Why does the air coming through a dirty/full air filter cause allergies to flare up, if there’s even less area for particles to get through than in a clean filter?

Question

Clearly a dirty filter would overall be less effective due to reduced airflow. But why is the air that *does* get through dirtier? Shouldn’t that air be even cleaner (albeit less of it)?

It is common for allergies to specifically flare up when under an A/C vent with a filter that hasn’t been replaced in a while.

In: 1

Answer 1

An old mostly-blocked filter will start breaking as air is forced through- creating new bigger holes that the accumulated dust can fit through.