Example:
By abuse of language, some say they are allergic to cat hair. In fact, they are allergic to Fel d 1 and Fel d 4 glycoproteins. Depending on temperature and pressure, the molecule will migrate into the air. The air we breathe will bring us into contact with the allergen.
In reality, this allergen is a chemical molecule that the cat needs to clean itself. This protein is emitted naturally, like that contained in pollens, mites, spores… But the modern petrochemical world has generated chemical pollutants, which are not natural, which make us sensitive and therefore allergic to compounds that existed naturally. This applies to most airborne allergies.