How Our Pollen Index Works

Every pollen level on AllergyNest comes from the Google Pollen API, part of Google Maps Platform. Here is what the number means, what it covers, how we show it and what it can’t tell you.

What the number means

Google reports pollen on its Universal Pollen Index, a scale from 0 to 5 with six categories:

Level	Category
0	None
1	Very low
2	Low
3	Moderate
4	High
5	Very high

Google gives a separate level for each pollen type and each plant it covers, as its Pollen API overview describes.

We always show a level as a number and a word together, such as “4 High”. Each level also has its own color, but color is never the only signal. When Google gives no value, we show “No index” or “Out of season” instead of a number.

Modeled, not measured

The index is not a pollen count. Google calculates it at a resolution of 1 x 1 kilometer (0.6 x 0.6 mile), according to its overview. The values are modeled estimates, not measurements of the pollen in the air, and they can be wrong.

Each metro’s values are for one point, the location of its main city. They are not an average across the metro.

What it covers

For the United States, Google’s coverage list names three pollen types (tree, grass and weed) and 11 plants: maple, elm, cottonwood, alder, birch, ash, pine, oak, juniper, grasses and ragweed.

• Ragweed is the only weed on that list, so other weeds have no plant-level values.
• There is no mold. Google’s pollen data has no mold, so we show no mold level. Our allergen guides describe common molds.
• The 48 contiguous states and DC only. Google returns no pollen values for Alaska, Hawaii or Puerto Rico, so we have no pollen pages there.
• Up to five days. Google’s overview says its forecast runs up to five days and that the number of days can vary by location and plant. We show today and up to four more days.

How we use it on AllergyNest

The headline level. A metro page leads with the highest of the three pollen type levels for the day. If tree pollen is at 2, grass at 1 and weed at 4, the page leads with 4 High. When a plant reaches that same level, the headline names the plant, as in “Ragweed is high in Houston today.” Otherwise it names the pollen type. Below it come the three types, highest first, each with its top plant, then the next four days at each day’s highest type level.

Plant by plant. Each metro page lists every plant Google reported for it that day, highest first, so not every plant appears for every metro. A plant with a level that Google marks as in season is labeled “In season”. A plant Google lists without a level shows “Out of season” when Google marks it out of season, and “No index” otherwise.

Lists of metros. On our metro list, on each state’s pollen page and on the home page, each metro shows its highest type level and the plant with the highest level that day. The home page headline names the plant or pollen type at the top level in the most metros, and where. If fewer than three metros reach the top level, it may lead with a lower level that more metros share.

Each metro’s own day. Every metro shows values for its own local date, so its day starts at local midnight.

How often we update. Our servers request each metro’s values from Google twice a day: soon after local midnight, so the page has the new day, and again each morning, local time, to pick up Google’s latest values. Each metro page shows the time its values were retrieved.

How long we keep values. Google’s service terms limit how long we may keep its values. We keep today’s values for at most 358 days from when we first retrieve them (Google allows 365) and forecast days for at most 22 hours (Google allows 24). An hourly job deletes expired values, and pages that show them are cached for at most one hour.

Missing values. If today’s values haven’t arrived, are late or can’t be read, the page says so. We never fill a gap with a guess.

Credit and limits. Every block that shows Google’s values carries Google’s credit line. The values appear only on our web pages, and our Terms of Use don’t allow you to reuse them. See data sources for details.

Station pollen counts are different

A pollen count is the number of pollen grains a counting station measures in the air where it stands, as National Allergy Bureau pollen stations do. Google’s index is a calculated level from 0 to 5 on a 1 x 1 kilometer grid, not a number of grains. The two measure different things, so a station’s count and the level we show for the same day may not match. Our guide to pollen index vs. pollen count explains the difference in more depth.

We don’t show station counts. We use National Allergy Bureau data only indirectly: CDC data on pollen-season timing at its stations helped decide which metros we cover.

What the level doesn’t tell you

The level describes pollen, not you. It can’t tell you how you will feel. People react to pollen differently, and a level that bothers one person may not bother another.

Google’s data also includes general health recommendations for each pollen type and level, according to its overview. We don’t show them.

If allergy symptoms bother you, see an allergist. Our allergen guides explain each allergen and when to see an allergist.

If a level looks wrong

We can’t correct the values in Google’s model ourselves, but please tell us anyway. If the problem is in how we show the data, such as the wrong metro, date or label, that is our error and we fix it. Our corrections policy explains how.