The reasons for buying an air purifier can be as numerous as the number of pollutants in the air — and indoors, that number tends to be bigger than most people would expect. Despite the best efforts of the filters in our air conditioners, pollen and dust particles can settle into carpets and upholstery. Mold spores can grow in moist areas. And if there are pets or smoke involved, expect dander and carcinogens to be part of every breath.

Whether you suffer from allergies or just want to eliminate the smell associated with all this particulate matter, a good air purifier can be an essential accessory in any room. But how are they different from the filters we already have in our regular AC?

All that depends on the purifier. Most models circulate air through a HEPA (High-Efficiency Particulate Air) filter, and often more than one. HEPA filters are certified to trap particulate matter as small as 0.3 microns, which covers dust, pollen and most bacteria. Some purifiers boast a medical-grade Hyper-HEPA filter that will go down to 0.003 microns — enough to pick smoke out of the air.

In conjunction with this, the more high-tech air purifiers can employ an extra line of defense. The Germ Guardian uses an optional UV light that renders many micro-organisms sterile, and an activated charcoal filter that can actually pull in and neutralize toxins through electrostatic attraction. There are also models that can ionize incoming particles, which are then sucked in by metal plates or other treated substances.

A word of caution on ozone, though. While they’re less common these days, some air purifiers generate ozone, which, according to the manufacturers, can deodorize and disinfect the air. The jury is still out on those claims, and in fact the presence of ozone in confined spaces can be more harmful than the pollutants you’re trying to get rid of.

For a measure of general effectiveness, you can look for a rating on most air purifiers called the CADR — Clean Air Delivery Rate. It’s a number that represents the cubic feet per minute (CFM) of air that has been cleared of all the particles of a given size. These ratings are recognized by the EPA and can be found on most devices. If you can’t find them, the number is fairly easy to calculate: Just take the CFM on the air purifier and multiply it by the percentage of a certain particle (smoke, pollen, etc.) that the machine can remove.