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 concentrations of hundreds of thousands per cubic metre of air. These biological particles are capable of traveling vast distances, spreading across regions. Seasonal changes, especially in spring and autumn, significantly increase the amount of these allergens, often triggering 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 confined to outdoor environments; sensitive individuals can also be exposed to a variety of indoor allergens. Common indoor allergens include those from mice, cockroaches, dust mite allergens, as well as cat and dog dander. Additionally, pollen and microscopic fungal spores may also be present indoors. Like outdoor allergens, indoor allergens, can be difficult to control as they often become embedded in a wide range of household items such as bedding, furniture, carpets, and even ventilation systems.