How Air Filters Work

Air filters remove contaminants and particles from air. Air filtering may be performed for a variety of reasons. Industrial processes may require air that has been cleaned to an exacting standard. Medical or biological research involves processes that are very sensitive to airborne contaminants. Hospitals may filter air to reduce the transmission or airborne bacteria and viruses. In living and workspaces, an air filter can improve quality of life by making the air easier to breathe.

Mechanism

All air filters involve processing air so that different contaminants are removed. The air is passed through some type of filter medium, which is selected for its ability to allow air to pass while blocking or stopping contaminants. The size, shape, and arrangement of the fibers that make up the filter are critical to the function of the air filter. Filters are manufactured with a maximum pore size. After all, a filter with a big hole wouldn’t be very good at air filtering.

Some particulate matter is simply caught between fibers, the same way coffee grounds are strained by a coffee filter. Larger particles are more likely to be filtered in this way than are smaller particles. Particles with a relatively high mass are also more likely than smaller mass particles to be filtered by being unable to change direction as quickly as the air they are moving in does. The inertia of the particle causes there to be a slight lag when the air is diverted around a fiber, and the particle runs into the filter media.¹

Even if a particle is small enough to make it through the gaps between fibers or change direction with the air, there is a chance that the particle will be stopped as the side of the particle touches a fiber. This may be because the particle is not centered in the air flow, like a boat running too close to the side of a river. It may also be that the particle is so small that the random jostling of atoms and molecules throws the particle into a fiber as it travels past.

Air filters can only filter the air in the space they are located. For an air filter placed in an HVAC system, that means they are able to filter the air that is used as an input for the system. Some HVAC systems draw air exclusively from outside of the space being treated, while others involve a recirculating mechanism so that air that has already been conditioned can be used again. Freestanding air filters will filter the air in the room they are located. If the room is able to exchange air with an adjacent room, some of the filtered air will make its way into the room without a filter. However, the majority of the filtered air will be in the room where the air filter is located.

Additional Measures

Some air filtration systems use electrostatic charges to trap particles. By imparting a charge to incoming particles with an electric field and passing the charged particles over surfaces that hold an opposite charge, particulate matter is removed from the air. These systems have the advantage of being able to have the electrodes washed for reuse, and they are also very effective at destroying fungal spores.

Many air filters include an activated carbon filter. Activated carbon is carbon that has been processed so it has a large surface area that can trap reactive chemicals. While the fibers in an air filter will capture solid particles and bacteria, activated carbon or charcoal can capture volatile organic compounds and other pollutants. Many air purifiers with carbon filters combine a HEPA filter with an activated carbon filter to offer improved air filtering.