What is pollen? How it gets indoors and how to reduce it

September 10, 2026 Contaminants

TL;DR: Pollen is a fine powder released by trees, grasses, and weeds so they can reproduce. For people with allergies, it triggers sneezing, congestion, and itchy eyes. It's an outdoor allergen that becomes an indoor problem: pollen gets in through open windows and doors and on clothes, hair, shoes, and pets, then settles into carpets and soft furnishings, so symptoms continue indoors. In the US, tree pollen typically peaks in spring, grass in late spring and early summer, and weeds from late summer until the first frost. Pollen seasons across North America have grown about 20 days longer since 1990. Consumer air quality monitors can't measure pollen directly, but you can cut indoor pollen by keeping it out, running a HEPA air purifier, and keeping humidity at 30–50% to limit the mold and dust mites that make allergies worse.

What is pollen?

Pollen is the fine, powdery material plants release to fertilize other plants of the same species. The grains are microscopic, light, and, in many species, built to travel long distances on the wind, which makes them very good at both getting into our homes and being inhaled.

The plants that cause most allergy problems are wind-pollinated: trees, grasses, and weeds. Showy, insect-pollinated flowers are rarely the cause, because their pollen is heavier and stickier. Grass and birch pollen cause far more issues than a rose garden.

Why pollen causes allergic reactions

For people with a pollen allergy, the immune system mistakes harmless pollen proteins for a threat and releases histamine. That response produces the familiar symptoms: sneezing, a runny or blocked nose, itchy and watering eyes, an itchy throat, and, for some people, worsening asthma. The reaction is specific to the protein, which is why someone can react badly to birch but not to grass.

When is pollen season?

"Pollen season" isn't one month of the year. Different plants release pollen at different times, many seasons overlap, and when you get symptoms depends on what you're allergic to. According to the Asthma and Allergy Foundation of America (AAFA), typical US seasons are:

Pollen type	Typical US season	Regional notes
Trees (e.g. birch, oak, cedar, maple)	February–April	Can start in December or January in the South
Grasses	April–early June	Can be year-round in warmer states such as California and Texas
Weeds (e.g. ragweed)	August–first hard frost	Ragweed is one of the most common triggers

Pollen levels also vary with weather. Warm, dry, breezy days generally push airborne counts up, while a few days of steady rain can clear pollen from the air. Plants keep releasing pollen, though, so wet weather gives only a short break.

Pollen seasons are getting longer

A 2021 study of 60 North American pollen stations found that from 1990 to 2018, pollen seasons grew about 20 days longer and pollen concentrations rose about 21%, closely linked to warming. In other words, you may well have a longer allergy season now than when you were a child.

The trend is expected to continue. A 2022 modeling study in Nature Communications projected that with 4–6°C of warming by the end of the century, spring pollen in the US would start 10–40 days earlier, summer and fall grass and weed seasons would run 5–15 days later, and total annual pollen emissions would rise by 16–40%.

How does pollen get into your home?

Many people associate pollen allergy with being outdoors, but symptoms often continue inside. Pollen grains are light enough to drift in on air currents and stick to almost anything:

Open windows and doors, particularly on warm, breezy days
On you: hair, skin, clothing, and shoes can carry a surprising amount indoors
On pets, whose coats are very efficient pollen collectors
Through ventilation systems with low-efficiency filters
On laundry dried outdoors, which collects pollen all day and carries it into your living room or bedroom

Once inside, pollen settles into carpets, rugs, bedding, curtains, and upholstery, and is stirred back into the air whenever you walk across a room, sit down, or make the bed. This is one reason indoor symptoms often feel worse in the evening or first thing in the morning.

How to reduce pollen indoors

Stop it coming inside

Keep windows closed on high-pollen days. Peak times vary by plant and weather, so check a local pollen forecast and air your home when counts are lower.
Dry laundry indoors during your pollen season.
Change clothes and shower after time outside. Washing your hair before bed helps keep pollen off your pillow.
Wipe down pets' coats and paws when they come in.
Upgrade your HVAC filter. If you have central air, the EPA recommends a filter rated at least MERV 13, or the highest rating your system can handle.

Remove what's already inside

Run a HEPA air purifier in your bedroom and the rooms you use most. Pollen grains are large particles, which HEPA filters capture very effectively. The Airthings Renew uses a HEPA-13 filter and is designed for everyday living spaces and bedrooms.
Vacuum often with a HEPA-filtered vacuum, and damp-dust instead of dry-dusting, which sends particles back into the air.
Wash bedding weekly in hot water during pollen season.
Cut back on soft furnishings in the bedroom if symptoms are severe, so pollen has fewer places to settle. See our guide to improving bedroom air for better sleep.

Control humidity

Keep relative humidity between 30% and 50%. This doesn't affect pollen directly, but dust mites and mold thrive in damp air and commonly trigger symptoms alongside pollen. A reaction that seems like "just pollen" may partly be caused by mold or dust mites. Read more about ideal indoor humidity levels.

Can an indoor air quality monitor detect pollen?

Not directly, but it can help you manage the indoor conditions that make allergies worse.

Pollen grains are relatively large, typically tens of microns across. Consumer particle sensors are built to measure fine particles such as PM2.5 (2.5 microns or smaller), so they don't measure pollen, and any product that implies otherwise is overstating its capabilities. For pollen counts, use a local forecast based on real pollen sampling.

The Airthings app's Insights feature includes a pollen forecast, so you can check likely pollen levels where you live.

Screenshot of the Airthings app showing a local pollen forecast

An indoor air quality monitor is still useful for managing your home around an allergy:

Humidity: keep levels between 30% and 50% to limit mold and dust mites.
Particulate matter (PM2.5): see what makes fine particles spike or drop when you open windows, vacuum, or run a purifier, and check that your filtration is working.
Trends over time: separate an outdoor pollen problem from an indoor air issue you might be blaming on allergies.

The Airthings View Plus measures particulate matter and humidity alongside radon, CO₂, and VOCs. For more on the particles it measures, see our guide to particulate matter, or learn how to test indoor air quality at home.

Frequently asked questions

Can an air quality monitor measure pollen?

No. Pollen grains are tens of microns across, while consumer particle sensors measure fine particles such as PM2.5 (2.5 microns or smaller). For airborne pollen counts, use a local pollen forecast based on real sampling.

Why are my allergy symptoms worse indoors?

Pollen comes inside on clothes, hair, shoes, and pets, then settles into carpets, bedding, and upholstery, where movement stirs it back into the air. That's also why symptoms often peak in the evening or on waking.

What time of day is pollen worst?

It depends on the plant and the weather, so there's no single rule. Check a local pollen forecast and open windows when counts are lower, for example during or after steady rain.

Does rain reduce pollen?

Steady rain washes pollen out of the air and generally brings counts down. The period right after rain can be worse, as warm, drying conditions lift grains back into the air.

Do HEPA air purifiers work for pollen?

Yes. Pollen grains are large particles, which HEPA filters capture very effectively. HEPA filters don't remove gases such as VOCs, which need an activated carbon filter.

Which plants cause the most allergies?

Wind-pollinated trees, grasses, and weeds. Grass and ragweed affect the most people. Showy, insect-pollinated flowers are rarely the cause, because their pollen is heavier and doesn't travel far on the air.

Are pollen seasons getting longer?

Yes. A 2021 study of 60 North American stations found pollen seasons grew about 20 days longer and pollen concentrations rose about 21% between 1990 and 2018, closely linked to rising temperatures.

Should I close my windows during pollen season?

On high-count days, yes, especially in the bedroom overnight. You still need ventilation for CO₂ and humidity, so air your home when pollen counts are lower, such as during or after rain.

Does humidity affect pollen allergy?

Not pollen itself, but damp air encourages dust mites and mold, which often trigger symptoms alongside pollen. Keeping humidity at 30–50% reduces that extra load.

Is it pollen or dust mites?

Pollen allergy tends to be seasonal and worse after time outdoors. Dust mite allergy is typically year-round and worst in the bedroom on waking. Symptoms that last through winter point away from pollen. An allergy test gives a definitive answer.

Sources

Written by Annabel Robertson

Marketing & Brand Manager, Airthings

press@airthings.com

September 11, 2026| Posted in Contaminants| Annabel Robertson| 8319

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