Health experts warned of unusually high pollen levels nationwide, as new data from the Leids Universitair Medisch Centrum and the Elkerliek ziekenhuis show a sharp rise in airborne tree pollen, with birch levels expected to peak in the coming days, Nature Today reports.

After a record-early start to the hazel season and record-high alder pollen levels, the past week saw very high concentrations from several tree species, including ash, poplar, elm, and hornbeam. While alder season has now ended, ash pollen remains a significant concern due to its allergenic properties.

Allergy testing conducted by the Reinier de Graaf ziekenhuis in 2021 and 2022 found that 26 percent of hay fever patients are allergic to ash pollen, while about half are sensitive to birch pollen. It remains unclear how many patients are allergic to both.

Ash pollen concentrations exceeded 300 grains per cubic meter of air, a level likely to trigger symptoms in many people. Although not a record, such high concentrations this early in the year are rare, having occurred only three times in March over the past 50 years—in 1998, 2003, and 2005. The total ash pollen count for March has already reached 2,955, surpassing last year’s record of 1,963, even though the month is not yet over.

Typically, peak ash and birch pollen periods overlap, making it difficult for patients to identify the cause of symptoms. This year, however, the flowering periods are more spread out. Hornbeam pollen levels have also reached unusually high March levels, recorded only twice before.

Meanwhile, birch pollen concentrations are rising. Measurements in Leiden reached 40 grains per cubic meter over the past week, just below the threshold of 50 at which symptoms commonly occur. In Helmond, levels reached 30. Most birch trees have not yet fully bloomed, which explains the relatively lower levels so far.

Based on temperature models from Wageningen University, researchers expect the birch pollen peak to begin in the coming week and continue until about April 17. Although recent night frost affected parts of the country, it likely caused limited damage because many birch trees had not yet started flowering. As a result, pollen concentrations are expected to rise significantly.

Health experts advise people with hay fever to take preventive measures early, including starting medication in time to avoid severe reactions. They also recommend consulting a general practitioner for appropriate treatment. Tracking symptoms daily and comparing them with pollen counts from monitoring institutions can help individuals determine which plants are triggering their reactions.