The dry and sunny spring weather causes the pollen counts to rise. The warmth makes the trees develop and release pollen from their catkins.
Warm and dry air also spreads pollen over larger areas, says Åslög Dahl, biologist and responsible for the pollen laboratory at the University of Gothenburg.
Hazel and grey alder have been in bloom since January, and now it's grey alder and goat willow that are next in line. In the coming days, there is a high risk of high levels of alder pollen in Götaland, Svealand, and the southern parts of Norrland, according to Pollenrapporten, which is run by the Swedish Museum of Natural History.
Many birch pollen allergy sufferers also react to alder and hazel. When birch arrives later, it will likely become worse than usual, so it's high time for those who need to start medicating now.
Birch pollen has a natural fluctuation that usually follows a two-year cycle, and last year it was very scarce in large parts of Sweden. So, the expectation is that it will be above average this year, says Åslög Dahl.
Since birch pollen travels easily with the wind, allergy sufferers can feel its effects long after the local blooming has ended.
Depending on how the air currents go, pollen can come from as far away as northern Finland, but also from further south, such as Ukraine.
Soon, goat willow pollen will also arrive, which some birch pollen allergy sufferers can also react to.
But it doesn't spread as far as birch, says Åslög Dahl
