Room Air Purifiers
Room air purifiers replaced tabletop units in the 1990s as the leading type of home air purifier. Tabletop units proved to be mostly ineffective, because of the inadequate volume of air being moved through the unit. Room air purifiers are almost as portable, and they are a lot more effective. There are three basic types of home air purifiers made for single-room use.

HEPA Filtered Air Purifiers
HEPA stands for High Efficiency Particulate Air. Most often made of charcoal, a HEPA filter is placed upwind of a high-velocity fan on the unit, which forces the contaminates into the filter, catching debris such as dust mites, tobacco smoke, pollen, and other pollutants.

Ion Generators
Ionic units work by sending negatively charged ions out into your home. The ions attach to chemicals and particles in the air, causing them to become attracted to the surfaces of your house as well as to each other. The pollutants simply drop right out of the air. You then wipe, mop, and vacuum the material off the surfaces, as part of your regular chores. Ion-generating home air purifiers produce some ozone, so be aware of emission rates.

Ozone Generators
Ozone units work by sending ozone out into your home. The ozone reacts with some chemicals in the air, effectively canceling them out, but producing some byproducts that can be harmful in greater or longer exposures. Buy a unit that emits 50 parts per billion or fewer.

Whole House Air Purifiers
Manufacturers design whole house air purifiers to be installed in a central heating, air conditioning, or HVAC system. The benefit of a whole house air purifier is that it is out of sight. A filter is located in a return air grille or plenum. Anytime the fan in your central system kicks in, air is moved through the filter, effectively cleaning some of the air throughout your house. A whole house air purifier doesn’t run all the time, since central air fans only kick in when the temperature drops too low or climbs too high. For an all-around air cleaning, install a whole house air purifier. But if you want to focus on one room, a room air purifier is much more effective.