The reason for the increase in airborne pollen levels is the widespread Japanese cedar afforestation that took place throughout Japan after the Second World War in order to provide building materials as well as for the purposes of flood control and forest conservation, and it was in the latter half of the 1960s that these trees began to reach the age of 30 years when they have the greatest pollen producing capacity.
飛散量の増加の原因は、戦後、建材および治水・治山の目的で全国に広くスギが植林され、それらが1960年代後半より花粉生産力の強い樹齢30年程度に達し始めたためである。