See Personal Protection Equipment The best-case scenario for filters that actually do achieve the HEPA standard is to filter particles down to 0.3 microns at 99.97% efficiency. How they work:

From: Dale
This is an article on Home Air Quality Monitors. - Other reviews say you need an activated charcoal filter for smoke & smell not just a HIPPA filter. In any case filters need to be changed regularly, an ongoing expense. Some say they will remove the larger smoke particles but not the smoke odor. Some ionizers produce ozone which is it's self a problem. I haven't looked up our Lasko fans to see if there is any comment on ozone from from them. We haven't used the ionizer other than a few times and it does tend to produce more dust on surfaces. It's not clear to me that it would be worth it for the cabin given the need for regular cleaning and filter changes. Some comments said that use with a dirty filter can be worse than no air purifier.

Electrostatic precipitators use positively and negatively charged plates to collect dirt and dust particles as they pass through the machine. A fan pulls the pollutants towards the electrostatically charged plates, and collects them.
You just clean the plates periodically.

Most immunologists recommend (HEPA) filters . The ultra-fine mesh on these filters traps all sorts of irritants

Ultrafine particles range from 0.1 microns all the way down to 0.003 particles – the tiniest that exist. Ultrafine particles are so small that, once inhaled, they move straight through the lung tissue and directly into the bloodstream.

High Efficiency Particulate Air (HEPA)
The best-case scenario for filters that actually do achieve the HEPA standard is to filter particles down to 0.3 microns at 99.97% efficiency.
HEPA filters work in mechanical Home Air Quality Monitors and are made with randomly arranged micro-glass fibers.
Some so-called HEPA filters are made of ordinary synthetic fibers. Synthetic fiber media is a far less dense structure and is much less efficient at trapping particles than media made of fiberglass or specialty synthetic fibers.

Clean Air Delivery Rate (CADR) ratings were developed by the Association of Home Appliance Manufacturers (AHAM) in the 1980s as a way for the general public to navigate the claims made by Home Air Quality Monitors.
The CADR rating system has many weaknesses that dramatically limit its usefulness.

IQAir HealthPro Series $800

Recommended Room Home Air Quality Monitors | Consumer Reports

Models with mechanical filters: This is the type that CR tests. Home Air Quality Monitors with pleated filters use fans to force air through a dense web of fine fibers that trap particles. Filters with very fine mesh are HEPA filters—those certified to collect 99.97 percent of particles of a certain size (0.3 micrometers in diameter—smoke and paint pigments, for example)

Models with activated carbon filters: Rather than catch particles like mechanical filters, sorbent filters use activated carbon that can capture some odor-causing molecules from the air. They may also tackle some gases, but they’re not particularly effective against formaldehyde, ammonia, or nitrogen oxide. Because activated carbon filters don’t combat particles, many Home Air Quality Monitors will have both an activated carbon filter and a pleated filter for catching particles. Activated carbon gets saturated faster than a pleated filter, though, and requires replacement more frequently—every three months, as opposed to every six to 12 months for pleated filters.

Electronic Home Air Quality Monitors: Electrostatic precipitators and ionizers charge particles in the air so that they stick to plates on the machine or to nearby surfaces by a magnetic-like attraction. CR doesn’t typically test electronic Home Air Quality Monitors or recommend them, because they can produce ozone which is a health threat.
The plates in those with plates to capture the particles quickly become “loaded,” and the air purifier efficiency often decreases by even 50% in just a few months. Some are passed off as HEPA.

Scores 91-62
Rated primarily on Dust, Pollen and Smoke removal
91 Alen BreatheSmart 75i Pure $750.00
90 Samsung Cube Stack $1,400.00
88 Blueair Classic 605 $830.00
85 Blueair Blue Pure 211+ $300.00
 Pre-filter – The fabric pre-filter  can catch the large particles.
 This pre-filter is washable or vacuumable.
 Particle filter – Instead of using a True HEPA filter,
 Blue Pure 121 uses a polypropylene fibers filter to help capture 99% of all allergens like dust,
 dust mites, pet dander, pollen and fungi (HEPA technology).

Activated Carbon Filter – The Activated Carbon filter is added inside the Particle filter and effective against smoke, odors, gases and VOCs.
77 Blueair HealthProtect 7470i $640.00
77 Blueair Blue Pure 211+ Auto $340.00
76 Samsung Cube $700.00
74 Honeywell InSight HPA5300B $250.00