The first - and most important - piece of knowledge is that no one kind of purifier will work against every toxin. All of them excel at some time of air pollutants - but not others. It's critical that you understand the air quality of your home and environment before you make a choice.

There are three types of pollutants that are addressed by air purifiers:

* Particulate - pollen, air pollution

* Microorganisms - biological dangers like mold and germs

* Chemicals

Here are the five most popular types of air purifiers. Learn their strengths and weaknesses before even considering a purchase.

1. Ultraviolet air purifiers:

Effective: If you have a basement subject to mold, ultraviolet is perfect for you. Ultraviolet air purifers are terminators when it comes to micro-organisms like mold spores, bacteria and viruses. This makes UV particularly valuable for anyone suffering from a respiratory disease, colds or the flu.

Useless: Ultraviolet air purifiers are impotent against dust, dirt or smoke and other airborne particles and chemicals are immune to its effects.

2. Carbon air purifiers:

This is the oldest kind of air purifying technology, as it dates back to the early 20th century. Carbon filters were used in World War I to protect soldiers against poison gas used by enemy armies.

Effective: Carbon filters excel at any kind of gaseous pollutants: fumes, odors and deadly gases.

Useless: If your problems are micro-organisms and allergens, forget the carbon air purifier. They will endure and even prosper in the presence of carbon filtration. It cannot deal with particulates, either, as they are not captured by carbon.

3. Ozone air cleaners:

Ozone air purifiers are the new kids on the block. We are all familiar with ozone because we hear about it often as the ozone layer that protects us from the deadlier rays of our sun. We are exposed to that wonderful ozone smell during a thunderstorm.

Effective: Ozone is a molecule that contains three oxygen atoms, rather than the usual two atoms in the usual air we breathe. This extra molecule makes it unstable, and therefore it's eager to rid itself of the third oxygen atom and it does so by combining with chemicals in the air. This makes chemicals oxidize and thus the ozone air purifier is dramatically effective at eliminating smells and gases.

There has been talk about these ozone air purifiers being dangerous. However, it's important to know that there are two kinds. The first pulls chemical pollutants into the ozone air cleaner and oxidizes them. These, such as the famous Ionic Breeze from Sharper Image, are not considered hazardous.

There are others that are actual ozone generators, though, and these have been found to be dangerous to small animals due to the amount of ozone they produce. Reason suggests, then, that they aren't good for human lungs, either.

Useless: Clearly, while these air purifiers are powerful against chemicals, they can't handle particulates at all.

4. HEPA filters:

HEPA filters are among the oldest air purifier technology and are immensely popular, being used even in related technology like vacuum cleaners.

Effective: they are awesomely effective against particulates, often being able to stop particles as small as 0.3 micron.

They are also extremely effective against all kinds of allergens, which are substances that are recognized by some immune systems as "dangerous" or "foreign", thus causing an allergic reaction. These allergens can be particulates like perfume or dust, or they can be microorganisms like viruses and cat dander.

Useless: HEPA filters are ineffectual against chemical pollutants of any kind.

5. Ionic air cleaners:

Effective: Ionic purifiers create negative ions which changes the electric charge of all particulate which then clump together and fall from the air. Further, this ionic charge causes them to adhere to the surface of the ionic filter.

Useless: if you have smells or chemical pollution in your home, ionic air cleaners will do nothing to remedy your problem.

Clearly, choosing the "best" air purifier involves a clear-headed assessment of your situation and likely a combination of at least two air purifier technologies. Don't let a salesman, no matter how persuasive, make this critical decision for you!