Air Filtering

The air on Earth is a mixture of gases, suspended solids, and biological forms. The gas mixture is primarily nitrogen and oxygen. A very small portion of the gases are argon, an unreactive gas. An even smaller portion is carbon dioxide, a by-product of biological organisms and human activities.

The suspended solids include soot, soil, pollen, mold, bacteria, viruses, and various other materials that can be harmful. A variety of methods have been developed to make the air easier to breathe by filtering out the suspended solids. Air filtering aims to remove solid contaminants in the air.

How air filters work

Air filtering is a simple and straightforward process. Air is forced through a mat of fibers which are packed tightly enough to trap miniscule particles, but not so tightly that air cannot pass. Particulate matter that collides with a fiber will stick to the fiber, meaning that when the air comes out of the other side of the filter, any particles that collided with a fiber will be left behind.

This air cleaning system can be incredibly effective. HEPA filters remove at least 99.97% of particulate matter that is 0.3 microns in diameter. For particles larger or smaller than 0.3 microns, HEPA filters remove more than 99.97% of the particles. That means for every 3,000 particles that enter a HEPA filter, less than one makes it through.

One drawback of fiber filtering is the need to change the filter periodically. This can prove expensive over time, and if there are no spare filters on hand the risk of running an air filter with a dirty filter is increased. Also, if there is sufficient moisture in the system, mold and bacteria can grow in the filter medium, posing potential health risks until the filter is swapped out.

Electrostatic filters dispense with the need to replace physical filters by imparting an electric charge to incoming air. The particles in the air are then attracted to an oppositely charged plate, or to the surrounding environment. The plates can be cleaned and reused, meaning that the problem of replacing a filter is avoided.

The fibrous and electrostatic filters take care of solid pollutants, but as far as removing volatile organic compounds goes, other steps are required. Activated carbon is specially processed carbon, often charcoal, that has a very large surface area. Volatile organic compounds readily adhere to the surface of the activated carbon. This process, called adsorption, differs from absorption in that the molecules only stick to the surface of the solid carbon instead of absorbing into the body. Air purifiers with carbon filters include an activated carbon layer that captures gaseous and suspended liquid pollutants from the air.

Other considerations

Air filters can only filter the air that passes through them. This is why it is recommended to place multiple air filters throughout a home. An alternative arrangement for air filtering an entire home is to place the filter within the HVAC system. The air filter can be placed between the intake of outdoor air and the rest of the system, and can also be placed in a recirculation line to help filter already conditioned air.

In the close quarters of a hospital ward, an infection can quickly move from patient to patient. Transmission of airborne diseases between patients in hospitals can be reduced by ensuring that air delivered to different rooms has been through an air filtration system.¹