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Last reviewed on: June 29, 2009
Arrow BulletEnvironmental Allergies

What causes allergies?

What causes allergies?

Many environmental allergies are caused by pollen from:

  • trees,
  • grasses, and
  • weeds.

Environmental allergies can also be caused by:

  • dust mites,
  • animal dander,
  • cockroaches, and
  • mould;
  • and in the workplace from,
    • cereal grain,
    • wood dust,
    • chemicals, or
    • animals.

The types of pollen that most commonly cause allergic reactions are produced by plants (trees, grasses and weeds) that do not have large flowers. These plants create small, light, dry pollen granules that can travel long distances through the air. Most allergenic pollen comes from plants that produce it in huge quantities. A single ragweed plant for instance can generate a million grains of pollen a day.

The chemical makeup of pollen is the basic factor that determines whether it is likely to cause hay fever. For example, pine tree pollen is produced in large amounts by a common tree, which would make it a good candidate for causing allergy. However, because pine pollen is heavy, tends to fall straight down and rarely reaches human noses, it appears to cause fewer allergies than other types of pollens.

The release of pollen takes place seasonally.

  • Tree pollen is released in early spring;
  • grass pollen in early June; and
  • ragweed in August and early September.
  • Mould from leaf litter occurs in the fall.

This web page is for information purposes only.
It is NOT a substitute for professional medical advice.

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