Inertial Impaction in Air Filters: Why Small Particles Are Captured

Core Principle

When particle-laden air flows through a filter, the air stream bends around the filter fibers or other structures in its path. Because of their inertia, however, particles tend to continue moving in their original direction and cannot turn as quickly as the airflow. If a particle's inertia is strong enough, it leaves the airflow path and strikes the surface of a filter fiber directly. Adhesive forces on the fiber, such as van der Waals forces, then capture the particle and prevent it from continuing through the filter with the air.

Key Factors

Particle mass and size: Larger and denser particles, including PM10, pollen, and some bacteria in the approximate range of 0.5–10 μm, have greater inertia and are more likely to collide with fibers.

Airflow velocity: The faster the airflow, the greater the particle's momentum. This increases the likelihood that it will leave the curved airflow path, making inertial impaction more pronounced.

Filter structure: Thicker fibers and tighter spacing force the airflow to bend more sharply. Particles are then more likely to lag behind the turn and collide with the fibers because of their inertia.

How It Differs from Sieving

Inertial impaction does not depend on a particle being larger than the filter opening. Even when a particle is much smaller than the gap between fibers—for example, a 5 μm particle passing through a 20 μm fiber gap—it can still be captured if its inertia carries it into a fiber. This is one of the main reasons medium-efficiency filters, including F-class filters, can effectively capture particles such as PM5.

In simple terms, inertial impaction is like a passenger being thrown forward when a fast-moving vehicle suddenly turns. The particle cannot follow the airflow's change in direction quickly enough, so it collides with a filter fiber and is captured.