Ambient airborne allergens are specific proteins typically released from the pollen of wind-pollinated plants or from microscopic fungal spores. Pollen from trees, grasses, and weeds, along with spores from over 100 fungi species, can reach hundreds of thousands per cubic metre of air. These biological particles can travel long distances, spreading from one region to another. Seasonal changes, especially in spring and autumn, increase the amount of these allergens, making it possible to trigger allergic reactions such as hay fever and asthma. Weather conditions, including wind and humidity, play an important role in their concentration and distribution.

Allergens are not limited to outdoor air; sensitive individuals can also be exposed to various indoor allergens. Mice, cockroaches, dust mite allergens, and cat and dog dander comprise the indoor allergen load. Pollen and microscopic fungal spores may also be present indoors. Indoor allergens, like outdoor allergens, can be challenging to control as they can be trapped in a wide range of household items such as bedding, furniture, carpets, and even ventilation systems.