If you have allergies, then you are aware of how serious they can get when you are exposed to something you have an allergy to. While there are many different types of allergies, and quite a range in severity, they can change and get worse, in some cases due to exposure to allergens.
When you buy a house from a pet owner, you might not even notice it at first, especially if the home recently was cleaned, but pet dander and hair can be hiding out in many places, only to appear after the home has been lived in for a while.
Changing out filters in the HVAC system can help, and if you have a vacuum with a HEPA filter installed, you can clean up the dander without risking releasing it back out into the air again.
Sometimes, it might take more than one cleaning before you have the problem under control.