While the arrival of spring brings joy to many, it marks a difficult time of year for people with allergies, as it signals the start of pollen allergy season. Pollen is nearly imperceptible in the air, yet it affects living organisms more than ordinary dust particles.

Pollen allergy most commonly manifests as hay fever. Symptoms include red, itchy and watery eyes, a stuffy and itchy nose caused by swollen mucous membranes and sneezing triggered by irritation. Depending on a person's sensitivity, the severity of symptoms can range from mild discomfort to severe allergic rhinitis and allergic asthma.

Elen Haasmaa recently defended a master's thesis at the Estonian University of Life Sciences, in which she compared an earlier pollen calendar with current data. She said her motivation for the research came from her child, who suffers from a strong pollen allergy.

"It seems like everyone knows someone who has a pollen allergy, so it's a very relevant topic. Studies also show that the number of people with pollen allergies is steadily increasing," she said.

Estonia's first pollen calendar was compiled in the 1990s by aerobiologist Maret Saar and geologist Anneli Poska, based on nine years of data collected at a monitoring station in Tartu.

Pollen levels are measured using a special device that collects pollen grains from outdoor air onto an adhesive tape, which is then analyzed under a microscope. While data for the 1990s calendar was gathered and analyzed at what was then the Institute of Zoology and Botany (now part of the University of Life Sciences), today, aerobiological monitoring is carried out by the Estonian Environmental Research Center, commissioned by the Ministry of Climate. Alongside other types of air pollution, the center also measures airborne pollen concentrations.

Each spring, the Environmental Research Center publishes an aerobiological monitoring report that summarizes the previous year's pollen data. However, unlike the 1990s calendar, no long-term overview using updated data had been compiled until now.

"That's understandable — there's no point in creating a pollen calendar every year. But once you've collected data over ten years or more, it makes sense to start looking for patterns and to compare them to earlier periods," said Haasmaa, the thesis author.

For her research, Haasmaa used the methodology developed by Maret Saar and created a new pollen calendar for the years 2012 to 2022, using data from the Environmental Research Center's Tartu monitoring station. This time frame was available when she began her work. Using the same methodology was essential for tracking how pollen levels and distribution have changed over time.

"People with allergies aren't concerned with the exact number of pollen particles in the air — they care about the health risk at any given moment. That's what the pollen calendar is designed to show," she explained.

Though people often refer to "pollen allergies" in general terms, only a small number of plants actually trigger allergic reactions. In Estonia, the culprits are trees like alder, birch and hazel, and among herbaceous plants, grasses and mugwort. Elsewhere in the world, ragweed is a major allergen, but it is not widespread in Estonia and currently does not pose a significant problem for local allergy sufferers.

More pollen in Tartu than Pärnu or Tallinn

Pollen monitoring stations in Estonia are located in Tallinn, Tartu, Pärnu, Jõhvi and Kuressaare.

In her thesis, Haasmaa focused on the first three cities, where the population is highest.

"One interesting pattern emerged — something previously noted in an international study co-authored by Maret Saar: pollen concentrations are lower in coastal areas compared to inland regions. In other words, pollen levels in the Tartu area were higher than in Tallinn and Pärnu," she said.

According to Haasmaa, one explanation could be the presence of large bodies of water near the monitoring stations. Pollen that falls into water doesn't rise back into the air. Another possible factor is wind direction, which in Estonia tends to blow inland from the Baltic Sea.

The collected data revealed that some plant species are now blooming about a month earlier than they did in the 1990s. "This clearly correlates with climate warming. When I compared the average annual temperatures in the climate portal, I found that during the years used to create the 1990s pollen calendar, the highest annual average temperature was 6.8 degrees Celsius. In the period from 2012 to 2022, the average was 6.8 degrees, but the highest annual average reached 8.4 degrees. So it's evident that average temperatures have significantly increased, which explains why pollen now appears in the air earlier," she explained.

Scientific literature indicates that a mild and snowless winter is typically followed by an earlier, warmer spring — though that didn't hold true this year. Nonetheless, trees confirm the trend toward earlier springs, as early-flowering species are now blooming sooner than they did in the 1990s.

"Previously, alder trees started blooming around the second week of February. Now, in some years, they begin as early as the end of January," Haasmaa said.

For people with pollen allergies, this means starting allergy treatment earlier. "It's easier to prevent symptoms than to treat them once they've already developed. That's why this thesis is also a valuable resource for allergists, helping them guide patients on when to begin taking their medications," she explained.

Does this mean the pollen season is now longer? According to Haasmaa, the opposite is true. The intense period of high allergy risk has actually become shorter and less severe.

Location also plays a role in monitoring results. In the 1990s, Tartu's monitoring station was located on what is now the site of Lõunakeskus shopping center, in an open landscape surrounded by agricultural fields, pasture and meadows. Today, the Tartu station is located on Tähe tänav, in a more urban environment surrounded by four- to five-story buildings.

Current data show that pollen concentrations are lower than they were in the 1990s. This doesn't mean that plants are producing less pollen, but rather reflects the reduced vegetation in urban areas and the restricted movement of air.

"In spring, if you look out the window, you'll often see a green film on the glass. That pollen is no longer circulating in the air — it's stuck to the surface," Haasmaa explained. In addition, urban greenery is now well-maintained and many grassy areas are mowed before plants have a chance to flower.

In recent years, both in Estonia and elsewhere in Europe, cities have begun moving away from intensive mowing of green spaces in an effort to support biodiversity. Haasmaa said it would be interesting to see in a few years whether this shift leads to an increase in herbaceous plant pollen. "That doesn't mean we should return to intensive mowing, but rather that we should aim for more informed urban planning," she said.

While reviewing earlier scientific literature for her thesis, Haasmaa found examples from Central Europe where urban green space planning took air quality into account.

"If we want to make life easier for allergy sufferers and keep city air cleaner, we should consciously choose insect-pollinated species for new green areas. These plants produce less pollen and their pollen doesn't spread on the wind. Of course, we can't just cut down existing trees, but by making thoughtful choices going forward, we could reduce the amount of airborne pollen in cities and make life more comfortable for allergy sufferers," she said.

Insect-pollinated plants also have beautiful flowers and attract butterflies and other insects. "Their presence would amplify the goals we're trying to achieve today with insect hotels and unmown meadows — supporting greater biodiversity," the author of the thesis added.

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