The dirty air goes in one side of a filter and clean are comes out the other. Where the air is actually purified can be explained with the four basic mechanisms of HVAC air filtration.
Large, high-density particles are typically caught by inertial impaction. When the air travels through the filter media, it passes around the fibers. But inertia causes the particle to separate from the air flow and collide with the fiber.
Interception captures medium-sized particles. These particles are too small to possess inertia. That's the reason of following the path of the air flow. If the particle comes into contact with the fiber, it adheres to the media and is caught.
The smallest particles captured by diffusion. The tiny particles travel along the irregular paths, similar to a gas molecule. This is known as Brownian Motion and does not follow the air flow lines. This greater movement increases the probability that the particle will collide with the filter fiber.
Electrostatic attraction is primarily found in filters with a synthetic media. The particles follow the path of the air flow, but if it comes to close, an electrostatic force pulls the particle in. It collides with the strands of media and is retained in the filter.