The reasons for this line of thought are perfectly clear: no matter how spic-and-span your home might be, dust particles, pet dander, and pollen will remain in the air.
Allergic rhinitis, which is sometimes called atopic rhinitis, may be either seasonal (hay fever or rose fever) or nonseasonal (caused by dust, mold spores, pet dander, cigarette smoke, and other household allergens).
When seniors have more restrictive living arrangements, they may miss caring for a pet, or may have had animals as a child, but married to someone allergic to dander.
Instead of ignoring pet dander or pollen, it mistakenly identifies it as a foreign body such as a virus, and trigger's the body's immune system response to combat it.
This is especially important when cleaning up after household pets, given their dander plus all the other things they typically carry into the house from outdoors.