Article Credit: Canadian Association of Optometrists ( opto.ca ) What causes allergies? 1-3 Allergies are common and can develop at any age. Seasonal allergies occur during specific times of the year, generally in spring when allergens such as pollen from trees, grass and weed is released into the environment. Perennial allergies are present year round and are triggered by allergens such as dust. If you have an allergy, these allergens are perceived as foreign particles by yo