Pollen index vs. pollen count

Both numbers describe pollen in the air, but they are made in different ways. A count is what a station actually caught: the number of pollen grains in a cubic meter of air, counted under a microscope.¹ An index is a level on a scale, and Google’s Universal Pollen Index comes from a multilayer model.⁹ Below is how each one is made, and why the two can tell different stories about the same day.

What a pollen count measures

In the United States, the National Allergy Bureau, part of the Aeroallergen Network of the American Academy of Allergy, Asthma & Immunology (AAAAI), reports pollen and mold spore levels from counting stations across the country, and each station had to meet strict certification standards.³ A 2022 study put the number of stations certified by the bureau in the United States at 74.⁵

The instrument usually sits on a rooftop, where it collects for a 24-hour period, and is then taken to a lab to be analyzed for pollen types and concentration.³ In a 2019 study of the bureau’s data, researchers wrote that its stations were required to sample at least three days a week from an unobstructed rooftop at least one story above the ground, with no local pollen sources.⁴ Counts are reported for trees, grasses and weeds, and for mold spores.³ Mold spore counts explained covers the mold side.

A person does the counting: a certified technician identifies and counts, under a microscope, the pollen collected over the previous 24 hours.⁵ The daily count is the number of grains divided by the volume of air sampled over those 24 hours, which makes it a daily average in grains per cubic meter.⁴

How the samplers work

Stations in the bureau’s network use one of two instruments: a Burkard volumetric air sampler or a Rotorod rotation impaction sampler.⁴ The Rotorod is a rotating arm that collects airborne particles on a rapidly spinning polystyrene rod.⁵ Some samplers run in short bursts: the Rotorod in the 2022 study, in Marietta, Georgia, collected for 15 minutes of each hour over a 24-hour period.⁵ North Carolina’s state sampler, at a laboratory in Raleigh, collects pollen for 1 minute of every 10.¹

The two instruments don’t catch pollen equally: the Burkard collects higher counts than the Rotorod, particularly for smaller particles, although daily counts from the two methods are positively and significantly correlated.⁴

Schedules vary as well: North Carolina’s sampler collects each sample for 24 hours, from 9 a.m. to 9 a.m., on weekdays except state holidays, and each sample represents the previous 24 hours.¹ In the bureau’s data, many stations don’t sample every day, and most don’t sample year-round.⁴

How counts get labels like “high”

The University of Texas at Tyler, which provides pollen and mold counts as a service, prints a scale on its count reports and notes that trees, weeds, grasses and molds all have different ranges.⁸ Its report for May 7, 2026, gives these pollen ranges, in grains per cubic meter of air:⁸

Level	Trees	Weeds	Grasses
Low	0–15	0–10	0–5
Moderate	16–90	11–50	6–20
High	91–1,500	51–500	21–200
Very high	Above 1,500	Above 500	Above 200

North Carolina’s air quality agency took a different approach. It classifies counts as low, moderate, high or very high based on frequency distributions of historical pollen counts, with the intent that the four categories correspond roughly to the four quartiles of typical data.²

What a pollen index is

Google designed its Universal Pollen Index to offer a globally unified scale for comparing pollen levels across different areas.⁹ It is based on a multilayer model that considers a concentration model for each plant, many local pollen indexes used around the world and how allergenic each plant’s pollen is, to predict the number of pollen grains per cubic meter of air per day.⁹

The index has six categories, from 0 (None) through Very low, Low, Moderate and High to 5 (Very high).⁹ Google says each category is based on specific concentration levels of pollen in a particular area and their effect on the severity of the allergic symptoms people usually experience.⁹

Google calculates its values at a resolution of 1 x 1 kilometer (0.6 x 0.6 mile) and gives a forecast of up to five days, though the number of forecast days may vary between locations and plants.¹⁰ Its plant data may also vary across locations.⁹

On AllergyNest, metro pages show Google’s index. How our pollen index works explains which plants it covers and how we show it.

Why the two can disagree

Place is the first reason. A count describes the air at one rooftop, and local sources of pollen have a significant impact on what a sampler catches.⁴ Stations are also few: a monitoring committee of the National Atmospheric Deposition Program describes the bureau’s stations as sparsely located.⁶ University of Arizona Cooperative Extension made the point for its state in a column dated January 15, 2024: it wrote that Arizona’s only official observation station was in Scottsdale, and that counts there were not likely to forecast airborne pollen at the mid and higher elevations of Yavapai County.⁷ Google’s index, by contrast, is calculated at a resolution of 1 x 1 kilometer.¹⁰

Time is the second. A count covers a day that has already passed, and manual counting is labor-intensive and comes with a considerable time delay.⁵ Google’s data looks ahead, with daily values for up to five days.¹⁰

Method is the third. AAFA describes pollen forecasts as estimates, “just like a weather forecast,” and says pollen counts are more accurate.¹¹ Labels add one more layer: North Carolina’s categories come from frequency distributions of historical pollen counts, while Google bases each of its categories on concentration levels and their effect on symptoms.²⁹

What neither number can tell you

North Carolina’s air quality agency notes that people vary in their sensitivities to pollen, and says its categories can also be thought of as indicators of the share of people affected by allergy symptoms: very few people are adversely affected by low counts, while rather many may be affected by very high ones.²

A low level is no guarantee: Missouri’s health department notes that some people with sensitivity to some types of pollen and mold can have allergy symptoms even when the amounts of pollen and mold in the air are low.¹²

The kind of pollen matters too: according to AAFA, people with pollen allergies only have symptoms when the pollens they are allergic to are in the air.¹¹

Our hay fever page describes the symptoms pollen can cause. For questions about your own symptoms, talk to an allergist.