What are HEPA filters?
HEPA filters are High-Efficiency Particulate Air filters that meet specific levels of particle removal efficiency. In the USA (ASME or U.S. DOE), a HEPA filter must remove 99.97% of 0.3µm particles, with the filtration efficiency increasing for particles smaller and larger than 0.3µm. In Europe, HEPA filters
must have a collection efficiency >99.95% (ISO designation H-13) or 99.99% (ISO designation H-14) for particles at the most penetrating particle size(typically 0.1µm to 0.3µm). HEPA filters can remove all kind of particles including solid dust, smoke, fumes, pollen, liquid aerosols, bacteria, fungi, and viruses (including COVID-19).

What are ULPA filters?
ULPA filters are Ultra-Low Penetration Air filters capable of removing >99.999% of particles in the air stream. They work just like HEPA filters but have a denser porous structure and a higher pressure drop, resulting in nearly 100% efficiency at 0.12µm.

How are the particles collected by air filters?
Air filters have a three-dimensional internal structure made by fibers,  or a porous matrix, arranged randomly . When particles go thru this porous structure, they can be collected by several physical mechanisms including diffusion, interception, inertial impaction(see below figure), and in some cases electrostatic attraction. The combined overall collection efficiency of an air filter has a behavior like the one shown in the figure below. This means that air filters have a minimum fractional efficiency at a size typically between 0.1 and 0.3µm, known as the most penetrating particle size (MPPS). For this reason, HEPA and ULPA filters are rated at these particle sizes, where they are known to be least efficient.

How do air filters catch very small nanoparticles and viruses?
Particles in the 1nm to <100nm size range are collected mainly by diffusion. These particles exhibit what is known as Brownian motion due to collisions with air molecules that make these tiny particles move with a random path. The smaller the particles the longer distance these particles can move, so they are collected very efficiently by the internal filter structure.

How do air filters catch particles larger than 0.3µm?
Particles larger than 0.3µm are collected by interception and inertial impaction, and the collection efficiency increases with particle size. The figure below shows a diagram of how these two collection mechanisms work. For interception the particle is collected by just being close enough to the fibers. For the inertial impaction the particle deviates from the streamline and hits the fiber. Both mechanisms become dominant at larger particle sizes.

By Andrew Jarvis - Own work using: Current Strategies for Engineering Controls in Nanomaterial Production and Downstream Handling Processes. U.S. National Institute for Occupational Safety and Health pp. 14–15 (November 2013)., CC BY-SA 4.0

What is Penetration for air filters?
Penetration is the fraction of particles that are not collected. It is equal to 1 minus the fractional efficiency. A 99.9% efficiency corresponds to a 0.1% or 10-3 penetration. For very high efficiency filters it is common to use the penetration instead of filter efficiency.

What happens with collection efficiency as the filters gets used?
As the filter loads with particles, these are deposited within the internal filter structure. The filter gets progressively more efficient, but its resistance to air flow increases (larger pressure drop), and the air flow starts to decrease. This loading continues and can eventually restrict the air flow significantly. It is recommended to replace the filter before it gets clogged with particles. The time interval for changing a filter could be weeks or months, depending on the concentration level of the particles being removed, and the allowable pressure drop of the filtration system.

What is the typical pressure drop of HEPA filters?
HEPA filter media is rated at a face velocity of 5.3 cm/s thru the flat filtration area, at which the pressure drop is typically about 1 in of water or 250 Pa. However, this is not a requirement for all HEPA filters, as the actual pressure drop will vary with the application and the specific type of filter, and how the media is laid out in the filter. HEPA filters are pleated to increase the filtration area and reduce the pressure drop for a given flow rate. Filters used for hospital ventilators may have a pressure drop of only 1 in of water at 100 L/min flow rate. For any filter, the pressure drop will increase as the flow rate increases.

Can air filters also remove gaseous contaminants?
No, air filters cannot collect contaminants that are in gas or vapor form, as they can go thru the filter just as the air does. However, there are special filters made with a bed of activated carbon or other gas adsorbing materials designed to remove gaseous contaminants.