the coldest, driest metros to a near-year-round 365 days in the warm, humid South.
When does ragweed season end?: Ragweed peaks in late summer and typically runs 6 to 10 weeks, ending when the first hard freeze (around 28°F) kills the plants — usually October or November across most of the country, and later in the warm South.
How many Americans have seasonal allergies?: About 25.7% of U.S. adults and 18.9% of children had a seasonal allergy in 2021, and roughly 50 million react to ragweed each fall.
Are pollen seasons getting longer?: Yes. Research on North American pollen trends found that warming lengthened the season by about 8 days and moved its start roughly 20 days earlier between 1990 and 2018, with ragweed season lengthening up to 27 days at northern latitudes since 1995.
Where does WeatherStory get these numbers?: Pollen-count thresholds come from the AAAAI National Allergy Bureau; prevalence from CDC/NCHS; and season-trend figures from peer-reviewed studies (Anderegg 2021 and Ziska 2011, both PNAS) and the US EPA. Season-length and ranking figures come from the WeatherStory Allergy Index, a modeled seasonal pollen-load model — see the methodology page. Each stat above lists its own source and year.

More pollen & allergy data

See the WeatherStory Allergy Index 2026 for the full 100-metro ranking and season-onset map, the worst cities for allergies ranking, or browse pollen counts by city.