In Europe, the areas most infested with A. artemisiifolia and therefore the most important sources of ragweed pollen are: the Pannonian Plain, especially Hungary, Romania, Serbia, the Rhône Valley in France, parts of northern Italy and Ukraine.
The flowering period of ragweed is from June to October, with a maximum release of pollen in August, when more allergic patients become symptomatic. Ragweed pollen has been identified at long distances of up to 600 km and more than 3 km high in the atmosphere.
Due to its increased allergenicity, even a small number of pollen grains (6-10 / m3 of air) can trigger allergic reactions.
High pollen exposure or high pollen concentration may lead to a significant increase in sensitization frequency and occurrence of symptoms.