Dirty ducts don't create allergens, but they can absolutely make an allergic reaction worse. Here's how.
Pollen, dust mite waste, pet dander, and mold spores are the four biggest household allergens. All four can accumulate on the interior surface of ducts. Every time the HVAC runs, that material gets stirred and pushed back into the living space.
Cleaning the ducts doesn't cure allergies. But for people who are sensitive, it removes one of the biggest daily sources of exposure inside their own home.