Pollen Magnified: What Pollen Looks Like Under a Microscope

To the naked eye, pollen is just yellow dust. Under a scanning electron microscope, each grain turns out to have its own shape: spiky balls, honeycomb spheres, beans and three-cornered grains. The image below shows pollen from six common plants magnified about 500 times.

Pollen size also explains a lot about allergies. Whole grains are usually too big to get past your nose, which is why pollen mostly causes hay fever. But when grains burst into tiny fragments, as can happen in thunderstorms, those pieces can reach deep into the lungs. For more, see our seasonal and pollen allergy guides.

Jump to: Quick answer · What you’re seeing · How big is pollen? · Thunderstorm asthma · Which pollen causes allergies · FAQ

Quick answer

• Shapes vary by plant: under a microscope, pollen grains can be spiky, smooth, ridged, bean-shaped or three-cornered.
• Size: intact grass pollen grains are about 35 to 40 micrometers across, and the lily grain in the image is about 50 micrometers long. A micrometer is a thousandth of a millimeter.
• Why it matters: whole grass pollen is usually too big to get beyond the nose, but fragments from burst grains are small enough to reach the lungs.
• Thunderstorm asthma: in Melbourne, Australia, in 2016, a thunderstorm during grass pollen season led more than 3,400 people to seek emergency care, and 10 people died.
• The worst offenders are wind-pollinated plants such as trees, grasses and ragweed.

What you’re seeing in the image

The image comes from the Dartmouth College Electron Microscope Facility, which released it into the public domain. The colors were added afterward (the image is “colorized”) so the different types stand out.

How big is pollen, and why does size matter?

Most pollen grains are far too small to see one at a time. A 2019 review in the Journal of Asthma and Allergy notes that intact grass pollen grains are about 35 to 40 micrometers across, which is usually too large to be breathed into the lower airways. To reach the lower airways, particles generally need to be smaller than about 5 micrometers.

That is why pollen mostly lands in your nose and eyes, causing the classic hay fever symptoms: sneezing, a runny or stuffy nose, and itchy, watery eyes. The American Academy of Allergy, Asthma and Immunology (AAAAI) puts it simply: grass pollen is usually too big to get beyond the nose, but pollen fragments are small enough to get into the lungs.

When pollen breaks apart: thunderstorm asthma

During some thunderstorms, pollen grains are swept up into moist clouds, where they burst. The 2019 review explains that a ruptured grass pollen grain can release up to 700 starch granules, each smaller than 5 micrometers. Storm winds then bring these fragments down to ground level, where people breathe them deep into their lungs.

The most serious known event happened in Melbourne, Australia, in 2016: more than 3,400 people sought emergency care and 10 people died. More than half of the people affected had never been diagnosed with asthma. Many had hay fever.

Thunderstorm asthma is rare and doesn’t happen with every storm. But if you have hay fever, especially grass pollen allergy, it’s a good reason to:

• Stay indoors with windows closed when a thunderstorm hits during grass pollen season.
• Get any wheezing, chest tightness or coughing checked by a doctor, since undiagnosed asthma was common in Melbourne.
• Keep your asthma medicines on hand if you have asthma.

Read more in our asthma guides.

Which pollen causes allergies?

The American College of Allergy, Asthma and Immunology (ACAAI) says plants whose pollen is carried by the wind are the most likely to cause seasonal allergies. The usual culprits are trees, grasses and weeds rather than the garden flowers in the image above.

Ragweed deserves a special mention. ACAAI says it blooms from August into November, usually peaks in mid-September, and can cause hay fever in as many as 25 million people in the U.S. During ragweed season, pollen levels peak in the morning.

If you’re allergic to tree pollen, some raw fruits may make your mouth itch; see birch pollen and food. To cut your exposure, read our pollen avoidance guide and whether masks help with pollen.

This guide is general information, not medical advice. Read our medical disclaimer.

Frequently asked questions

What does pollen look like under a microscope?

It depends on the plant. Under a scanning electron microscope, sunflower and hollyhock pollen look like spiky balls, morning glory pollen like a honeycomb sphere, lily pollen like a bean and primrose pollen like a three-cornered tripod.

How big is a pollen grain?

Many are a few tens of micrometers across. Intact grass pollen grains are about 35 to 40 micrometers, and the lily grain in our image is about 50 micrometers long. You can see pollen as dust when lots of grains collect, but not one grain at a time.

Why does pollen cause allergies?

In people with pollen allergy, the immune system treats proteins in pollen as a threat and releases chemicals that cause sneezing, a runny nose and itchy eyes. Because whole grains usually stay in the nose, symptoms center on the nose and eyes.

Can pollen get into your lungs?

Whole grains usually can’t get past the nose, but fragments can. When pollen bursts, for example during a thunderstorm, the fragments are small enough to reach the lungs and can trigger asthma attacks.

What is thunderstorm asthma?

Sudden asthma attacks in many people at once after a thunderstorm during high pollen season, caused by burst pollen fragments. It’s rare, but people with hay fever should stay indoors during these storms and get any new wheezing checked.

Sources

• Dartmouth College Electron Microscope Facility. Misc pollen (public domain), via Wikimedia Commons.
• Harun NS, Lachapelle P, Douglass J. Thunderstorm-triggered asthma: what we know so far. J Asthma Allergy. 2019;12:101-108.
• American Academy of Allergy, Asthma and Immunology. Your questions answered on air pollution, wildfires and asthma.
• American College of Allergy, Asthma and Immunology. Gardening with allergies.
• American College of Allergy, Asthma and Immunology. Ragweed allergy. Reviewed June 28, 2023.

Last reviewed: October 21, 2026.