When a person who is allergic to cats is even in the same home as a domestic cat they might rapidly show symptoms of their allergy by itching, sneezing and coughing etc.. The reason for the rapid onset of these symptoms of asthma and rhinitis is the minuscule size of the allergen that comes off the domestic cat – Fel d1, a protein. It is in cat dander which is dried saliva and skin cells containing the allergen.

The size is mindbogglingly small at between 2.5 and 5 millionth of a meter or between 2.5 and 5 thousandth of a millimetre.
The small size of this allergen means that the particles stay airborne for long periods. Long after the cat has gone outside, for example, the allergen particles are still floating around the home ready to fall on the unsuspecting person who is allergic to cats.
The combined weight of the Fel d1 particles in a cubic litre of air is between 2 and 20 nanograms (ng). One ng is one billionth of a gram.
A bit of cat science for those who are interested 🙂
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