The machines on this list combat airborne contaminants using High-Efficiency Particulate Air (HEPA) filters, activated carbon filters, ionic technology, and/or UV light.  HEPA air purifiers are the most popular. To comply with the current baseline standard, HEPA filters are tested and certified to remove at least 99.97% of dust, pollen, mold, bacteria, and any other airborne particles as small as 0.3 microns. There are different levels of HEPA filters, but in general, you want to make sure the model you buy meets the baseline standard (many that do have a "True HEPA" label) and stay away from those marketed as "HEPA-like," "HEPA-type," or "HEPA-style," which might not be as effective.  As Wirecutter reported in 2020, a NASA study found that HEPA filters can actually capture almost 100% of particles as small as 0.01 microns (10 nanometers), well exceeding the baseline standard. To put that in perspective, COVID-19 is about 60 to 140 nanometers, H1N1 influenza is about 80 to 120 nanometers.  Meanwhile, activated carbon filters are filled with porous carbon, which traps gases such as NO2 and VOCs, as well as foul smells, from the air. Activated carbon filters are often combined with HEPA filters, which do not remove gases. Our top overall choice, the Dyson Purifier Cool TP07, features a 360-degree filter with a HEPA layer that captures allergens, bacteria, mold spores, pollen, and other pollutants, and an activated carbon layer that removes gases and odors. It also doubles as an oscillating fan. The version that destroys formaldehyde, called the Purifier Cool Formaldehyde TP09, costs $100 more. Dyson says its latest air purifiers, including the TP07 and TP09, not only meet the baseline HEPA filter standard, but are also fully sealed to the HEPA H13 standard, meaning they can remove 99.95% of particles down to 0.1 microns. The LG PuriCare 360 is another model that combines HEPA and active carbon. The only downside to HEPA and active carbon models is that the replacement filters can be costly (around $50 to $100) and wasteful.   Other models, like the Sharp FXJ80UW and Aura Air, combine HEPA, active carbon, and ionic technologies. Sharp's model features an ion-generating unit that creates positively and negatively charged ions using water and oxygen molecules in the air, and disperses them throughout the room. The ions then actively attach to and break down airborne pollutants like bacteria, mold, and viruses before returning to the air as invisible water vapor. Sharp says the technology reduced 94% of E.coli bacteria, up to 87.7% of mold fungus, and 90.3% of MS2 virus in a closed laboratory over four hours. Aura Air's patented Sterionizer disinfection technique works similarly. The primary concern about ionic air purifiers is their possible production of ozone gas molecules, a lung irritant that can exacerbate asthma symptoms and cause a host of other health issues. The safety of ionic air purifiers came into question in the mid-2000s after it was revealed that the then-popular Sharper Image Ionic Breeze air purifier produced ozone (an issue that eventually bankrupted the company).  As the California Air Resources Board (CARB) warns, some ozone-producing devices are still marketed as air purifiers. The clean air agency maintains a list of potentially hazardous models to avoid.  All models on our list that use ionic technology are EPA-compliant, CARB-certified, and considered safe to use in your home for extended periods of time.