Pollen count explained — what the numbers actually mean
A pollen reading of "4" means something very different from tree pollen to ragweed to mold. Here's how pollen is actually measured, what the 0–5 index hides, and how to read a forecast like it's for you.
The short answer
Pollen is measured in grains per cubic meter of air (grains/m³), sampled at certified counting stations. Those raw grain counts get compressed into a 0–5 scale — the Universal Pollen Index, or UPI — which is the headline number most forecasts (including ours) show you.
The catch: the underlying thresholds for "low," "medium," or "high" are different for each pollen type. A tree-pollen reading of "high" means more than 90 grains per cubic meter. A grass-pollen reading of "high" means more than 20. Same word, very different amounts of actual pollen in the air.
How pollen is actually counted
Most US pollen counts come from stations certified by the National Allergy Bureau (NAB) — about 80 active stations as of this writing, most staffed by a trained volunteer counter, often a board-certified allergist.
The hardware is surprisingly analog. Two common devices:
- Rotorod samplers — two thin metal arms rotate at high speed for short intervals (typically one minute out of every ten), catching airborne pollen on a sticky surface.
- Burkard volumetric traps — a small vacuum pulls air through a narrow slit onto a rotating drum coated in adhesive, producing a continuous seven-day pollen record.
At the end of each sampling period the collection surface is stained and examined under a microscope. A human counter identifies pollen by grain shape — oak and ragweed and grass look nothing like each other — and tallies the total as grains per cubic meter of air.
It's slower and more human than most people assume. The Google Pollen API and other forecast services blend these ground-truth measurements with satellite imagery, plant-bloom models, and weather forecasts to generate predictions for places that don't have a nearby station.
The 0–5 scale, decoded
The Universal Pollen Index compresses a wide range of grain counts into six buckets that mean roughly the same thing across pollen types. The color-coding below is exactly what you see on the home-page forecast bars:
- 0 — None. Nothing measurable in the air.
- 1 — Very Low. Detectable but below the threshold where sensitive individuals typically react.
- 2 — Low. Minor symptoms possible for highly sensitive people.
- 3 — Medium. Most allergy sufferers will notice symptoms.
- 4 — High. Nearly all sensitive people will have symptoms. Worth limiting outdoor exposure.
- 5 — Very High. Severe reactions likely even for people with moderate sensitivity.
Thresholds by pollen type
Here are the NAB-published grain-count thresholds that drive most public forecasts. Units are grains per cubic meter of air (for mold, spores per cubic meter).
| Type | Low | Moderate | High | Very High |
|---|---|---|---|---|
| 0–14 | 15–89 | 90–1,499 | 1,500+ | |
| 0–4 | 5–19 | 20–199 | 200+ | |
| 0–9 | 10–49 | 50–499 | 500+ | |
| 0–6,499 | 6,500–12,999 | 13,000–49,999 | 50,000+ |
A few observations worth internalizing:
- Tree pollen thresholds are the widest. "Very high" for trees starts at 1,500 grains/m³ — an order of magnitude above any other category — because oak, birch, and pine produce enormous absolute volumes.
- Grass pollen peaks low in absolute numbers. A "high" grass day is only 20 grains/m³, but grass pollen is unusually efficient per grain at provoking symptoms.
- Ragweed sits in between. Its combination of concentration (50+ grains/m³ for "high") and long-distance wind transport is what makes it such an effective trigger for such a large share of the population.
- Mold is counted in much bigger numbers because spores are smaller, lighter, and more abundant than pollen grains — they're also measured separately.
Why the forecast can feel off
"The forecast says low, I feel terrible." A few likely reasons: microclimate (a single large oak next to your apartment can dominate your personal exposure in April, regardless of the regional average); timing (pollen counts peak between 5 AM and 10 AM, so afternoon reports describe a different part of the day); or multi-trigger overlap (tree pollen is winding down, grass is ramping up, and a mold spike after last night's rain pushes everything higher at once).
"The forecast says very high and I feel fine." You may not be particularly sensitive to the pollen in question. A "very high" tree-pollen day still leaves roughly three-quarters of the population unbothered, because only about 25% of Americans have a clinically significant tree allergy. If you've never reacted to oak-laden April days, you probably aren't tree-allergic — and even a "5" day won't bother you.
Personal sensitivity is highly specific. Two people can live on the same block, breathe the same air, and have completely different symptom profiles. This is why allergist testing — skin-prick or specific-IgE blood test — is so useful: it tells you which of the 40+ common pollen types actually trigger you, which lets you read a forecast with your own sensitivity in mind rather than the population average's.
How to actually use a pollen forecast
Rules-of-thumb for turning UPI numbers into daily decisions:
- UPI 0–1. Treat it as a normal day. No precautions.
- UPI 2. Open windows if you like, keep doing what you'd do. Consider starting a daily antihistamine if you know your threshold is low.
- UPI 3. Take your daily antihistamine if you aren't already. If outdoor plans can be shifted to a 0–2 day, shift them.
- UPI 4. Close windows in the morning (the peak window). Shower before bed. Consider adding Flonase if it isn't already in your rotation.
- UPI 5. Indoor day if you can manage it. If you can't, keep errands short, wear sunglasses (pollen gets in the eyes), and shower immediately after coming inside.
Don't just check today. The 5-day forecast shown on the home page tells you when the next drop is coming. If Thursday is a 5 and Sunday is a 2, that's the day to schedule the yard work, the long run, or the outdoor event you can move.
And if you're curious about how each pollen type behaves across the year — why tree season in your area starts when it does and how it hands off to grass and then ragweed — our guide to when allergy season starts walks through the whole calendar.
Sources
- National Allergy Bureau. NAB-certified pollen-counting stations and reporting methodology.
- American Academy of Allergy, Asthma & Immunology. Pollen-count thresholds and interpretation.
- Google Developers. Google Pollen API documentation — Universal Pollen Index methodology.
- Asthma and Allergy Foundation of America. Allergy prevalence statistics.
- International Association for Aerobiology. Minimum Recommendations for Aerobiological Monitoring Programs (Galán et al., 2014).
Check today's reading and 5-day forecast for your area on the home page, or browse the rest of our reference guides.