The short answer is that it depends on what your Indoor Air Quality (IAQ) issues are and what type of home you have. And the answer may be different for someone with asthma and allergies, or someone in a wildfire-prone region, than it is for someone trying to reduce the effects of VOCs or gasses.
Whole Home or Portable?
Air purifiers can be in-duct/whole-home systems, wall mounted, or they can be portable. Each has advantages. An in-wall system can be centrally controlled and doesn't require you to purchase a unit for each room. One big issue, however, limits their effectiveness: in duct systems aren't turned on all of the time so they aren't filtering 24 x 7. And many older homes simply can't use in-duct or whole-home purification due to the limitations of the heating system in place.
Portable air purifiers are easily adaptable and can work alone and or complement an existing in-duct system.
Zeroing in on Particle Pollution
Particulate pollution, from a range of sources including pet dander, pollen, dust smoking, cooking, viruses, germs, and forest fires, to name a few, is high on the list for IAQ improvement.
The dominant particle removal technology of the past many years has been mechanical filtration, in which dust and other particles are drawn by a fan through a fiber or paper filter. This simple mechanical filter and fan technology, also known as HEPA or HEPA-style has been around since the 1940's and is found in most portable air purifiers on the market today.
A more sophisticated solution, Advanced Particle Removal Technology (APART™), offers an effective, alternative to HEPA filtration.
Both are viable options although the new, patented APART technology, developed by research scientists and air quality experts at the University of Washington, offers consistent air flow and effectiveness over time, less maintenance, and a lower cost to own than comparable HEPA/mechanical filter-based systems.
Mechanical filters physically block and trap particles as they pass through it. Most mechanical filters are HEPA (High Efficiency Particulate Air) or more commonly they are HEPA-style filters. Not all filters marketed as HEPA are true HEPA, and the term is used commonly used to mean a HEPA-style, high-efficiency filter. HEPA filters were designed in the 1940's to protect research scientists from radiation particles, removing 99.97% of the fine particles, at 0.3 microns, which pass through the filter, as well as larger particles such as mold spores, pollen, and dust. HEPA and HEPA-style filters can be very efficient in particle removal, however because the filter blocks and traps, the filter also clogs, airflow drops, and effectiveness declines. So very frequent filter changes may be needed to get close to maintaining the original effectiveness.
Efficiency? Effectiveness? When it comes to air purifiers, what's the difference?
The distinction is key to understanding air purifier performance. Efficiency measures whether an air purifier does a good job of blocking particles. Effectiveness measures whether it cleans the air well. HEPA filters are, by design, efficient. As a by-product of that design, they have decreasing effectiveness as they clog with trapped particles. For your health, effectiveness (it cleans the air well) is what matters.
Electronic Air Purifiers /Electrostatic precipitators (ESPs) use a fan to draw particles past internal ionizing wires. The wires electrically charge the passing particles, so that they become attracted to collection plates inside the purifier. ESP is a well-established, highly effective, flow-by particle removal technology. Old-style or traditional ESP purifiers lost popularity with consumers as they had difficult-to-clean metal plates, and in some cases introduced unacceptable amounts of ozone into the air.
Now, patented APART™ technology from Agentis Air brings ESP into the 21st century, building on its effectiveness while eliminating the issues related to manual cleaning and ozone. In addition to staying at peak effectiveness over the filter life, APART technology eliminates manual cleaning (collection cartridges are disposable) and is certified ozone-safe by the California Air Resources Board. When compared to HEPA/HEPA-style purifiers, APART technology delivers peak effectiveness with a lower cost to own (with long-lasting collection cartridges) and less maintenance. Find out more about APART here.
Ion generators also charge particles but don't collect them. As particles are released into the room, they are attracted to walls, furniture, and floors. Ionizers are promoted as filter-free, but because particles are deposited, not collected, secondary cleaning of these surfaces is required. This secondary cleaning can cause particle reentry into the air and renewed exposure to pollutants. As a result, ion generators are not recommended.
Filters for VOCs and Gasses
Air purifiers for VOCs and gasses work differently and serve a different function--eliminating non-particulate pollution that is too small to be captured by ESP or mechanical air purifiers. The more effective of these technologies can be used to supplement a particle pollution air purification device.
Sorption filters which use activated charcoal or activated carbon to trap odors and some VOCs, are commonly sold as an add-on filter to a mechanical or ESP particle collection filter. Their effectiveness depends on the quantity of activated charcoal they use and also on the frequency of filter replacement. In general, they are considered a safe technology, however, it takes many pounds of activated charcoal to be effective, and frequent filter changes are required as the charcoal gets spent. Combination HEPA/Carbon filters or add on filters rarely have enough charcoal to be effective. A separate, activated charcoal filter system may be a better option of your primary issue is VOCs.
Photocatalytic oxidation (PCO) oxidizes or neutralizes gaseous pollutants using ultraviolet radiation and a catalyst such as titanium dioxide. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) in a position paper reports, "Some PCO technologies are ineffective in reducing gas concentrations significantly, and there are PCO technologies that have also been shown to generate harmful contaminants during the air-cleaning process." The AHSRAE position paper also notes: "Different UV lamps used in many PCO devices can emit significant ozone." PCO technology isn’t considered effective for particulate matter.
Ozone generators release ozone into the air, which is supposed to remove VOCs by converting them into less harmful gases such as carbon dioxide. According to the EPA, the low levels of ozone used don't effectively achieve this goal, and yet are significant enough to exceed acceptable federal and California Air Resources Board limits. Due to the many health issues linked to increased ozone levels--reduced lung function, chest pain, lung tissue inflammation, as well as exacerbation of asthma, emphysema, and bronchitis—the EPA and most consumer health organizations do not recommend ozone generators.
UVGI purifiers using ultraviolet germicidal irradiation (UVGI or UV-C) kill airborne bacteria, viruses, and fungal spores. This technology has been proven to work when it has several minutes or more of exposure to kill the contaminants, however, most portable UVGI purifiers for home use only provide a few seconds of exposure as air passes through the purifier, limiting effectiveness.
PCO and UV-C or UVGI, may be effective for gaseous pollution but are not considered effective for particle pollution. These purification technologies may be useful in addition to, but not instead of, particle air purifiers such as HEPA- style or APART.