Illustration of pollen grains from the common ragweed (Ambrosia artemisiifolia), a very invasive species of the family Asteraceae. Once native on the American continent, this annual plant is now distributed over nearly all temperate countries. One plant can release one billion pollen grains per season. This pollen can cause strong allergic reactions including rhinitis, hay fever or asthma-like symptoms. Only a small pollen-concentration of 6-11 grains per metre cubed is enough to provoke an allergic reaction.