I am trying to understand the science behind air purifiers like the Levoit Core series and how they actually clean the air. I know they use HEPA filters but I am confused about how a fan pushing air through a paper-like material can capture microscopic particles like pet dander and pollen without getting instantly clogged. Is it purely a mechanical sieve where particles get stuck in the fibers or is there some kind of electrostatic charge involved in the process? Also what is the difference in how the HEPA part works compared to the activated carbon section which handles odors? I want to be able to explain this to my spouse so can someone give me a simple breakdown of the physics of air filtration?
HEPA filtration is not just a simple sieve where large particles get stuck in holes. It works primarily through three physics principles: interception impaction and diffusion. Interception is for medium-sized particles that simply touch a fiber and stick to it. Impaction is for larger particles that are too heavy to follow the air stream around a fiber and crash directly into it.
The most interesting principle is diffusion which is what captures the tiniest particles like viruses and ultra-fine smoke. These particles move randomly in a zig-zag pattern known as Brownian motion. Because they move erratically they are highly likely to eventually collide with a fiber and get trapped instead of following the air flow straight through the filter.
The air purifier is essentially a two-stage cleaner. The HEPA component which is a dense mat of randomly arranged fibers uses the methods described above to physically trap solid particles like dust pollen and dander. The activated carbon part which is a bed of porous charcoal uses a chemical process called adsorption to trap odor and gas molecules.
Adsorption is different from absorption. In adsorption the gas molecules stick to the surface of the charcoal material like Velcro not getting soaked up like a sponge. The vast surface area inside the carbon pellets allows it to chemically bond with pollutants like volatile organic compounds or cooking smells removing them from the air stream.
You can think of the HEPA filter as a huge spider web where particles are caught based on their size and movement. The filter is designed to be highly effective at capturing both the largest particles and the very smallest particles down to $0.3$ microns which is the Most Penetrating Particle Size or MPPS.
The fan is critical to the process because it is what determines the Air Changes Per Hour or ACH rate. The air must be pulled through the dense filter media multiple times to ensure all particles are captured. A stronger fan helps increase the effective cleaning power of the filter itself.
So the HEPA is not really a sieve it is a trap.
Air purifiers generally do not rely on an electrostatic charge to capture particles unless they specifically use an ionizer or electrostatic precipitator technology. Standard HEPA purifiers are based on these purely mechanical and physical trapping methods which is why they are so reliable and effective.
Most consumer purifiers include a pre-filter which is just a coarse mesh designed to capture the largest debris like human hair and pet fur. This protects the much more expensive and delicate HEPA filter from getting prematurely clogged by the big stuff allowing the HEPA layer to focus on the truly microscopic particles.
The reason the filter eventually needs replacement is because the accumulated particles start to restrict the airflow through the filter media. When the flow is restricted the fan has to work harder to push the same volume of air which stresses the motor and reduces the cleaning efficiency. This restriction is what triggers the replacement light.