Warmer winters allow plants to flower earlier and species such as ambrosius do better, so that species continue to flower in September and secrete the pollen that causes allergic reactions. Drought also means that pollen hangs longer in the air. A year now also has more summer days when the ozone concentration is high. Lungs then become more sensitive to allergens, while plants simultaneously secrete more pollen as a stress response.
About 20 percent of the Dutch suffer from hay fever complaints, according to the Leiden University Medical Center (LUMC). Data from healthcare knowledge center Nivel shows that since 2018 there has been no significant increase in the number of hay fever patients who visit their GP. Most patients usually report around the current time of year, when the concentration of the highly allergenic grass clump peaks. In the first two weeks of May, for example, about one in 285 Dutch people (350 per 100 thousand) visited the doctor because of hay fever. That is more than one and a half times more than last year, but less than in the three years before. 2021 also had a very high peak in June, an indication of the weather dependence of complaints. But while nothing is as changeable as the weather, the numbers from recent years make one thing very likely: June is often itchy, sniffling and sneezing. So, presumably, the worst isn’t over yet.