What is the Universal Pollen Index?
The Universal Pollen Index (UPI) is a 0–5 scale used by Google's Pollen API to describe how much pollen is in the air. It is reported per pollen type — tree, grass, weed — and per individual plant, at roughly 1 km resolution.
The six levels
| UPI | Category | What it means in practice |
|---|---|---|
| 0 | None | Nothing of that type in the air. |
| 1 | Very Low | Fine for almost everyone. |
| 2 | Low | Only the very sensitive will notice. |
| 3 | Moderate | Most people allergic to that pollen will feel it. |
| 4 | High | A rough day if you react to what is out there. |
| 5 | Very High | Stay in if you can. |
How it is built
Google describes the UPI as a representation of pollen level that accounts for the concentration per plant model, many different local pollen indices used worldwide, and the allergenicity of different pollen types — that is, how strongly a given pollen tends to provoke symptoms. It is a modelled prediction of grains per cubic metre, not a direct count.
Two consequences follow. First, it is a forecast, so it can be wrong. Second, because allergenicity is folded in, a 3 for ragweed and a 3 for pine are not the same number of grains — they are a similar amount of trouble.
Why you cannot convert it to the 0–12 scale
Several US apps use a different 0–12 index. It is tempting to divide by 2.4 and move on. That does not work: across 175 place-days in 35 US cities, the best possible straight-line conversion removed only 14% of the variation, and barely beat ignoring the other source altogether. The full comparison is here.
Practical rule: never compare a number from one app to a number from another. Compare an app only against itself, over time.
Reading a UPI number well
- Check the type, not the total. Grass 4 matters enormously if you react to grass and not at all if you do not.
- Watch the direction. A rise from 2 to 4 is more informative than the 4 on its own.
- Expect rain to knock it down. Heavy rain washes pollen out of the air, so a wet day usually under-runs its forecast.
- In season matters. Google flags whether a plant is in its season; an out-of-season plant showing a 1 is usually background.
Pollenary shows the UPI per plant per day, so you can see a Ragweed 4 sitting next to a Tree 1 instead of one blended score.
Pollen levels are a forecast, not a medical measurement. Pollenary is not a medical device and does not diagnose or treat any condition. Talk to a doctor about your allergies.