Pollen is a powdery substance that ranges in consistency from fine to coarse. It contains the sperm cells and other elements of seed-bearing plants, and it is carried in the air. There are several dozen types of pollen grains, and several of them can affect your allergies as well as have use in scientific applications.
In temperate climates, birch pollen is prevalent as a common allergen. Although the grains of birch pollen are large, once they are wet, they crumble and are easier to inhale. The crumbled pollen grains can settle in your airways and cause asthma, as well as other conditions like conjunctivitis and hives.
Carya pollen comes from pecan trees and other relative deciduous trees and shrubs. The pollen is rather large and isn't ordinarily a problem to people unless a person has a specific allergy to it. It is often carried by birds and insects; the hard casings make it possible to be carried in the fecal matter of mammals like squirrels and bears as well.
Erodium is a type of pollen grain that is native to parts of Europe and Asia, but it is also from a common weed in the United States. The erodium weed produces several attractive purple rosettes, but the pollen they release can affect a person's asthma or other allergies.
Goldenrod pollen comes from the flowers of the same name that thrive in sunny parts of North and South America. The pollen grains produced by the goldenrod flower are large and are usually only carried by insects and birds for pollination.
Ragweed, from the ambrosia family of pollens, is widely responsible for a number of allergies in people throughout the United States and parts of Europe and Asia. It is an invasive weed that thrives in temperate climates, and it is very hard to keep ragweed from growing over home gardens and landscapes. Problems caused by ragweed pollen include asthma, dermatitis, pinkeye and pneumonia.