In order to find a suitable answer to this question, we need to have a clear idea about pollen. Pollens are microscopic structures produced by plants for the purpose of reproduction. In some plants reproduction can happen only through cross-pollination. This means that pollens need to be transferred from flowers of one plant to that of other plants belonging to the same species. For some flowering plants, insects carry out this process, whereas the others depend on winds.
Generally, the pollens that act as allergens are produced by flowerless plants like weeds and grasses. These pollen grains are light, small and dry. This ensures that they are easily transported through wind.
The chemical compositions of pollens also play a major role in determining whether it will cause hay fever or not. For example, in spite of the fact that pine pollen have to travel a long distance for causing fertilization in pine trees, they are not very much allergenic. The reason behind this is that these pollen gains are quite heavy and never get scattered. This proves that people can get pollen allergy only if they get exposed to lighter pollen grains, which can get scattered here and there while transported by the wind.
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