The American Medical Association reports that half of all illness today is due to, or aided by, polluted indoor air.

A variety of small machines are available for use in the home environment (some much larger ones treat industrial buildings), all aimed at either reducing the pollutant quantity in the air, or completely removing the damaging substances.

Many factors influence someone's decision to purchase an air purifier:

-noise level
-frequency of filter replacement
-visual appeal
-air quantity that is treated per unit of time
-pollutant type
-cost of operating the machine
-warranty
-how simple is it to use

A whole range of filters exist. Cleaning them can be done by hand, vacuuming or with water, the filters needing to be replaced after a few months or years, depending upon the model.

Many air purifiers have a 'Clean Air Delivery Rating' or CADR, this is a comparison rating between a range of other air purifier makes. Comparisons like this allow customers to make more informed choices on which air purifiers to purchase. The greater the CADR number the more effective the purification is.

A standard in the U.S. is met by many air purifiers, it is set by HEPA. HEPA filters are able to filter out 99.97% of all pollutants down to the size of 0.3 micrometers in the air column. Other filter machines that claim to be a HEPA-type or HEPA-like often do not filter at as high an efficiency.

HEPA filters are extremely expensive, replacements costing virtually as much as the purifier cost. To lower the costs significantly a significant amount of air purifiers use both HEPA and less expensive 'pre-filters' so that HEPA replacement is not so frequent.

Fans move air through the machine and its cloth-like filter, the vast majority of harmful airborne substances being removed, more so than the highly effective ionic machines. A major advantage of this machine is allergens are efficiently contained within the machine, also filters can take a few years before replacement is necessary. Cigarette smoke, odors, chemical fumes and gasses are not totally eliminated.

Some air purifiers create ozone, notably those that use ions, which has led to a long standing debate over whether these purifiers really are lowering the air pollutants. Ozone is known to be a very toxic substance, potentially lethal in some cases. When it reacts with other substances it often breaks down into smaller pollutants.

Highly reactive ozone is mixed inside the machine with the air destroys various bacteria and chemicals. Running the machine on a low setting, if that exists, is typically what the manufacturers suggest, lessening the chances of ozone poisoning. The best thing about ozone air purifiers are their huge success at removing odors. Unfortunately most chemicals and allergens remain in the air when using this machine.

Ionic air purifiers, as with HEPA machines, use a cloth-type filter. Air is ionized and makes the pollutant particles passing through become positive or negatively charged. Electrically charged plates of the opposite charge exist to then attract the charged particles out of the air. These particles become stuck to the plates. Also, oppositely charged polluting particles are attracted to each other, their increased weight means they readily fall out of the air. These machines are very quiet, especially in comparison with HEPA air purifiers. Ionic air purifiers can remove very fine particles from the air, even when they are a few feet away. Not so good is the lack of odor removal, also microbes are removed but not killed.

There is a range of air purifiers that use activated carbon. Activated carbon has a huge surface area that is highly chemically reactive and adsorbent, causing odors and the like to bond to the carbon. Of all the air purifying filter types this is the most absorbent. Additionally, HEPA filters can also be added making the resulting machine highly effective. Smoke, gases, odors and chemicals in the air are removed easily. Micro-organisms and allergens are not effectively removed.

Ultraviolet (UV) lights have been used in some air purifiers. These are really most effective on micro-organisms. Mould, viruses and bacteria are prevented from reproducing and growth, so long as the UV duration of exposure is high. Chemicals, odors, allergens and smoke are not removed.

The last type of air purifier uses hydroxyl radicals (positively charged single oxygen and hydrogen molecule). These molecules are extremely reactive, apparently 1000,000 times quicker at removing pollutants from the air than with the use of ozone. Mold, viruses and bacteria are most effectively reduced with this type of machine. Every type of pollutant is removed when this machine is working in tandem with UV and negative ion usage. The machine its self is not expensive and doesn't require any filters. The one disadvantage is one manufacturer produces it.

Machines such as these are best left on for most if not all of the day, since air in rooms is perpetually changing and therefore requiring constant cleaning. Remember that the only air that is cleaned is that which the air purifier consumes. Place the air purifier in a region that best takes in all the surrounding air, e.g. not under your desk. Lastly, if you create less pollution (e.g. from aerosol, glue or paint use) in the air in the first place your machines and healthy lifestyle will continue for far longer!